It pays to have a spring in your step

Abstract

In humans, a large portion of the mechanical work required for walking comes from muscle-tendons crossing the ankle joint. Elastic energy storage and return in the Achilles tendon during each step enhance the efficiency of ankle muscle-tendon mechanical work far beyond what is possible for work performed by knee and hip joint muscle-tendons.

Figures

Figure 1. Stance limb mechanical power during walking

Bottom panel shows the instantaneous rate of mechanical energy production (Watts) for each of the lower limb joints (ankle, knee and hip) of a single limb over the stance phase of level walking at 1.52 m/s. Positive values indicate energy generation and negative values indicate energy absorption. Top panel is the sum total power produced by the ankle, knee and hip. The ankle joint generates a large portion of the positive mechanical power during stance. Figure is adapted from Devita et al. (5).

Figure 2. Ankle joint ‘catapult mechanism’ during walking

Ultrasound imaging data from humans walking shows the instantaneous rate of mechanical energy production (W/kg) for gastrocnemius muscle fibers and Achilles tendon separately. The muscle-tendon power (bold gray) is the sum of the gastrocnemius muscle power (dotted gray) and the Achilles tendon power (black dashed). Positive values indicate energy generation and negative values indicate energy absorption. The muscle fibers contribute very little to total muscle tendon mechanical power output but the Achilles tendon stores and returns a significant amount of mechanical energy. Figure is adapted from Ishikawa et al. (9).

Figure 3. Ankle joint powered exoskeleton hardware and control

Light weight carbon fiber bilateral ankle-foot orthoses (i.e. exoskeletons) designed to drive ankle plantar flexion with artificial pneumatic muscles during walking. Exoskeletons were controlled using the subjects’ own soleus surface electromyography with proportional myoelectric control. Figure is adapted from Sawicki and Ferris (23).

Figure 4. Mechanical power and net metabolic cost during powered walking at 1.25 m/s

Top panel (A) shows mean (thick black) and one standard deviation (thin black) mechanical power delivered by the ankle joint over the stride from heel-strike (0%) to heel-strike (100%). The mean contribution of the exoskeleton mechanical power (thick gray) + 1 standard deviation (thin gray) is overlaid. Mechanical power is computed as the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass. Positive power indicates energy transferred to the user and negative power indicates energy absorbed from the user. Middle panel (B) shows the mean positive mechanical power delivered by the sum of the ankle, knee and hip joints (black) and ankle joint (white) during unpowered walking and the exoskeletons (gray) during powered walking. Error bars are standard error of the mean. Brackets indicate the percentage contribution comparison between bars (right to left). For example, the exoskeleton average positive mechanical power was 63% of the ankle joint average positive mechanical power over the stride. Bottom panel (C) shows the mean net metabolic power during unpowered (white) and powered (gray) walking. Error bars are standard error of the mean. Right bracket indicates the change in net metabolic power as a percentage difference from unpowered walking (−10%). Mechanical and metabolic power values are normalized by subject mass. Figure is adapted from Sawicki and Ferris (23).

Figure 5. Exoskeleton mechanical power, net metabolic cost and ankle joint apparent efficiency with increasing walking speed

Bars indicate nine subject mean (A) exoskeleton average positive (black), negative (white) and net (gray) mechanical power over a stride for powered walking, (B) change in net metabolic power (powered - unpowered) due to powered assistance from bilateral ankle exoskeletons, and (C) ankle joint apparent efficiency. Apparent efficiency is computed as the ratio of average exoskeleton positive mechanical power to the resulting reduction in net metabolic power and assumes that artificial muscle work directly replaces biological muscle work. For all panels, walking speed increases from left (1.25 m/s) to right (1.75 m/s). All metabolic power values are normalized by subject mass. Error bars are standard error of the mean. Figure is adapted from Sawicki and Ferris (24).

Figure 6. Schematic of joint level mechanics and energetics of human walking

Pie charts indicate the relative amounts (%) of ankle (light gray), knee (medium gray) and hip (dark gray) positive mechanical work (left column) and metabolic energy (right column). The area of each pie chart reflects the magnitude of the total energy per stride that is also displayed numerically below each pie chart. The apparent efficiencies used to convert mechanical energy values to metabolic energy values are indicated in the center column. In panel A, we assumed that all the joints performed positive mechanical work with identical efficiencies set at 0.25 (16). This results in an overestimate of the measured metabolic cost. In panel B, we set the efficiencies so that the ankle joint efficiency matches our measured value of 0.61 (23). The knee and hip were assumed to operate with equal efficiencies (0.24) such that the metabolic energy matched the measured total net metabolic cost per stride (308 J). In panel C, we computed values for a compensated gait where half of the ankle joint work was redistributed to the hip joint. This is meant to represent a hypothetical clinical patient with reduced ankle plantar flexor power at pushoff (e.g. amputee, stroke, or spinal cord injury). Reducing ankle plantar flexor work by 50% and compensating with extra hip work results in a 15% increase in metabolic cost.

Figure 7. Joint mechanical powers at the hip, knee and ankle during gait in transtibial amputees

Transtibial amputees walked at 1.27 m/s with a Solid Ankle Cushion Heel (SACH) prosthetic foot. The graph shows the joint powers for the intact (dashed) and prosthetic limbs (solid). The power generated by the prosthetic “ankle” at pushoff is significantly reduced compared to the intact ankle. Conversely, the corresponding hip power generation is markedly increased demonstrating the shift from ankle power to hip power. Figure was adapted from Sadeghi et al. (21).

Figure 8. Joint mechanical powers at the hip, knee and ankle during gait with natural and increased pushoff

Healthy subjects walked at 1.25 m/s with natural gait (black) and consciously increasing plantar flexion pushoff (gray). The shift between ankle and hip joint powers is apparent as hip power generation at the end of stance is lower in the increased pushoff condition compared to natural pushoff. The figure is adapted from Lewis and Ferris (14).