The effect of intraspecific variation in calcaneal tuber length on endurance running economy

Abstract

Intraspecific- and interregional variation in human endurance running performance are both long studied topics. Endurance running is defined as the ability to run long distances (5>km) using aerobic metabolism. As today, athletes from East Africa dominate endurance running competitions, the proposed reasons for the observed variations are numerous and in some cases ethically questionable. As today, no consensus exists on the matter. The answer as to why humans are efficient endurance runners at all, as opposed to many other mammals, can be found in the rich literature concerning human evolution. The process of human feet evolution especially has led to the development of specific traits that enhance endurance running; these traits emerge after the divergence between Chimpanzees and the ancestral hominin. This paper focuses on the Achilles tendon moment arm as specific variable trait that may affect endurance running performance. The fossil record provides evidence for the transition from arboreality to bipedal walking and endurance running, in the form of fossil morphology and trackways. Early hominins, such as the Australopithecines, are thought to represent the transition between arboreality and bipedal walking, while Homo erectus is thought to be the first capable of endurance running. The main selection pressure for endurance running is related to the persistence hunting theory. The persistence hunting theory states that the human thermoregulatory efficiency, combined with increased musculoskeletal efficiency would enable early hominins to hunt prey to exhaustion. Early hominins adapted musculoskeletal and thermoregulatory traits that would favor persistence hunting and grant first access to prey. One such musculoskeletal trait is the Achilles tendon, the Achilles tendon behaves as an elastic spring, in which energy can be stored and reutilized. The amount of energy stored in the Achilles tendon is a function of mainly the Achilles tendon moment arm length. A fossil proxy for Achilles tendon moment arm is the calcaneal tuber length. In this paper it is hypothesized that a smaller tuber increases the energy storage in the Achilles tendon and reduces the cost of transport (J/kg/m) of endurance running at 4 m/s. Calcaneal tuber length was measured on two sample groups (Haya, Kenya and Sedgeford, England) to evaluate intraspecific but also interregional differences in cost of transport for running at 4 m/s. The Kenyan group as opposed to the English group demonstrated a statistically significant smaller average calcaneal tuber lengths, t(32)= 1.69, p =0.026. This experimental data was implemented in the biomechanical software program OpenSim, and combined with optimization theory in MATLAB. 3D predictive simulations were performed to test the hypothesis. Predictive simulations, as opposed to non-predictive simulations allow both the kinematics and kinetics to change due to a change in calcaneal morphology. Sensitivity experiments were performed to evaluate the effect of scaling, locomotive speed, tendon stiffness and initial guess mode. Due to limited sample control, no conclusions can be based on interregional differences. Based on maximal observed difference in calcaneal tuber length (0.338 cm), a smaller calcaneum reduces the cost of transport with 4.5% at 4 m/s and 0.78% at 1.33 m/s. The reduction in cost of transport is attributed to a reduction muscle energy consumption of the peroneus brevis and peroneus longus muscles. No significant relationship was found between tendon energy storage in the Achilles tendon and calcaneal length. The scaling procedure proved to have limited effect on the triceps surae muscle tendon unit. A more compliant tendon (20 N/cm instead of 35 N/cm) resulted in lower cost of transport for running and higher cost of transport for walking. The fossil record is too fragmentary to provide any conclusive evidence on calcaneal evolution. It appears that the general foot evolution is directional towards enhanced bipedalism, but that for specific endurance running either many reversals occurred or that hominin species developed specific traits individually.