The Influence of a Bout of Exertion on Novice Barefoot Running Dynamics

Barefoot, forefoot strike (FFS) running has recently risen in popularity. Relative to shod, rear-foot strike (RFS) running, employing a FFS is associated with heightened triceps surae muscle activation and ankle mechanical demand. Novice to this pattern, it is plausible that habitually shod RFS runners exhibit fatigue to the triceps surae when acutely transitioning to barefoot running, thereby limiting their ability to attenuate impact. Therefore, the purpose was to determine how habitually shod RFS runners respond to an exertion bout of barefoot running, operationally defined as a barefoot run 20% of mean daily running distance. Twenty-one RFS runners performed novice barefoot running, before and after exertion. Ankle peak torque, triceps surae EMG median frequency, foot-strike patterns, joint energy absorption, and loading rates were evaluated. Of the 21 runners, 6 maintained a RFS, 10 adopted a mid-foot strike (MFS), and 5 adopted a FFS during novice barefoot running. In-response to exertion, MFS and FFS runners demonstrated reductions in peak torque, median frequency, and ankle energy absorption, and an increase in loading rate. RFS runners demonstrated reductions in peak torque and loading rate. These results indicate that a short bout of running may elicit fatigue to novice barefoot runners, limiting their ability to attenuate impact. Key points In response to exertion, novice barefoot runners demonstrate fatigue to their soleus. In response to exertion, novice barefoot runners demonstrate a reduction in ankle energy absorption In response to exertion, novice barefoot runners demonstrate an increase in loading rate Key words: Fatigue, Footwear, Foot-Strike, Loading Rate, EMG, Torque Introduction Barefoot running has recently risen in popularity, largely due to its association with a forefoot-strike (FFS) contact pattern (Lieberman, 2012). Moving from a rear-foot strike (RFS) – the predominant pattern among shoe runners – to a mid-foot strike (MFS) or FFS, results in a shift in mechanical demand from the knee to the ankle, and presumably, a reduced likelihood of knee joint injury (Bonacci et al., 2013; 2014; Williams et al., 2012). The increase in work by the ankle allows for a slower lowering of the body over the compliant ankle-foot complex and a reduction in the vertical loading rate (Lieberman et al., 2010; Shih et al., 2013; Squadrone and Gallozzi, 2009). Control about the ankle during a mid-foot strike MFS or FFS is likely attributed to passive tension of the Achilles/Tibialis Posterior tendons and/or muscle action of the triceps surae (gastrocnemii and soleus) (Ahn et al., 2014). A recent investigation of 1065 runners indicates novice barefoot runners are more likely to maintain a RFS rather than adopt a MFS/FFS (Nunns et al., 2013). Yet, similar to their FFS counterparts, novice barefoot RFS runners contact the ground in a more plantar-flexed foot posture relative to traditional shod running (Williams et al., 2012). This presumably increases reliance upon the triceps surae to help mitigate ground reaction forces, albeit to a lesser extent than when employing a MFS/FFS. This claim is supported by recent findings of Williams et al. (2012) whom instructed novice barefoot runners to RFS and reported an increase in ankle mechanical demand relative to when they ran shod. (Williams et al., 2012) High-level exertion elicits muscle fatigue, which is characterized by a failure to generate the required force/power output for a given task (Fitts, 1994; Phinyoomark et al., 2012). As measured during maximal isometric contractions, in response to fatigue, muscles exhibit a reduction in peak tetanic tension and prolonged fiber contraction and relaxation times (Bigland-Ritchie and Woods, 1984). This leads to lower rates at which muscle fibers shorten and develop tension (Fitts, 1994) and is correlated with reduced motor unit firing frequency (as measured by an EMG median frequency analysis) (Stulen and DeLuca, 1981). Consequently, the ability for lower-extremity muscles to attenuate forces diminishes following fatigue (Milgrom et al., 2007). This possibly explains reports of altered ground reaction force profiles (Zadpoor and Nikooyan, 2012) and reduced shock attenuation (Mercer et al., 2003) among fatigued runners. The aforementioned findings indicate to an increased potential for injury among fatigued runners, and a heightened demand to the triceps surae during barefoot running, regardless of preferred foot-strike pattern. The findings of Morio et al. (2012) – whom reported an increase in lower limb stiffness among novice barefoot runners following an exhaustive stretch shortening cycle exercise to the plantar-flexors – further indicate to a heightened potential for injury in response to a controlled fatiguing task of the triceps surae (Morio et al., 2012). Similarly, Paquette et al. (2016) suggested an increase potential for repetitive load injury among non-RFS runners due to their findings of reduced foot contact angle variability following 40 minutes of continuous running (Paquette et al., 2016). Habitually shod RFS runners who employ a MFS or FFS may over-exert their triceps surae during novice barefoot running, inducing localized muscle fatigue. Theoretically, this would result in a reduced capability to slowly lower the heel (e.g. under eccentric control), and thereby attenuate impact, possibly leading to injury and/or a reversion in running kinematics. This is deemed deleterious as it exposes the calcaneus to high frequency collisions without a shoe sole to attenuate the impact (Lieberman, 2010; 2012). Despite this conjecture, no study to date has examined the potential for triceps surae fatigue among novice barefoot runners, the influence this has on their running dynamics, and how this may differ across foot-strike patterns. A previous investigation conducted by the authors demonstrated variability in lower-limb dynamics among habitually shod RFS runners who perform an acute (i.e. within day) transition to barefoot running (Hashish et al., 2015). Expanding upon these findings, the purpose of the present investigation was to determine how these runners respond to an exertion bout of novice barefoot running performed at their self-selected running speed. We hypothesized that novice barefoot runners would exhibit fatigue to the triceps surae in response to exertion, resulting in altered lower-extremity movement patterns and a reduced ability to attenuate impact.