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First published online August 31, 2007
Journal of Experimental Biology 210, 3255-3265 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000950

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Mechanical power and efficiency of level walking with different stride rates

Brian R. Umberger^1,2,* and Philip E. Martin^2,3

¹ Department of Kinesiology, University of Massachusetts, Amherst, MA 01003, USA
² Department of Kinesiology, Arizona State University, Tempe, AZ 85287, USA
³ Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA

View larger version (24K): [in this window] [in a new window]   Fig. 1. Group mean sagittal plane joint angles (A–C) and angular velocities (D–F) for the hip, knee and ankle during walking with stride rates above (+20%), below (–20%), and at the preferred rate. The ±10% conditions have been omitted for clarity, but were generally intermediate to the preferred and ±20% conditions. At lower stride rates, there was a greater range of motion at the hip and a lower range of motion at the knee. The joint angular velocities were similar across stride rates for all three joints during the early to mid stance phase (0–50% of the gait cycle), but were higher for the hip and the knee during the late stance phase and the swing phase (50–100% of the gait cycle). Flex, flexion; ext, extension; DF, dorsiflexion; PF, plantarflexion.

View larger version (9K): [in this window] [in a new window]   Fig. 2. Group mean vertical (A) and anterior–posterior (B) ground reaction forces (normalized to body weight, BW) during walking with stride rates above (+20%), below (–20%), and at the preferred rate. The ±10% conditions have been omitted for clarity, but were generally intermediate to the preferred and ±20% conditions. At higher stride rates there were greater fluctuations in the vertical force, reduced peak anterior-posterior forces, and the stance phase lasted a greater proportion of the gait cycle. Zero and 100% of the gait cycle correspond to right heel strike, and the end of the stance phase coincides with the forces going to zero.

View larger version (24K): [in this window] [in a new window]   Fig. 3. Group mean sagittal plane net joint moments (A–C) and powers (D–F) for the hip, knee and ankle during walking with stride rates above (+20%), below (–20%), and at the preferred rate. The ±10% conditions have been omitted for clarity, but were generally intermediate to the preferred and ±20% conditions. Moments have been normalized to body weight (BW) and leg length (LL), and powers have been normalized to body mass. At higher stride rates there were larger moments and more power produced at the hip joint, and smaller moments and less power produced at the ankle joint. The knee joint was the major site of energy absorption, with more absorption occurring at higher stride rates. Flex, flexion; ext, extension; Gen, generation; Abs, absorption; PF, plantarflexion; DF, dorsiflexion.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Net rate of metabolic energy expenditure (A), average positive (B) and negative (C) mechanical power (summed over the hip, knee and ankle joints) and net mechanical efficiency (D) during walking with the preferred stride rate (0), and with stride rates above (+) and below (–) the preferred rate. Metabolic and mechanical powers have been normalized to body mass. Values are means ± 1 s.d. and the vertical arrows indicate the location of the predicted minimum or maximum for each variable. The horizontal bars are the confidence interval for the predicted minimum or maximum. Net metabolic power was minimized near the preferred stride rate, while mechanical power exhibited a plateau that spanned stride rates lower than preferred, and mechanical efficiency exhibited a plateau that spanned stride rates higher than preferred.

View larger version (5K): [in this window] [in a new window]   Fig. 5. Net mechanical efficiency computed two different ways for walking with the preferred stride rate (0), and with stride rates above (+) and below (–) the preferred rate. Efficiency was computed either including (closed circles) or excluding (open squares) the magnitude of the negative power in the denominator of the efficiency expression (see text for details). When negative power was ignored, the computed efficiency was higher, with the effect being greater at higher stride rates. The overall dependence of mechanical efficiency on stride rate was similar for both expressions.