Background
Plantar heel pain (PHP) is a common condition affecting the lower limb. It is most prevalent amongst middle aged adults affecting between 4 and 7% of the general population as well as accounting for up to 8% of running related injuries [
1‐
3]. Importantly, PHP can have detrimental impacts including pain, reduced function and negative psychological effects [
1,
4,
5].
Typically, those most affected with PHP are more likely to be obese and stand for longer periods of time [
6,
7]. In addition, athletically active individuals (e.g. runners) are also commonly affected [
3,
8,
9]. However, recent research has also found that reduced muscle function, strength and size is associated with people with PHP compared to people without [
10]. Commonly used interventions [
11‐
14] do not seem to address this association and they only have limited effectiveness with 45% of cases reporting they still have symptoms 10 years after the initial onset of symptoms [
15]. It is possible, therefore, that addressing the associated reduced muscle function and strength may improve treatment outcomes.
Improving muscle strength has been used successfully for managing other lower limb complaints [
16]. However, while existing strengthening programs for PHP are frequently prescribed, these programs are inconsistent and have undergone limited rigorous evaluation of their effectiveness [
17‐
20]. These limitations include: no consensus on which muscles to target, what exercises to use, and what program of repetitions, resistance, and frequency to implement. Accordingly, clinicians have no evidence-based guidelines to follow when recommending strengthening programs for PHP. Without such guidelines, clinicians can only base their recommendations on anecdote and what little literature is available.
Therefore, to begin the process of acquiring an evidence base and developing guidelines, experts in the field need to combine their expertise to develop a strengthening program for PHP that can be rigorously evaluated and refined if necessary. Expert development will provide the essential initial step to rigorously investigate such a strengthening program; that is, to initially define and then refine the muscles to target, the exercises to use, and the program of repetitions, resistance, and frequency to apply.
With this in mind, this study aimed to seek information from experts to develop a consensus-driven progressive strengthening program for PHP. The findings of this study may inform future clinical trials that evaluate the effectiveness of strength training for PHP.
Discussion
We conducted this Delphi study to gain consensus from a panel of experts on a program of strengthening exercises for PHP. Experts initially completed a questionnaire about important inclusions and exclusions in a progressive strengthening protocol. Before the next rounds, three vignettes were created to broadly represent three common but different patient types, recognising that one program may not be suitable across a range of sub-populations. Experts were then asked to agree or disagree, and provide feedback, for each of the proposed progressive strengthening programs. By the end of three rounds there was consensus on all three programs.
When experts were asked for their opinion on exercises (in Rounds 1 and 2) to be included three exercises were consistently recommended throughout the Delphi study, which were heel raises, digital plantarflexion and the short foot exercise. However, there was significant variation in how these exercises were described and applied. Heel raises, or exercises to improve calf strength, were the most commonly suggested exercise. However, a recent systematic review found that there is no difference in heel raise capacity between those with and those without PHP [
10], so this recommendation may diverge from current evidence. Interestingly, both the heel raise exercise and the heel raise with the digits dorsiflexed exercise variation were occasionally not recommended by some experts due to perceived difficulty performing these exercises or provocation of symptoms. This inconsistency indicates that there is a need to better understand the role of exercises for PHP, including the barriers to using them. Furthermore, there is little robust evidence for the benefit of the exercise selections for those with PHP.
To ensure this program meets the optimum requirements of a strengthening program [
31], we wanted to ensure that there was provision to progress the exercises and the programs with a focus on increasing strength. Muscle strength and electromyographic muscle activity can be increased by escalating exercise complexity towards more functional tasks and increasing the loads applied to the muscles during exercise [
32,
33]. The results of this study recommend increases in weight (within each stage of the progressive strengthening program) and functionality (between individual stages of the program). The current American College of Sports Medicine (ACSM) Progression Models in Resistance Training for Healthy Adults Guidelines state that these loading principles should be applied to all strengthening programs if the goal is to increase strength [
31]. We believe that the final programs in this Delphi study address techniques to progress patients with increased loads and functionality. However, further research evaluating their effectiveness would be beneficial.
In order to provide progression for increasing load on muscles, experts were given an opportunity to choose a repetition maximum to guide the weight choice for a given exercise. However, one concern from some of the experts in this study was the use of the expression ‘repetition maximum’, they stated this phrase was counterintuitive as it determines both the weight and the number of repetitions a patient could achieve for a given exercise. Upon reflection, we believe it is a limitation of the programs provided in our study and could cause some confusion. However, we wanted to quantify the amount of weight a patient could use to provide a method of progression in the program. A solution, for the clinician, may be to use an appropriate weight to meet the repetitions specified or use the repetition maximum in its place. The ACSM guidelines suggest a weight that is 60% of a one repetition maximum should be used to increase strength. However, a recent network meta-analysis suggests higher-load (> 80% of single repetition maximum) prescriptions maximise strength gains, and all prescriptions included in the analysis promote muscle hypertrophy [
34].
Another method of progression, training frequency, was also debated within the expert panel. Some experts questioned whether daily or every second day was the most appropriate timeframe to facilitate increases in muscle strength with particular exercises. The ACSM guidelines do not specifically recommend how frequently exercises should be performed for maximum strength gains. The guidelines suggest up to four sets of an exercise per week for an untrained individual and up to 10 sets per week for trained individuals [
31]. However, similar to the increasing load guidelines above, a recent network meta-analysis suggests that performing resistance training 2–3 times per week achieves the greatest muscle strength and hypertrophy gains [
34].
Several strengths underpin this study. The experts selected were from a range of countries and were well distributed across professions dealing with PHP. In addition, the majority of experts had completed a PhD at the time of enrolment in the study, implying a deeper understanding of research and evidence. Finally, a relatively even distribution between clinical and academic experts in the study ensured a spread of knowledge between practical and theoretical approaches.
However, this study should also be viewed considering four limitations. Firstly, the overall response rate of those experts initially approached to participate was 67% and the retention rate across the three rounds was 75%. Delphi studies frequently have low response rates, however a retention rate of at least 70% or greater for each round is generally considered satisfactory [
21,
22,
35]. It is possible that a larger sample may change the results of this study, although we believe our sample was broadly representative of the experts available. Secondly, we do not know the effectiveness of the exercise programs developed in this study. Future clinical trials that evaluate the effectiveness of these programs may change our understanding, which will subsequently influence future expert opinion. Nevertheless, as our knowledge currently stands, the programs we gained consensus for in this study provide a basis for further evaluation and offer clinicians a consensus-driven program to use for PHP. Thirdly, similar to many Delphi studies, we only considered items that achieved > 70% agreement to have an acceptable level of agreement, but we did not assess other factors such as percentage disagreement of, and variability in, answers from participants. Fourthly, this study only canvassed the opinions of experts, so we did not survey patients for their opinions regarding patient preferences. Indeed, one qualitative study found that participants with PHP reported that they ‘don’t feel as strong in (their) whole body’ and they are ‘frustrated with the exercises provided’ [
36]. This highlights that patient preference could be critical when developing and refining strengthening programs. Accordingly, it would be worthwhile if future research investigated patient preferences in exercise prescription for PHP.
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