Does human foot anthropometry relate to plantar flexor fascicle mechanics and metabolic energy cost across various walking speeds?

Abstract

Foot structures define the leverage in which the ankle muscles push off against the ground during locomotion. While prior studies have indicated that inter-individual variation in anthropometry (e.g. heel and hallux lengths) can directly affect force production of ankle plantar flexor muscles, its effect on the metabolic energy cost of locomotion has been inconclusive. Here, we tested the hypotheses that shorter heels and longer halluces are associated with slower plantar flexor (soleus) shortening velocity and greater ankle plantar flexion moment, indicating enhanced force potential as a result of the force-velocity relationship. We also hypothesized that such anthropometry profiles would reduce the metabolic energy cost of walking at faster walking speeds. Healthy young adults (N=15) walked at three speeds (1.25, 1.75 and 2.00 m s-1), and we collected in vivo muscle mechanics (via ultrasound), activation (via electromyography) and whole-body metabolic energy cost of transport (via indirect calorimetry). Contrary to our hypotheses, shorter heels and longer halluces were not associated with slower soleus shortening velocity or greater plantar flexion moment. Additionally, longer heels were associated with reduced metabolic cost of transport, but only at the fastest speed (2.00 m s-1, R2=0.305, P=0.033). We also found that individuals with longer heels required less increase in plantar flexor (soleus and gastrocnemius) muscle activation to walk at faster speeds, potentially explaining the reduced metabolic cost.

Keywords: Biomechanics; Locomotion; Muscle.

Fig. 1.

Schematic diagram of the experimental apparatus, setup and procedure. Participants (N=15) completed shod walking on an instrumented treadmill at three different walking speeds (1.25, 1.75 and 2.00 m s⁻¹). (A) In one visit, we collected kinematic, kinetic, muscle activation (left leg) and ultrasound muscle (soleus; right leg) imaging data. Metabolic measurements were collected during the other visit. The order of the visits was randomized. (B) Participants stood still on a wooden ruler block, and we obtained foot geometric data. Heel length was defined as the horizontal distance from the lateral malleolus to the line of action of the Achilles tendon (posterior aspect of the heel). Hallux length was measured as the horizontal distance from the metatarsophalangeal joint to the distal end of the great hallux.

Fig. 2.

Time-series soleus fascicle mechanics and ankle moment data. (A–C) Average soleus fascicle length (A), velocity (B) and ankle plantar flexion moment (C) time-series data (grand mean±s.d.) during the stance phase of different walking speeds (1.25, 1.75 and 2.00 m s⁻¹) of all participants (N=15). We analyzed the shortening velocity of soleus fascicles both as an average over the stance phase and as the instantaneous velocity at the peak ankle plantar flexion moment. (D–F) Average soleus fascicle length (D), velocity (E) and ankle plantar flexion moment (F) time-series data during the stance phase of different walking speeds of participants in the 1st quartile (dashed lines) and 3rd quartile (solid lines) for heel lengths.

Fig. 3.

Regressions between normalized foot anthropometric measurements and dependent variables. (A) Average shortening velocity, (B) shortening velocity at peak ankle plantar flexion moment, (C) peak ankle plantar flexion moment and (D) cost of transport against heel and hallux length (normalized to height) for each participant (N=15). The soleus shortening velocity at peak ankle plantar flexion moment was negatively associated with heel length, but only for 1.75 m s⁻¹ walking speed (P=0.028); however, when a detected outlier was removed, the correlation was non-significant (P=0.166). Additionally, heel length was significantly associated with the cost of transport only for the fastest walking speed (P=0.033). No other associations were statistically significant.

Fig. 4.

Stance-integrated muscle activation of four left-leg muscles. (A) Lateral gastrocnemius, (B) medial gastrocnemius, (C) soleus and (D) tibialis anterior integrated activation against heel and hallux length (normalized to height) for each participant (N=14). Heel length was statistically significantly associated with soleus and medial gastrocnemius activation (medial gastrocnemius: 1.75 m s⁻¹: R²=0.676, P<0.001; 2.00 m s⁻¹: R²=0.376, P=0.020; soleus: 1.75 m s⁻¹: R²=0.306, P=0.040; 2.00 m s⁻¹: R²=0.263, P=0.061). None of the other stance-integrated muscle activations was statistically significantly associated with anthropometric measurements.