The foot comprises of 26 bones, 33 joints, and a network of muscles, tendons, and ligaments that are an integral component of human movement in both habitual and athletic endeavors. Structure determines function, a notion that is reflected well in the foot, enabling wide-ranging degrees and planes of motion attributable to the complex yet the malleable arrangement of structures in the foot (1). Given that the foot serves as a medium between the forces imposed upon but also generated by the body during athletic movement, functions of the foot are not singular, playing a key role in balance, force/impact absorption, and propulsion. (2) The latter two functions form the beginning and end of the spring-like actions that occur in the stance phase that support the performance and economy of running and sprinting (2–5). This spring-like mechanism is not only exemplified across the limb but also in the foot through the windlass mechanism. The windlass mechanism explains how the structures of the medial arch work to store and release mechanical energy, economically and effectively (6,7). However, this mechanism is also reinforced by the contractile contributions of the intrinsic muscles and tendons of the foot, often referred to as the ‘foot core’. During the first phase of ground contact, these muscles lengthen in an attempt to dampen the vertical forces through eccentric lengthening (8). Through this mechanism, elastic energy is stored and returned in the second phase of stance to contribute to the propulsive forces generated by the hip extensors that propel the body forward during running and sprinting (8–10). Subsequently, the strength of the muscles that make up the ‘foot core’ and medial arch is instrumental in the amount to which these stretch-shortening actions are achieved during running and sprinting. Despite this somewhat intuitive and important role the foot has in running and sprinting it is both a poorly assessed and developed aspect in sports performance.
Therefore, the discussion will be focused on reviewing the literature surrounding the impact foot strength has on running and sprinting to understand the reasons for the paucity of foot strength training in physical preparation.
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Given that the determinants of sprinting performance are centered around the ability to produce high force within a short time frame, minimal energy loss or force dissipation at the point of ground contact is not desirable (1,11). With increases in speed the intrinsic foot muscles and longitudinal arch showing a linear relationship, whereby both the magnitude and velocity of activation of these muscles increase significantly (9). This could also be indicative of a greater increase in force generated by the foot muscles in attempts to mitigate compression and maximise energy storage in the arch, particularly when ground reaction forces also follow this trend (12). As mentioned, it is hypothesised that the contributions of the foot in propulsion result from adding to the exhibited extensor moments generated by the rest of the limb. More specifically this refers to the plantarflexion moments local to the fore and midfoot during late stance that suggested aid propulsion. Adaptions to the activation level and muscle size are just two methods to improve this fore/midfoot plantarflexion moment. Yet contrary to this Tanaka et al (13) reports that despite sprinters exhibiting larger foot muscle size than non-sprinters, this difference did not necessarily account for quicker sprint performances. However, it is important to acknowledge the size of the muscles in the foot is an indirect measure of the muscular strength which may explain the contrasting evidence to that presented above and amongst model-based investigations. More specifically Hashimoto and Sakuraba (13) investigated the effect of a course of 8 weeks of intrinsic foot exercise had on foot strength and sprint performance. Participants were able to significantly improve 50M sprint times and vertical performance following foot strength training. Vertical Jump performance is predominately a propulsive action which in conjunction with 50M sprint performance may indicate that these improvements are a result of a greater contribution from the foot in the propulsive portion of stance (late stance). Similar improvements in speed have also been reported amongst long-distance running individuals following a short intervention period (14). Although these findings show evidence of the importance of foot strength to improve running and sprinting performance, there are fundamental limitations of these studies that may compromise the ecological validity of its conclusions. Hashimoto and Sakuraba’s assessed the changes in physical performance amongst a small sample size of 12 male participants increasing the chances of a type 2 error and thus influencing the power of the study. Whereas although Sulowska’s et al used a larger sample size in this study, foot strength exercises were also coupled with other exercises not specific to the foot. Consequently, improvements in running and sprint performance can not be purely attributable to increased foot strength. Ultimately, it is clear that more experimental research and methodological rigor is needed to better understand the role and effect of foot strength in the propulsion portion of stance beyond model-based investigations.
Nevertheless, the impact of foot strength is more popularly researched in the context of running-related overuse injuries, such as medial tibial stress syndrome (MTSS) and plantar fasciitis to name a few. Foot strength comes into focus as a preventative method amongst these pathologies, due to the relationships between these pathologies and reduced force absorption qualities at the foot, the other main function of the foot in running and sprinting (15–17). Specifically, the reduced force absorption qualities are demonstrated in navicular drop, excessive pronation, and/or pronation velocity in both dynamic and static tasks (15–17). Evidence suggests that these motions at the foot are a consequence of greater compliance in the medial arch, in turn impeding the absorption capabilities but also the transference of force through the foot (1,18). Furthermore, Takashi et al (19) investigations show that increased stiffness of the medial arch negates this, which explains the use of foot strengthening exercises to increase the eccentric control of pronation and therefore increasing stiffness and absorption qualities. Recent experimental studies into the impact of foot strengthening exercises have proved successful in improving these risk factors. Following a course of 4 – 6 weeks of foot strengthening exercises across three different studies, participants were able to reduce navicular drop considerably (19–21). However, only one study presented contrasting results with no significant difference in navicular drop (22). The difference between results could be due to the variation in the assessment of navicular drop when compared to the other study, as Lynn et al only measured navicular drop in standing. The gold standard for measuring navicular drop is the recorded difference in navicular height between both standing and sitting posture, this is so that active muscular contributions from the medial arch can be accounted for when assessing navicular drop. Yet, these methodological errors are also present in Mulligans & Cook’s study, only taking into account non-weight bearing navicular height which is not reflective of the weight-bearing conditions of running or sprinting. Nonetheless, it would seem that improving foot strength does has the capability to improve force absorption (and thus reduce running-related injury) of the forefoot and midfoot, but these findings remain challenging to apply to running and sprinting. To better measure whether this relationship exists, more research is needed that can assess foot strength in isolation and review the long-term impact on running-related injury incidences, following foot strength training.
This discussion aimed to review the existing literature that surrounding the impact foot strength has on running and sprinting to understand the reasons for the paucity of foot strength training in physical preparation. Generally, research has begun to unearth the benefits of foot strength, both for performance and injury through biomechanical and experimental investigations. However, the inconsistencies in results, lack of methodological rigor, and scarcity of research (when compared to hip and knee research) can explain the absence of foot strength interventions in running/sprinting physical preparation. To improve this transition from literature to the field, future research should focus on conducting studies that utilise valid gold standard methods of assessing force absorption and propulsive capabilities of the foot and with bigger sample sizes.
References
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