The foot has been considered both as an elastic mechanism that increases the efficiency of locomotion by recycling energy, as well as an energy sink that helps stabilize movement by dissipating energy through contact with the ground. We measured the activity of two intrinsic foot muscles, flexor digitorum brevis (FDB) and abductor hallucis (AH), as well as the mechanical work performed by the foot as a whole and at a modelled plantar muscletendon unit (MTU) to test whether these passive mechanics are actively controlled during stepping. We found that the underlying passive viscoelasticity of the foot is modulated by the muscles of the foot, facilitating both dissipation and generation of energy depending on the mechanical requirements at the centre of mass (COM). Compared to level ground stepping, the foot dissipated and generated an additional -0.2 J kg 21 and 0.10 J kg 21 (both p , 0.001) when stepping down and up a 26 cm step respectively, corresponding to 21% and 10% of the additional net work performed by the leg on the COM. Of this compensation at the foot, the plantar MTU performed 30% and 89% of the work for step-downs and step-ups, respectively. This work occurred early in stance and late in stance for stepping down respectively, when the activation levels of FDB and AH were increased between 69 and 410% compared to level steps (all p , 0.001). These findings suggest that the energetic function of the foot is actively modulated by the intrinsic foot muscles and may play a significant role in movements requiring large changes in net energy such as stepping on stairs or inclines, accelerating, decelerating and jumping.

During human running, softer parts of the body may deform under load and dissipate mechanical energy. Although tissues such as the heel pad have been characterized individually, the aggregate work performed by all soft tissues during running is unknown. We therefore estimated the work performed by soft tissues (N = 8 healthy adults) at running speeds ranging 2 – 5 m· s−1, computed as the difference between joint work performed on rigid segments, and whole-body estimates of work performed on the (non-rigid) body center of mass (COM) and peripheral to the COM. Soft tissues performed aggregate negative work, with magnitude increasing linearly with speed. The amount was about −19 J per stance phase at a nominal 3 m· s−1, accounting for more than 25% of stance phase negative work performed by the entire body. Fluctuations in soft tissue mechanical power over time resembled a damped oscillation starting at ground contact, with peak negative power comparable to that for the knee joint (about −500 W). Even the positive work from soft tissue rebound was significant, about 13 J per stance phase (about 17% of the positive work of the entire body). Assuming that the net dissipative work is offset by an equal amount of active, positive muscle work performed at 25% efficiency, soft tissue dissipation could account for about 29% of the net metabolic expenditure for running at 5 m· s−1. During running, soft tissue deformations dissipate mechanical energy that must be offset by active muscle work at non-negligible metabolic cost.

The metabolic cost of human running is not well explained, in part because the amount of work performed actively by muscles is largely unknown. Series elastic tissues such as tendon can save energy by performing work passively, but there are few direct measurements of the active versus passive contributions to work in running. There are, however, indirect biomechanical measures that can help estimate the relative contributions to overall metabolic cost. We developed a simple cost estimate for muscle work in humans running (N = 8) at moderate speeds (2.2–4.6 m/s) based on measured joint mechanics and passive dissipation from soft tissue deformations. We found that even if 50% of the work observed at the lower extremity joints is performed passively, active muscle work still accounts for 76% of the net energetic cost. Up to 24% of this cost compensates for the energy lost in soft tissue deformations. The estimated cost of active work may be adjusted based on assumptions of multi-articular energy transfer, elasticity, and muscle efficiency, but even conservative assumptions yield active work costs of at least 60%. Passive elasticity can reduce the active work of running, but muscle work still explains most of the overall energetic cost.

The metabolic cost of human running is challenging to explain, in part because direct measurements of muscles are limited in availability. Active muscle work costs substantial energy, but series elastic tissues such as tendon may also perform work while muscles contract isometrically at a lower cost. While it is unclear to what extent muscle vs. series elastic work occurs, there are indirect data that can help resolve their relative contributions to the cost of running. We therefore developed a simple cost estimate for muscle work in humans running (N = 8) at moderate speeds based on measured joint energetics. We found that even if 50% of the work observed at the joints is performed passively, active muscle work still accounts for 76% of the net energetic cost. Up to 24% of this cost due is required to compensate for dissipation from soft tissue deformations. The cost of active work may be further adjusted based on assumptions of multi-articular energy transfer and passive elasticity, but even the most conservative assumptions yield active work costs of at least 60%. Passive elasticity can greatly reduce the active work of running, but muscle work still explains most of the overall energetic cost.