Intrinsic Muscle Strength in Plantar Fasciitis

Plantar fasciitis is something I have frequently blogged about. Plantar fasciitis is very common. I beleive plantar fasciitis is generally badly managed with a lot of snake oil being touted for it and a lot of one size fits all approaches to treatment from one hit wonders. At any one time, if you had a group of people with plantar fasciitis, x% will be about ready to come right due to the natural history of the condition regardless of the treatment given (so even the snake oil and useless treatments will appear to work sometimes). Just look at the response rates in the placebo groups in clinical trials. This means that we really probably should only be using treatments that have been shown to get a better response than a placebo or the natural history. Unfortunately a lot of the treatments used for plantar fasciitis have not been yet subjected to those proper studies or many of those that have have not been tested in well designed and controlled studies (I looked at that in more depth here). The next best thing to that is to use interventions that are consistent with the available evidence and have some biological plausibility and theoretical coherence in their use. To do that we need to rely on study designs that are not prospective clinical trials of the efficacy of an intervention. One such study, a case control study, was just published:

Intrinsic foot muscle volume in experienced runners with and without chronic plantar fasciitis
RTH Cheung, L.Y. Sze, N.W. Mok, G.Y.F. Ng
Journal of Science and Medicine in Sport ; Article in Press
Plantar fasciitis, a common injury in runners, has been speculated to be associated with weakness of the intrinsic foot muscles. A recent study reported that atrophy of the intrinsic forefoot muscles might contribute to plantar fasciitis by destabilizing the medial longitudinal arch. However, intrinsic foot muscle volume difference between individuals with plantar fasciitis and healthy counterparts remains unknown. This study examined the relationship of intrinsic foot muscle volume and incidence of plantar fasciitis.
Case-control study
20 experienced (> = 5 years) runners were recruited. Ten of them had bilateral chronic (> = 2 years) plantar fasciitis while the others were healthy characteristics-matched runners. Intrinsic muscle volumes of the participants? right foot were scanned with a 1.5 T magnetic resonance system and segmented using established methods. Body-mass normalized intrinsic foot muscle volumes were compared between runners with and without chronic plantar fasciitis.
There was significant greater rearfoot intrinsic muscle volume in healthy runners than runners with chronic plantar fasciitis (Cohen’s d = 1.13; p = 0.023). A similar trend was also observed in the total intrinsic foot muscle volume but it did not reach a statistical significance (Cohen’s d = 0.92; p = 0.056). Forefoot volume was similar between runners with and without plantar fasciitis.
These results suggest that atrophy of intrinsic foot muscles may be associated with symptoms of plantar fasciitis in runners. These findings may provide useful information in rehabilitation strategies of chronic plantar fasciitis.

This study compared a group of runners with chronic plantar fasciitis with a group of runners without and found that the bulk (~strength) of the proximal area of the plantar intrinsic muscles was less. This is assumed to indicate that these muscles are weaker. Nothing in the methods or analysis raises any alarms. The crucial thing in a case control designs is the recruitment of the control group and how well they match the cases. This study did match one-for-one. Personally, I prefer to see a lot more controls than cases in these types of studies (as they are easier to recruit) and the make up of that control group is so crucial to the outcome of the study, so you want to make sure it is a good representative group. However, I will not hold it against the above study as both groups do appear to be well matched on the criteria they assessed them on.

By their very nature, case control studies like the one above are observational and cross sectional and not prospective so no causal relationships can be determined. Is the weakness of the plantar intrinsics a risk factor for plantar fasciitis or did the weakness develop after the plantar fasciitis occurred due to the pain. You can not conclude either way based on this study. If I was forced to guess which one is more likely, I would probably pick the later. Whichever one it is, the weakness exists and probably should be addressed as part of the rehabilitation of plantar fasciitis. We do have the recent study for Rathleff et al that looked at stretching compared to strengthening in plantar fasciitis and showed that the strengthening group did better at one time point compared to stretching. However, both groups in that study had silicone heel inserts which work in plantar fasciitis (ie Pfeffer et al, 1999), so we do not know if their results from that study were due to them, natural history or the strengthening and stretching as they did not have an insert only group….even though I want to believe.

As this apparent weakness exists in those with plantar fasciitis regardless if it is a cause or a consequence, it probably does need to be addressed and the use of the exercises like the type advocated from the Rathleff et al study or exercises like the short foot exercise do make sense. It is consistent with the available evidence and is biological plausibility and theoretical coherent.

As always, I go where the evidence takes me until convinced otherwise …. and I will keep up the loading program for the management of plantar fasciitis.

Cheung, R., Sze, L., Mok, N., & Ng, G. (2015). Intrinsic foot muscle volume in experienced runners with and without chronic plantar fasciitis Journal of Science and Medicine in Sport DOI: 10.1016/j.jsams.2015.11.004

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