Mechanical work performed by the individual legs during uphill and downhill walking

Abstract

Previous studies of the mechanical work performed during uphill and downhill walking have neglected the simultaneous negative and positive work performed by the leading and trailing legs during double support. Our goal was to quantify the mechanical work performed by the individual legs across a range of uphill and downhill grades. We hypothesized that during double support, (1) with steeper uphill grade, the negative work performed by the leading leg would become negligible and the trailing leg would perform progressively greater positive work to raise the center of mass (CoM), and (2) with steeper downhill grade, the leading leg would perform progressively greater negative work to lower the CoM and the positive work performed by the trailing leg would become negligible. 11 healthy young adults (6 M/5 F, 71.0±12.3 kg) walked at 1.25 m/s on a dual-belt force-measuring treadmill at seven grades (0, ±3, ±6, ±9°). We collected three-dimensional ground reaction forces (GRFs) and used the individual limbs method to calculate the mechanical work performed by each leg. As hypothesized, the trailing leg performed progressively greater positive work with steeper uphill grade, and the leading leg performed progressively greater negative work with steeper downhill grade (p<0.005). To our surprise, unlike level-ground walking, during double support the leading leg performed considerable positive work when walking uphill and the trailing leg performed considerable negative work when walking downhill (p<0.005). To understand how humans walk uphill and downhill, it is important to consider these revealing biomechanical aspects of individual leg function and interaction during double support.

Figure 1

Dual-belt force measuring treadmill mounted at 9°. A force platform mounted under treadmill TM1 recorded the vertical, anterior-posterior, and medial-lateral components of the ground reaction force produced by a single leg. The inner edges of the left and right treadmill belts were separated by less than 2 cm.

Figure 2

Average perpendicular (A) and parallel (B) ground reaction forces (GRFs) and center of mass velocities during the stance phase of level, uphill (+9°), and downhill (−9°) walking. Vertical dotted lines indicate the instant of trailing leg toe-off. During double support, thick lines represent the leading leg GRFs and thin lines represent the trailing leg GRFs.

Figure 3

Average individual limb mechanical power over a complete gait cycle normalized to body mass. Vertical dotted lines indicate the periods of double support (DS) and single support (SS) during the stance phase. During double support, thick lines represent leading leg mechanical power and thin lines represent trailing leg mechanical power.

Figure 4

Average (standard deviation) positive (W⁺) and negative (W⁻) individual limb external mechanical work performed (A) by the trailing and leading legs during double support, (B) during single support, and (C) over a complete step normalized to body mass. Double support negative and positive work became negligible with steeper uphill and downhill grade, respectively, and are omitted for clarity (See Table 1 for values). Symbols distinguish mechanical work performed by the leading (triangles) and trailing (circles) legs during double support from that performed during single support or summed over a complete step (squares). Asterisks (*) indicate significantly greater than level walking (p<0.005).