Active Voice: Foot Orthoses and Neuromuscular Fatigue — A New Insight?
By Luke A. Kelly

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Luke A. Kelly is a sports podiatrist working at ASPETAR, Qatar Orthopaedic & Sports Medicine Hospital in Qatar. His main research interests are in the field of neuromechanics, with specific focus on how neuromuscular function in the lower limb can be modified to reduce injury risk and improve athletic performance. He is also a Ph.D. candidate at the School of Human Movement Studies at the University of Queensland in Australia. This commentary presents Kelly’s views associated with the research article he and his colleagues published in the Dec. 2011 issue of Medicine & Science in Sports & Exercise® (MSSE).

Prolonged running can lead to various neural (reduced drive to the muscle) and muscular (reduced contractility) adjustments, impairing muscle force generating capacity and culminating in significant modifications to lower limb biomechanics. Emerging evidence suggests that some of these biomechanical changes, such as increased joint loading and increased vertical impact forces, may be important in the pathogenesis of overuse running injuries.

Foot orthoses are often prescribed by sports medicine practitioners as part of treatment and prevention of lower limb overuse injuries in runners. It has been proposed that one such benefit of foot orthoses is their ability to reduce foot pronation forces (inward roll of the rear-foot and collapse of the medial arch), thus reducing the mechanical demand imposed on muscles that help control this movement. Recent evidence shows that running with foot orthoses does reduce the activation of muscles, such as tibialis posterior and tibialis anterior, leading to recruitment patterns that reflect similar activity levels of people with more neutral foot posture (see Murley et al. 2010). However, these studies have only reported data from participants in a non-fatigued state. As recreational runners typically run at levels of moderate exertion for sustained periods, we thought it beneficial to examine the impact of such sustained running with foot orthoses on neuromuscular control as well as the nature of post-exercise fatigue.

As reported in our recent MSSE article, we used a repeated-measures study design, where participants ran for one hour in neutral running shoes and one hour in neutral running shoes with foot orthoses. The runs were conducted seven days apart, and the order was randomized among participants. Based on electromyographic data, our results confirmed that less muscle activity is required to maintain the same constant submaximal speed with orthoses than is required when running without orthoses. Additionally, these neuromuscular adaptations persisted for the entire duration of the run. Immediately after the run, all subjects displayed characteristics of neuromuscular fatigue (reduced ability to generate force in the ankle plantar-flexors), regardless of whether they were using foot orthoses or not. However, there was a difference that favored running with orthoses, when we evaluated the ability to generate muscle force rapidly (an important characteristic in an activity such as running). Thus, after running with orthoses, rapid force production of the plantar flexors was reduced only by six percent, whereas the same participants suffered a 33 percent decline in in this same characteristic after running without orthoses.

This finding sheds light on a potentially important mechanism of the effects of foot orthoses. Runners with over-pronating feet require increased muscular activity, in order to stabilize the lower limb. Over the duration of a sustained run or training period, this may induce muscular fatigue, altered lower limb biomechanics and possibly lead to overuse injury. As runners with increased foot pronation are often prescribed foot orthoses to manage conditions such as patello-femoral pain syndrome and plantar fasciitis, part of the efficacy of foot orthoses may be due to reductions in muscular overload and subsequent protection against muscle fatigue induced by sustained running.