Variability of neural activation during walking in humans: short heels and big calves

Abstract

People come in different shapes and sizes. In particular, calf muscle size in humans varies considerably. One possible cause for the different shapes of calf muscles is the inherent difference in neural signals sent to these muscles during walking. In sedentary adults, the variability in neural control of the calf muscles was examined with muscle size, walking kinematics and limb morphometrics. Half the subjects walked while activating their medial gastrocnemius (MG) muscles more strongly than their lateral gastrocnemius (LG) muscles during most walking speeds ('MG-biased'). The other subjects walked while activating their MG and LG muscles nearly equally ('unbiased'). Those who walked with an MG-biased recruitment pattern also had thicker MG muscles and shorter heel lengths, or MG muscle moment arms, than unbiased walkers, but were similar in height, weight, lower limb length, foot length, and exhibited similar walking kinematics. The relatively less plastic skeletal system may drive calf muscle size and motor recruitment patterns of walking in humans.

Figure 1.

Morphometric measurements of the lower right limb. (a) Posterior view of the leg. The triceps surae complex consists of the medial gastrocnemius (MG), lateral gastrocnemius (LG) and soleus (SOL) muscles. The vertical grey bars denote the positions of the ultrasound probe for measurements [9], which were taken at 30% of lower leg length for the MG and LG muscles and at 50% of lower leg length for the SOL muscle. Lower leg length equalled the distance between the popliteal crease (PC) and the lateral malleolus (LM). (b) Ultrasonic image of the MG. Muscle thickness (MT) was the perpendicular distance between superficial and deep aponeuroses of the muscle. Pennation angle (PA) was the angle between the fascicle and deep aponeurosis of the muscle. (c) EMA of the right foot of each subject was determined by dividing the in-lever of the foot (r, shaded line) by the out-lever arm of the foot (R, dashed line). Measurements between the LM, medial malleolus (MM) and posterior surface of the calcaneal tendon (CT) were used to geometrically calculate r. R represents the average of the lengths between LM and the fifth metatarsophalangeal (MTP) joint and the MM and first MTP.

Figure 2.

(a,b) Representative motor recruitment patterns, or sEMG recordings, of the MG (black, upper), LG (blue, middle), and SOL (red, lower) muscles of an (a) MG-biased subject and (b) an unbiased subject walking at 0.9 m s⁻¹. (c) Muscle bias at all walking speeds. Individuals were denoted ‘MG-biased’ (open symbols) if their bias values exceeded 0.67 at most speeds. ‘Unbiased’ (closed symbols) subjects walked with nearly equal amplitudes of their MG and LG muscles (bias = 0.33–0.66) at most speeds. Each symbol type represents an individual (n = 10). (d) Relative heel length (squares) and relative MG thickness (pluses) vary with average bias. Heel length (r/R) decreases with average bias. The solid line represents the linear regression, heel length = 0.545 − 0.393 × bias (r² = 0.51; p = 0.01), while the dashed line represents the linear regression, relative MG thickness = 0.031 + 0.038 × bias (r² = 0.39; p = 0.009).