Authors: John W. A. Osborne (Discipline of Podiatry, School of Allied Health, Human Services and Sport, La Trobe University, Melbourne, Australia), Hylton B. Menz (Discipline of Podiatry, School of Allied Health, Human Services and Sport, La Trobe University, Melbourne, Australia), Glen A. Whittaker (Discipline of Podiatry, School of Allied Health, Human Services and Sport, La Trobe University, Melbourne, Australia), Matthew Cotchett (Discipline of Podiatry, School of Allied Health, Human Services and Sport, La Trobe University, Melbourne, Australia), Karl B. Landorf (Discipline of Podiatry, School of Allied Health, Human Services and Sport, La Trobe University, Melbourne, Australia)
Categories: Review, ankle, foot, resistance training
Source: Journal of Foot and Ankle Research
Doi: 10.1002/jfa2.70040
Muscle Strengthening Exercises for the Foot and Ankle: A Scoping Review Exploring Adherence to Best Practice for Optimizing Musculoskeletal Health
Authors: John W. A. Osborne, Hylton B. Menz, Glen A. Whittaker, Matthew Cotchett, Karl B. Landorf
Abstract
Background
Foot and ankle muscle strengthening exercises are common interventions for many musculoskeletal conditions that are associated with pain and limited function in the lower limb. The scientific literature has a multitude of strengthening exercises recommended, and they have been criticized for not adhering to best practice and for being poorly reported. The aims of this scoping review were to (i) describe what foot and ankle strengthening exercises have been recommended in the scientific literature, (ii) compare the prescription of these exercises to best practice recommendations, and (iii) assess the completeness of the reporting of these exercises and exercise programs.
Methods
This scoping review was conducted in accordance with the Joanna Briggs Institute methodology for scoping reviews. A systematic search of peer‐review journal articles was conducted on 23 February 2023. Study designs that were included were experimental, quasi‐experimental, feasibility, pilot studies, and observational. For each study included in the review, study design and participant details such as age, sex, and conditions treated were noted. To describe the foot and ankle strengthening exercises, each exercise was noted, which included its name, the number of sets and repetitions recommended, the load type and its magnitude, and whether there were any progression strategies. Exercises were grouped according to primary movement and a general exercise descriptor. To compare to best practice, each program's prescription parameters of frequency, intensity, and time were compared to the American College of Sports Medicine's (ACSM) guidelines. To assess completeness of reporting, each study was assessed with the Consensus on Exercise Reporting Template (CERT).
Results
The search yielded 1511 documents, and 87 were included after full‐text screening. Of the included studies, most were randomized controlled trials, and the most common participants were healthy adults (mean age 18–83 years). Across all studies, a total of 300 foot and ankle exercises were prescribed. The most common strengthening exercise category involved ankle plantar flexion (25% of 300 exercises), followed by plantar foot intrinsics (16%). The most common prescription of strengthening exercises included 3 sets (37%) of 10 repetitions (38%) performed 3 times per week (34%), often without a prescribed load (66%). Prescribed sets per muscle group met ACSM recommendations for novice lifters in 93% of studies. In contrast, load intensity (for increasing muscle strength) was prescribed at the recommended dose of 60% of 1 repetition maximum or greater in only 2% of exercises. The median score for completeness of reporting according to the CERT checklist was 31% of all items.
Conclusions
This scoping review found that the studies predominantly included ankle plantar flexion and plantar foot intrinsic muscle strengthening exercises, typically prescribed at 3 sets of 10 repetitions, 3 times per week. When compared to best practice recommendations, load intensity in exercise prescription is commonly less than recommended or is not reported. In addition, the review highlights deficiencies in the reporting of exercise programs. We propose using established best‐practice exercise prescription guidelines like those from the ACSM and the adoption of CERT for reporting exercises in the scientific literature.
Background
Foot and ankle muscle strengthening exercises are a common treatment modality for managing foot and ankle musculoskeletal conditions [1]. For example, strengthening exercises have been prescribed for several foot and ankle conditions to reduce pain and improve function [2, 3, 4]. They can also be used to improve strength for increased athletic performance and to provide general psychological benefits [5, 6, 7]. There are a multitude of strengthening exercises that are commonly prescribed, but there is ongoing debate about whether current exercise prescriptions effectively optimize strength gains [8, 9, 10, 11]. In addition, strengthening exercises, as well as exercises more generally, have been criticized for being poorly reported, which can lead to inconsistent prescription and poor reproducibility [12, 13, 14].
To ensure optimal exercise prescription, researchers and clinicians should consider how exercises are prescribed and the quality of their reporting. Fortunately, guidelines have been developed to provide structure for both the prescription and reporting of exercise [15, 16]. The American College of Sports Medicine (ACSM) has developed guidelines to provide recommendations for the optimization of strengthening exercise prescription [15]. These guidelines provide advice on the appropriate dosage of exercise training variables such as frequency (times per week), intensity (load type, magnitude, and progression), and time (sets and repetitions) to achieve maximal strength gains. In addition, for the reporting of exercises, the Consensus on Exercise Reporting Template (CERT)—a 16‐item minimum checklist to improve the reporting of exercise programs—was developed to improve transparency and reproducibility in clinical and research settings [16]. However, it is currently unclear whether exercises to strengthen the foot and ankle meet best practice guidelines or are being reported appropriately.
Therefore, the aims of this scoping review were to (i) describe what foot and ankle strengthening exercises have been recommended in the scientific literature, (ii) compare these exercise prescriptions to best practice recommendations, and (iii) assess the completeness of the reporting of these exercises and exercise programs.
Methods
Study Design
This study is a scoping review that utilized a systematic literature search. To provide a robust framework, the review was conducted in accordance with the Joanna Briggs Institute methodology for scoping reviews and is reported using the Preferred Reporting Items for Systematic Reviews and Meta‐Analyses Extension for Scoping Reviews [17, 18].
Eligibility Criteria
Document Type
Studies needed to have been reported in a peer‐reviewed journal.
Types of Studies
Study designs included the experimental and quasi‐experimental (including randomized controlled trials, nonrandomized controlled trials, and single‐group pre‐post intervention studies), feasibility and pilot studies (randomized and nonrandomized), and observational studies (including case series, case reports, case‐control, and cohort studies). Systematic reviews were not included; however, the original studies included in any systematic review were considered for inclusion.
Participants
Studies were included if foot and ankle exercises were prescribed as part of a strengthening program for adults (aged 18 years or over) with or without musculoskeletal conditions or injury, and the program was prescribed for 1 week or longer. Studies were excluded if participants had chronic or systemic neurological or cardiovascular disorders, such as diabetes, neuropathy, and Charcot–Marie–Tooth disease.
Types of Strengthening Exercises
Studies that provided foot and ankle strengthening exercises across multiple sessions were included. Studies where an exercise was prescribed for immediate pre‐post testing (e.g., assessing the intrinsic muscle activation during or immediately after the performance of the short foot exercise) or used special equipment that is not readily utilized in clinical practice (e.g., isokinetic strengthening machines) were excluded. If a study mentioned that rehabilitation was performed without specifying “strengthening” or the exercises themselves, these were also excluded. All foot and ankle strengthening exercises in a study were included in the analysis.
Search Strategy and Information Sources
A systematic search was conducted on 23 February 2023 (Supporting Information S1). The Ovid platform was used to search Ovid MEDLINE (1946‐present) and CINAHL (1980‐present). To broaden the search, some terms were truncated with wildcard symbols. All keywords were searched in title, book title, abstract, original title, name of substance word, subject heading word, floating subheading word, keyword heading word, organism supplementary concept word, protocol supplementary concept word, rare disease supplementary concept word, unique identifier, and synonyms. All titles and abstracts found in the search were imported to Zotero 6.0 (Digital Scholar, Vienna, Virginia, USA) for inclusion assessment. After duplicate removal, articles were then assessed based on the title and abstract by the primary author (J.W.A.O.) for inclusion in the review. Articles deemed appropriate for inclusion had the full text obtained and reviewed for eligibility requirements. Once eligibility was determined, each article was analyzed and relevant data extracted by the primary author with spot checks by two other reviewers (M.P.C. and K.B.L.). A PRISMA flow diagram (Figure 1) is included to document the phases of the systematic review. Gray literature and the reference lists of systematic reviews and articles that met inclusion criteria were also hand‐searched for additional relevant articles.
Samples and Exercise Descriptions (including Definitions)
Data were extracted from studies included in the review by the primary author (JWAO) using Microsoft Excel (Microsoft Corporation, Redmond, Washington, USA). The data included details about the study design, population targeted for recruitment, conditions treated, mean age, and sex of participants. Exercise data were extracted defining the exercises prescribed, including the name of the exercise. The primary author then grouped exercises by context to enable categorization of the primary movement and a general exercise descriptor. Further exercise prescription details were extracted including frequency (exercise frequency and duration of program), intensity (load intensity, load type, and progression of exercise), and time (sets, repetitions, etc.).
Populations Targeted
The population targeted for recruitment into each study was documented as stated in each article's text.
Conditions Treated
The conditions treated needed to be specified in the text. Specific conditions were grouped according to broad categories. For example, midportion and insertional Achilles tendinopathy were grouped as “Achilles tendinopathy” and chronic and functional ankle instability were grouped as “ankle instability”.
Primary Movement
Exercises were grouped according to the primary movement of the joint during the exercise. This included exercises for the ankle (plantar flexion, dorsiflexion, inversion, or eversion) or the digits (flexion, extension, adduction, and abduction). Exercises where more than one movement was needed were considered “multiple” (i.e., multiple movements). Exercises without movement occurring at a joint (e.g., standing still) were considered “static”.
Muscle Group
To calculate sets per muscle group, we determined that all exercises performing a primary movement, (e.g., ankle plantar flexion or ankle inversion), would be considered one muscle group. Therefore, if a study had three ankle plantar flexion exercises where the participant performed three sets of each, it was considered nine sets for the ankle plantar flexion muscle group.
Exercise Categories
We grouped exercises by category to provide them with context relative to their intended aim. These categories included targeted movements (ankle plantar flexion, ankle dorsiflexion, ankle inversion, and ankle eversion), specific structures of interest (plantar intrinsics, hallux, and lesser digits), and the desired outcome (balance and dexterity) of the exercise. For example, resisted ankle inversion targets the movement of ankle inversion, the short foot exercise targets the specific structures of the plantar intrinsics, and scrunching digits and writing the alphabet with the foot target the outcome of improving the dexterity of the digits, foot, and ankle.
Frequency
A tally of the program's training frequency (e.g., to perform the program or exercise three times a week) was documented. In some studies, different exercises in a program were performed at different frequencies, so the frequency was documented per exercise rather than per study.
Duration of Program
The duration of the program (e.g., an 8‐week exercise program) was documented.
Intensity (Load Intensity)
The intensity was also quantified as a percentage of repetition maximum (RM), and/or the raw value provided was documented. When a weight or resistance band type was provided with no context of its RM, it was only documented as the weight specified.
Load Type
Load types were documented, which included any equipment that created a resistance or load for the exercise (e.g., Thera‐Band, bodyweight, and weighted bags). Where nothing was specified, “not reported” was documented.
Progression of Exercise
Exercise progressions were documented and categorized as “increasing in volume only,” “increasing in load only,” “increasing in volume then load,” “increasing in load then volume,” “increasing in load and volume,” an “alternative” method that was neither load or volume, an “unspecified” method of progressing exercise, or “not reported.”
Time (Sets and Repetitions)
The tally of sets and repetitions prescribed for each exercise was documented independently. The repetition range (e.g., heel raises for 10 repetitions), time (e.g., inverting the ankle for 10 min), and distance (e.g., scrunching a 1‐m length of towel) were also documented.
Comparison of Exercise Prescription to the ACSM Guidelines Recommendations
Comparison was made to the ACSM guidelines for exercise testing and prescription, which provide recommendations for the best dosage of strengthening exercise variables (e.g., frequency, intensity, and time) to achieve strength gains in the following novice, intermediate, and advanced lifters [15]. The term “lifter(s)” refers to anyone performing resistance strength training; a novice lifter is someone who has little to no experience with strength training and an advanced lifter is someone who performs strength training regularly and has years of experience [15]. The guidelines were summarized to compare each study against best practice (Table 1). The summary provides the optimal number of sets per muscle group, load, or resistance relative to repetition maximum and the frequency of performing the exercise. Groups with different amounts of training experience have different recommendations for achieving optimal strength gains [15].
When comparing exercise prescription variables to the ACSM guidelines, the following structures were used. For repetitions, the prescribed dosage for each exercise was compared to the ACSM guidelines. For sets per muscle group, all exercises within each study were grouped according to primary movements (e.g., all ankle plantar flexion exercises were grouped together) and then the collective sets were compared against the ACSM guidelines. For load intensity and training frequency, the prescribed dose of each individual exercise was compared against the ACSM guidelines. Finally, every program within each study was compared to best practice recommendations to ascertain if it met all three domains of sets, recommended load intensity, and frequency.
Quality of Reporting
The primary author (JWAO) assessed the quality of reporting of exercises in each study using the CERT checklist [16]. The CERT is a 16‐item checklist developed by an international panel of experts, and was designed to improve the transparency and reproducibility of exercise programs used in scientific studies [16].
Results
Characteristics of Included Studies
The search yielded 1511 documents; after removing 62 duplicates, 1449 documents were screened by the title and abstract. Following the title and abstract review, there were 292 relevant documents, and 87 documents were included after a full‐text screening (Table 2). Most studies were randomized controlled trials [2, 4, 8, 19, 22, 23, 24, 27, 29, 31, 32, 34, 35, 36, 37, 39, 40, 43, 44, 45, 46, 47, 51, 52, 53, 55, 56, 57, 58, 59, 60, 62, 63, 65, 66, 67, 69, 71, 72, 73, 74, 75, 77, 82], followed by pre‐post intervention studies [20, 26, 28, 38, 42, 49, 61, 64, 68, 70, 76, 84, 87, 90, 91, 95, 96]. Also included were trial protocols [30, 33, 54, 79, 94], feasibility studies [80, 93, 101], nonrandomized controlled trials [21, 41, 48, 78], pilot studies [50, 83], case reports [25, 86], and a case series [81]. Where both a protocol and a trial were available, either the protocol or trial was used for results depending on which provided more details.
Populations Targeted
The mean age across studies was 39 years with a range of 18–83 years. Of the 87 studies, 38 (44%) used a general population of adults [2, 10, 20, 23, 24, 25, 27, 30, 31, 32, 34, 35, 36, 38, 39, 40, 41, 42, 45, 46, 49, 52, 55, 56, 59, 62, 63, 64, 66, 70, 73, 76, 79, 80, 81, 100, 101, 103], where specific subgroups of adults were used in 49 studies, including older adults in 12 of 87 studies (14%) [4, 21, 37, 54, 58, 78, 82, 84, 85, 87, 89, 95], females only in 5 (6%) [19, 29, 53, 72, 77] and males only [61], male sedentary adults [44], and young adults (between the ages of 18–30 years) [60] in 1 (1%) each. Twelve of 87 studies (14%) targeted athletes [22, 28, 33, 50, 65, 75, 83, 86, 91, 92, 93, 97]; 5 (6%) included runners [28, 83, 91, 92, 93], 3 (3%) included recreational athletes [33, 75, 86], 2 (2%) included unspecified athletes with a previous ankle sprain [50, 97], and 1 (1%) each for volleyball [65] and collegiate athletes [22]. Of the 87 studies, 7 (8%) included individuals with pes planus [43, 48, 51, 67, 71, 96, 102] and 6 (7%) post‐surgical patients [27, 57, 69, 90, 94, 98]. Four studies (5%) recruited college or university students [26, 47, 88, 99].
Conditions Treated
Of the 87 studies, 31 targeted healthy individuals (36% of 87 studies) [4, 21, 28, 32, 37, 38, 43, 44, 48, 54, 58, 59, 64, 67, 68, 70, 71, 73, 77, 78, 83, 84, 85, 87, 91, 92, 93, 95, 96, 102]. Where specific conditions were targeted, ankle conditions were commonly chosen with 18 studies (20%) recruiting individuals with ankle instability [22, 26, 34, 35, 36, 45, 47, 52, 55, 56, 60, 65, 75, 76, 88, 97, 99, 100], 5 (6%) ankle fractures [27, 57, 69, 90, 98], and 4 (5%) acute and chronic ankle injuries [23, 24, 41, 50]. Tendinopathies were also targeted with seven studies (8%) including Achilles tendinopathy [31, 33, 42, 62, 63, 74, 103] and three (3%) tibialis posterior tendinopathy [40, 49, 81]. Eight studies (9%) provided exercise as an intervention for plantar heel pain [2, 25, 30, 39, 46, 79, 80, 101]. Less common conditions included hallux valgus in three studies (3%) [19, 53, 72], falls and balance issues in two (2%) [82, 94], and metatarsalgia [20], general foot pain [89], and posterior impingement [86] in one (1%) each. There were several studies that targeted more proximal conditions including patellofemoral joint pain in two studies (2%) [51, 66], lower limb [29], and low back pain [61] in one (1%) each.
Exercise Details
Exercise Category
A total of 300 exercises were prescribed across all studies. The most common category of exercise was ankle plantar flexion exercises for 75 individual exercises (25%), followed by 50 exercises for plantar intrinsics (16%), 45 for ankle and digital dexterity (15%), and 27 for ankle dorsiflexion (9%). Ankle eversion (23, 8%) and ankle inversion exercises (22, 7%) had similar representation across all exercises prescribed. There were substantially fewer exercises targeted at the digits hallux (10, 3%) and lesser digits (1, < 1%). Other exercises included balance (15, 5%), plyometrics (3, 1%), and those classified in the other category (29, 10%) (Supporting Information S2).
Primary Movement
Out of the 300 exercises, the most common primary movement featured in exercises prescribed across all studies was ankle plantar flexion (84, 28%), followed by digital flexion (39, 13%), ankle dorsiflexion (34, 11%), arch lifting (32, 11%), ankle eversion (23, 8%), ankle inversion (22, 7%), and digital abduction (4, 1%). There were 14 (5%) static exercises that did not elicit a movement (i.e., balancing exercises or voluntary isometric contractions) and 44 (15%) that used multiple movements in one exercise (i.e., tracing the alphabet with the foot and ankle or curling a towel with the digits). Less common movements were digital extension (2, < 1%) and digital adduction (2, < 1%) (Supporting Information S2).
Exercise Prescription Variables
In relation to frequency, there was a large variation from once every hour (or hourly) to once per week. The most common frequency to perform exercises was three times per week for 101 of 300 exercises (34%), followed by daily (68 exercises, 23%), three times per day (20, 7%), five times per week (19, 6%), and two times per week (14, 5%). Other lesser prescribed frequencies included 30 min per week (9 of 300 exercises, 3%), every other day (3, 1%), once per week (4, 1%), 2 times per day (13, 4%), 2 to 3 times per day (8, 3%), 3 to 4 times per day (6, 2%), 4 times per day (2, < 1%), 4 to 5 times per day (4, 1%), hourly (2, < 1%), and a frequency was not reported 27 times (9%) (Supporting Information S2).
In relation to intensity of load, 198 of the 300 exercises (66%) did not have any specified type of load and 13 (4%) specified no load to be used. Of the remaining 89 exercises, 12 different types of load were prescribed. The most common equipment or objects to provide resistance or load were Thera‐Band, (51 of 300 exercises, 17%), followed by bodyweight only (11, 4%), elastic tubing (8, 3%), a weighted bag (5, 2%), dumbbells (4, 1%), pin‐loaded weights (2, < 1%), ankle weights (1, < 1%), the contralateral foot (1, < 1%), furniture (1, < 1%), gel toe separators (1, < 1%), and a Pilates reformer (1, < 1%) (Supporting Information S2).
A specified load intensity was not provided in 234 of the 300 exercises (78%). For the 66 exercises that it was provided, the load intensity, according to a participant's RM, was reported for 9 exercises (3%) [2, 39, 58, 78, 79, 80, 87]. Specifically, it was reported as a percentage of a 1RM for 1 exercise (< 1%) [58] or a percentage of 3RM for 2 exercises (< 1%) [87]. Alternatively, it was reported as a specified number RM without a percentage for 6 exercises (2%) [2, 39, 78, 79, 80]. Commonly, load intensity relative to an individual's RM was not reported, although a prescribed load was still provided for 56 exercises (18%). When a load intensity was prescribed not relative to a repetition maximum, the most common were red or medium Thera‐Band (23 exercises, 8%) and bodyweight (11 exercises, 3%). Other load intensities included varieties of Thera‐Band from extra heavy for seven exercises (2%) to light for two exercises (< 1%), a one to 2 kg (kg) weight for one exercise (< 1%), 3 kg weight for one exercise (< 1%), and no weight at all for six exercises (2%).
In relation to progression, 176 of the 300 exercises (59%) used progressions. Quantifiable progressions (i.e., using a prescribed quantity of load or volume) were used in 146 exercises. When quantifiable progression was used as part of an exercise prescription, the most common method was increasing load only (81 exercises out of 300, 27%), followed by volume only (28, 9%), volume then load (23, 8%), load then volume (12, 4%), volume or load (11, 4%), and volume and load (2, < 1%). Alternative progressions, such as changing the surface people, were using or giving people a choice of progression strategy, occurred in 8 exercises (3%), and an unspecified progression strategy occurred in 11 exercises (4%). It was unclear whether the alternative and unspecified progression offered by the program contributed to a load or volume increase or required a greater skill level to complete the exercise (Supporting Information S2).
In relation to repetitions, using a fixed number was the most common approach when prescribing exercises, which was used in 221 of the 300 exercises (74%). The most common repetition value used was 10 repetitions for 115 of 300 exercises (38%), followed by 15 repetitions (41, 14%), then 30 repetitions (19, 6%). Other repetition values included 5 (10, 3%), 8 (9, 3%), 12 (15, 5%), 20 (7, 2%), 100 (2, < 1%), and 200 (2, < 1%) repetitions. For one exercise, 26 repetitions [41] were used; this accounted for writing each letter of the alphabet once. Repetitions were not specified for 41 (13%) exercises (Supporting Information S2).
Some studies used a repetition range instead of a fixed number. Ten to 15 repetitions were the most common range (7, 2%), followed by 5–15 (4, 1%), 5–10 (2, < 1%), and 20–25 (1, < 1%) (Supporting Information S2). Other studies did not use a number or range, electing instead to use time or distance. The most common repetitions for time values were to perform an exercise for 10 min (8, 3%), followed by 3 min (2, < 1%), 1 min (2, < 1%), and 10 s (2, < 1%). All other timed approaches were only used once (< 1%), including 30 s, 45 s, 30–60 s, and 2, 4, 5, 6, 20, and 30 min (Supporting Information S2). One exercise prescribed a distance or length measure of a one‐m towel for toe scrunches (Supporting Information S2).
In relation to sets, most studies reported the number of sets for exercises. The most common number of sets prescribed per exercise was 3 sets across 110 of 300 exercises (37% of exercises), followed by a single set for 94 exercises (31%), 2 sets for 27 exercises (9%), 5 sets for 4 exercises (1%), 8 sets for 3 exercises (1%), and 4 sets for 3 exercises (1%). Ranged sets were also prescribed with ranges of two to three sets for seven exercises (2%), two to four sets for four exercises (1%), and one to three sets for two exercises (< 1%). Two studies prescribed one exercise each to be performed “as many [times] as possible” (< 1% of all exercises). Sets were not specified for 57 exercises, accounting for 19% of all prescribed exercises (Supporting Information S2).
Comparison of Exercise Prescription to the ACSM Guideline Recommendations
Firstly, we compared the training frequency of programs in the included studies to the ACSM guidelines [15] (Figure 2).
The recommended frequency for a novice or untrained individual in the ACSM guidelines is two to 3 days per week [15]. The frequency of exercise (i.e., how often to perform them) was reported 273 times across the 300 exercises (91% of all exercises). The prescribed frequency to perform an exercise ranged from once a week to hourly. The most common frequency was 3 times a week across 101 of 300 exercises (33%), which meets the novice and untrained lifter recommendations. Recreationally trained and athletic individuals are recommended to train less frequently at 2 days per week [15]. Training 2 times per week was prescribed for 14 of the 300 exercises (5%) and for once a week for 4 exercises (1%). Other frequencies prescribed per exercise of the 300 exercises included daily (69 of 300 exercises, 23%), not specified (27, 9%), 3 times a day (20, 6%), and 5 times a week (19, 6%). Aside from “not specified,” all of these prescriptions were more than any of the recommendations in the ACSM guidelines.
Secondly, we compared the intensity of load prescribed in the studies included in this review to the recommendations provided by the ACSM guidelines [15] (Figure 3).
The guidelines report all load intensity according to a RM, as it provides a valid and consistent reference point for determining load intensity for an individual [104, 105]. The recommended load intensity for a novice or untrained lifter is 60%–70% of a 1RM (60%–70% 1RM) or 8 to 12RM [15]. None of the 87 studies included in this review reported a percentage of loading between 60% and 70% 1RM. However, an 8RM load intensity was reported for 5 of the 300 exercises (2%) [2, 78, 79, 80] and 1 study reported a 10RM for 1 exercise [39]. This load intensity would meet recommendations for novice lifters. One exercise was prescribed at 50%–60% of 1RM [58], and 2 were prescribed at 50%–55% of 3RM [87]; both are less than the recommendation for a novice lifter. The recommended load intensity for intermediate and advanced lifters is between 80% and 100% 1RM [15], which was not met in any study or for any exercise.
Thirdly, the number of sets per muscle group prescribed in the studies included in this review was compared to the recommendations for strength training in the ACSM guidelines [15] (Figure 4).
To achieve optimal strength gains, the guidelines outline sets per muscle group and not per exercise. For this review, the muscle group was defined as a primary movement (e.g., plantar flexion and ankle inversion). According to the ACSM guidelines, the recommended number of sets per muscle group for novice lifters is between one and three [15]. In 28 of the 87 studies included in this review (32%), one set of exercises per muscle group was prescribed [4, 20, 27, 29, 30, 37, 41, 48, 51, 53, 57, 59, 60, 61, 64, 66, 67, 68, 73, 77, 89, 91, 92, 93, 94, 98, 100, 102]. In 16 studies (18%), 2 sets per muscle group were prescribed [4, 30, 35, 38, 41, 49, 53, 58, 66, 75, 77, 82, 92, 93, 97, 103]. The largest prescription per muscle group was 3 sets per muscle group, which was prescribed in 40 studies (46%) [2, 21, 22, 25, 32, 34, 35, 39, 41, 42, 43, 44, 45, 46, 47, 50, 52, 54, 56, 57, 62, 63, 65, 66, 70, 71, 75, 76, 78, 84, 85, 88, 89, 92, 93, 95, 96, 99, 101, 103]. For more experienced lifters, the ACSM guidelines recommend “multiple sets to be used with systematic variation of volume and intensity over time.” Of the 87 studies, 4 sets or more per muscle group were prescribed in 21 studies (30%) [4, 19, 20, 21, 28, 30, 31, 33, 34, 36, 40, 41, 49, 55, 66, 74, 86, 87, 89, 95, 97] with the number of sets ranging from 4 to 16. Of the 26 studies providing 4 or more sets, 4 of those studies specify athletes (i.e., advanced lifters) as participants [28, 33, 86, 97]. No specific sets were specified for 35 primary movements across 10 of the 87 studies (11%) [23, 24, 26, 69, 72, 79, 80, 81, 83, 90], with 2 of these studies prescribing “as many sets as possible,” which could achieve the recommendations for novice, intermediate, or advanced lifters [79, 80].
Finally, we assessed each program to assess how many met all recommendations for frequency, intensity, and time. For novice or untrained lifters, this would equate to 1 to 3 sets, at 60%–70% of a 1RM, 3 times per week. This was met in 5 of 87 studies (6%) [2, 39, 78, 79, 80]. For the intermediate or advanced lifter, this would equate to greater than 3 sets (as an increase in volume and intensity from a novice), at 80% or more of 1RM, 2 to 3 times a week. No programs met these recommendations.
Quality of Reporting
Of the 87 studies, the number of items reported on the CERT ranged from 1 to 15 (out of a maximum of 16). The median CERT score of all included studies was 5, equating to only 31% of the items on this checklist. Eighty‐three of the 87 studies (95%) provided a “detailed description of the exercise intervention including, but not limited to, number of exercise repetitions/sets/sessions, session duration, intervention/program duration, etc” [2, 4, 6, 19, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60], 63 studies (72%) provided “detailed description of the type of exercise equipment (e.g., weights, exercise equipment such as machines, treadmill, and bicycle ergometer)” [2, 4, 7, 21, 23, 24, 26, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 39, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 54, 58, 59, 61, 62, 63, 65, 66, 67, 68, 69, 70, 71, 72, 76], and 58 (66%) provided a “detailed description of the decision rule(s) for determining exercise progression and a detailed description of how the exercise program was progressed” [2, 4, 8, 21, 23, 25, 26, 33, 34, 35, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 52, 54, 55, 56, 58, 59, 60, 61, 62, 63, 64, 65, 66, 68, 69, 70, 73, 75, 77, 78, 79, 80]. A “detailed description of any home program component (e.g., other exercises, and stretching)” was reported in 18 studies (24%) [23, 29, 33, 34, 37, 57, 61, 63, 66, 67, 69, 74, 79, 84, 87, 92, 93, 95] and a description of “whether there are any nonexercise components (e.g., education, cognitive–behavioral therapy, and massage,)” was reported in 8 studies (9%) [2, 4, 33, 37, 57, 79, 80, 87]. Seven studies (8%) provided sufficient information for the item “detailed description of motivation strategies” [2, 41, 42, 79, 80, 92, 93], “describe the type and number of adverse events that occurred during exercise” [4, 30, 42, 57, 58, 79, 80], and for “describe the decision rule for determining the starting level at which people commence an exercise program (such as beginner, intermediate, advanced, etc)” [2, 30, 70, 79, 80, 82, 85]. Sufficient information was provided for the item “Describe whether the exercises are generic (one size fits all) or tailored to the individual and detailed description of how exercises are tailored to the individual” in 20 studies (23%) [2, 4, 23, 30, 32, 45, 46, 49, 50, 58, 63, 66, 70, 78, 79, 80, 86, 89, 92, 93] and “how adherence or fidelity to the exercise intervention is assessed/measured and the extent to which the intervention was delivered as planned” in 17 studies (20%) [2, 21, 29, 30, 32, 34, 43, 45, 46, 47, 49, 50, 71, 74, 79, 80, 91]. No studies fulfilled all criteria (Supporting Information S3).
Discussion
Foot and ankle muscle strengthening exercises are a common intervention. They can be used for healthy populations to maintain adequate strength, although they are often prescribed by clinicians for managing lower limb musculoskeletal conditions [93, 106]. Our review set out to describe and critique the exercises outlined in the literature. Specifically, we aimed to (i) describe what foot and ankle strengthening exercises have been recommended in the scientific literature, (ii) compare these exercise prescriptions to the ACSM guidelines for prescription of strengthening exercises, and (iii) assess the completeness of the reporting of these exercises and exercise programs using the CERT.
Exercises Prescribed
For the studies included in our review, the ankle was the most commonly targeted region for strengthening exercises at the foot and ankle. Ankle plantar flexion exercises comprised 25% of the total exercises and were the most frequently prescribed exercise category and primary movement. This may be partly due to most conditions being related to ankle pathology. However, more than one third of the samples included healthy adults, demonstrating that most foot and ankle muscle strengthening programs, regardless of pathology, emphasize enhancing ankle plantar flexion movements. In addition, there was a high frequency of prescription of plantar intrinsic exercises, highlighting the significance attributed to intrinsic foot musculature. Interestingly, a relatively large percentage (15%) of dexterity exercises were prescribed, such as ankle alphabet (an exercise tracing letters using the ankle as the primary mover) and toe yoga (exercises using the digits to move independently of one another).
For loading intensity of strengthening exercises, studies used various types of resistance equipment, but red medium‐strength resistance bands (such as Thera‐Band) were found to be the most frequently used. Interestingly, while the use of resistance bands was a common method to provide resistance, no specified resistance or no resistance at all was used for most exercises. Achieving maximal strength gains requires adequate load intensity to stimulate change [10]. Inadequate load intensity has the capacity to not provide adequate stimulus to facilitate gains in strength [8, 107]. Prescribing inadequate loads is a consistent theme in this scoping review.
Exercise progression was incorporated into the majority of foot and ankle strengthening programs. This highlights that the investigators of the studies included in our review acknowledged that strengthening exercises require progression to achieve the best possible gains. This approach also aligns with the fundamentals of strength training and the recommendations outlined in the ACSM guidelines [15]. Most programs opted to increase load before increasing the volume, which is consistent with the ACSM recommendations that suggest increasing loads by 2%–10% for smaller muscle exercises (such as at the foot and ankle) when an individual can perform 2 or more repetitions beyond the initial prescription without reaching fatigue [15]. While load before volume may have been the most common, there was a large variety of approaches used to progress exercises across all studies, including increasing the perceived skill difficulty of an exercise and leaving the progression to a practitioner's discretion. Accordingly, further refinement and understanding of the effects of foot and ankle strength training exercise selection and prescription protocols on foot and ankle musculature should be a priority for future research.
For time, a diversity of approaches was used, but fixed repetition was by far the most common prescription method. Sets and repetitions were often prescribed as 3 sets of 10 repetitions per exercise. However, this prescription is a 'one size fits all’ or generic approach and may not consider strength training fundamentals such as specificity or variation for optimum benefits at the foot and ankle [105]. Furthermore, like all strength training protocols, this prescription structure was designed for training the entire body (including large muscle groups such as the hips or chest) and has been adapted for use with proximal structures rather than being specifically designed for achieving maximal strength gains at the foot and ankle.
Comparison to Best Practice
The ACSM guidelines have established dosage recommendations for strengthening exercise prescriptions across different populations [15]. For novice lifters (with some mild reduction in volume for certain older populations), it is recommended that optimal strength gains occur when training 4 sets per muscle group (or primary movement), using 60%–70% 1RM with 2–3 training sessions per week [8, 9, 10, 15]. For intermediate and advanced lifters, higher load intensity and an increase in training volume are recommended for increased strength gains with the ACSM recommending advanced individuals cycle loads of 80% 1RM or greater [15]. These recommendations are supported by several meta‐analyses to estimate the optimal number of sets and load intensity for beginners, the recreationally trained, and athletes to achieve strength gains [8, 9, 10].
The prescription of training frequency, in the studies included in our review, often exceeds best practice recommendations, with some exercises prescribed excessively (e.g., hourly and daily) compared to the ACSM guidelines of three times per week for a novice and less than that for advanced lifters [15]. While frequency may impact strength gains less than load intensity or set volume, adequate recovery time is an important programming consideration for strength adaption [108]. Increasing frequency must always be viewed in the context of the collective volume of load intensity, sets, and exercise selection [15]. Although occasionally, high frequency training might be beneficial, several meta‐analyses have shown that the highest yield of strength gains occurs at two times per week for advanced individuals [8, 10].
Regarding load intensity, higher loads are needed to achieve optimal strength gains compared to hypertrophy or muscular endurance [8, 11]. However, load intensity, in relation to a RM, was only reported in 3% of exercises in our review. Repetition maximum is not only a valid measure to determine an individual's strength capacity, but also the most appropriate method to determine the prescription of load intensity for that individual [109, 110, 111]. There may be some balance exercises (e.g., single leg balance) where it is difficult to determine a load based on RM, but balance exercises only accounted for 5% of the exercises included in our review. When loads were reported, they rarely met the ACSM guidelines, with 2% of exercises reporting a load intensity that would be considered acceptable for novices or for more frail populations aged over 65 years (i.e., between 60% and 80% 1RM) and none meeting recommendations for intermediate or advanced lifters (i.e., 80% RM or greater) [15]. Moreover, load intensity was infrequently reported. Load was not reported in 76% of exercises; although we cannot be certain, some possibly used bodyweight only. If this was the case, these exercises may have been under‐loaded (or over‐loaded) to optimize for strength gains compared to the recommendations outlined by the ACSM guidelines. Furthermore, many exercises provided resistance bands and weights under 3 kg, which could also represent loads that do not meet recommendations. As the ankle joint complex can produce a force up to five times an individual's body weight during running, this amount of load is likely to be inadequate [112]. Future studies could avoid this by prescribing loads as a percentage of RM at an appropriate load intensity, as suboptimal loads will not lead to maximal strength gains.
Most programs met the set volume recommendations for novice lifters, with 62% prescribing between 1 and 3 sets per muscle group. Considerably, fewer studies included programs with four or more sets. The investigators concluded that athletes (advanced lifters) should consider increases in volume up to eight sets per muscle group to achieve optimal strength gains [8, 9, 10]. This is a significant increase in volume when compared with the recommendations for novice lifters [8, 9, 10], but is consistent with the recommendations set by the ACSM guidelines [15]. However, of the 12 studies that recruited athletes included in our review, only 2 prescribed 8 or more sets for a primary movement [28, 86]. Therefore, in the majority of studies, the advanced lifter or athletic population was prescribed inadequate volume to optimize strength gains. Regarding the 12 studies designed for the older populations, the prescriptions were between 1 and 6 sets per muscle group, which is consistent with all the other studies aimed at healthy adults that ranged from insufficient to more than recommended for older or more frail populations [15]. There is an opportunity that researchers consider prescribing a greater volume of sets per primary movement to optimize outcomes for strength gains, especially if targeting athletic populations. This may also be the case for clinicians if they are basing their prescriptions on current scientific literature, that is, the studies included in our review.
Finally, only 5% of the studies met all ACSM recommendations for a training program for novice lifters in relation to sets, load intensity, and frequency. These programs prescribed 3 sets per primary movement (plantar flexion), with loads of 8 to 10RM, performed 3 times per week [1, 2, 3, 4, 5]. By meeting the needs of novice lifters, these recommendations may better represent the requirements of the general population. However, the small percentage of studies meeting these guidelines highlights that most foot and ankle strengthening programs in the studies included in our review did not meet recommendations for optimal strength gains.
Overall, while all strength training yields positive outcomes, adherence to best practice principles can optimize strength gains [10, 11]. Discrepancies between what was prescribed in the studies included in our review and best practice suggest that more tailored and informed strengthening programs should be used for the foot and ankle to optimize strength gains, with a focus on increasing load intensity (i.e., % RM) as the priority.
Quality of Reporting
As previously found [12, 13, 14], exercises were generally poorly reported. We found that nearly all studies included in our review reported a name for each exercise. In more than half of the studies, the names of exercises were synonymous with how the exercises are performed. However, many of the exercises were difficult to understand due to the complexity of the description. For example, the “short foot exercise” is challenging to explain and demonstrate. Indeed, due to its complexity, this exercise also is also referred to by other names, including “doming” and “plantar arch raise,” and the descriptions of how to perform the exercise also varies widely. Certainly, many exercise names do not create a clear understanding of how to perform the exercise. This raises the question of whether simply providing a name or description of the exercise is sufficient. Supporting images and videos of how an exercise is performed would be helpful, which has previously been suggested by Christensen et al. [12]. A package of information that includes consistent naming and descriptions of specific exercises that also has accompanying audio and/or visual information would likely provide optimal information for an individual to successfully perform the exercises. While this would be worthwhile for patients, it would also be helpful to improve consistency and repeatability in research.
The establishment of a tailored approach to an exercise program was generally under reported. When considering a tailored approach, future studies may consider tailoring prescription variables to individuals' capacity using a % of RM rather than giving everyone the same prescribed load (i.e., 3 kg or medium Thera‐Band). Such an approach would allow for each program to be adequately tailored for each participant, according to their capacity or pain levels. It would also be easier for clinicians to translate the program into practice and would provide some basis to determine if the program requires modification based on a patient's level of discomfort. This could easily be achieved by determining a participant's RM for any given weight at the start of the program and provide an adapted weight relative to the participant's %RM prescribed. Further, this RM method can be translated to different equipment, from Thera‐Bands to dumbbells, and is not a new concept to strength and conditioning literature. The authors acknowledge that the RM method has only been established for bench press, squat, and deadlift, but it has not yet been established for foot and ankle exercises [111]. Nonetheless, the principle can still be applied to the foot and ankle until more research is conducted. Based on this, the resistance (or load) can be tailored for an individual's capacity (i.e., use more or less resistance or load), and doing so, increases the likelihood that the prescription, including the progression of load, is optimized to achieve strength gains.
When considering conditions of the foot and ankle such as plantar heel pain and tendinopathies, everyday walking and other loads contribute to the cumulative load on tissues [113]. Therefore, the inclusion of the reporting item “whether there are any home program” or “nonexercise components” is useful in understanding the overall loads to which participants are subjected. These items are important as additional loads, outside of those applied in a strength training program, may negatively influence pain and function [114]. Accordingly, we recommend that future studies include a statement about how much incidental exercise or other loading exercises may or may not be included to provide a more complete indication of the entire loading regime.
With the above in mind, our findings indicate that future research offering strength training as an intervention should include detailed descriptions (with online video instructions likely being even more effective), including dosage of exercises and programs for completeness, transparency, and optimal benefit.
Strengths and Limitations
There are several strengths of our review, which collated 87 studies and 300 exercises to summarize the literature of foot and ankle strengthening exercises. The review includes studies from various countries and covers a range of conditions, providing a broad perspective on what is currently being prescribed. It also compared their exercise prescriptions to best practice guidelines and evaluated the reporting of these exercises. We believe this is the first review of this kind to be published.
However, our review has a number of limitations that need to be considered. The authors acknowledge that only two databases have been searched, and relevant studies may have been missed. However, to account for this, hand searching of reference lists, gray literature, and studies included in systematic reviews was performed.
Only one author performed all screening and data extraction of included articles and data. Additional reviewers would have improved the rigor of the search, screening and data extraction process. Future reviews should consider two or more reviewers to screen and extract data.
Exercise prescription is complex and can include many variables that were not assessed in our review (e.g., rest, time between sets, and exercise tempo). Instead, we sought to focus on the key variables of sets, repetitions, and frequency as they are common prescription variables used by researchers and clinicians.
The ACSM best practice guidelines were developed for the whole body and were not specific to the foot and ankle. Understanding the impact of exercise choice and its prescription variables (sets, loading magnitude, and training frequency) on the foot and ankle specifically would be beneficial.
A recommendation of our review is to use the CERT as a method for reporting exercise, which aligns with previous research [13, 14]. However, we are aware that one study [12] has suggested that the CERT lacks enough specific details to allow adequate translation to clinical practice.
Finally, the ACSM guidelines and CERT checklist were published after many of the studies in our review were conducted, so the low adherence to these was not unexpected.
Conclusion
This review found that prescriptions of foot and ankle muscle strengthening exercises predominantly recommend exercises targeted at the ankle plantar flexion and intrinsic muscles, typically prescribed at 3 sets of 10 repetitions 3 times a week, often using no prescribed load. In addition, foot and ankle exercise programs do not provide adequate load intensity to maximize strength gains when compared to recommendations from the ACSM. There are also substantial deficiencies in the reporting of foot and ankle exercises, making them difficult to understand and implement. We suggest adopting established best practice recommendations like the ACSM guidelines for exercise testing and prescription and the CERT for improved exercise reporting.
Author Contributions
John W. A. Osborne: conceptualization, formal analysis, investigation, methodology, visualization, writing–review and editing. Hylton B. Menz: conceptualization, formal analysis, investigation, methodology, writing–review and editing. Glen A. Whittaker: investigation, methodology, writing–review and editing. Matthew Cotchett: investigation, methodology, writing–review and editing. Karl B. Landorf: conceptualization, formal analysis, investigation, visualization, writing–review and editing.
Ethics Statement
The authors have nothing to report.
Consent
The authors have nothing to report.
Conflicts of Interest
H.B.M. is the Emeritus Editor and Professor K.B.L. is a member of the Editorial Board of the Journal of Foot and Ankle Research. It is a journal policy that editors are removed from the peer review and editorial decision‐making processes for manuscripts they have coauthored.