The correlation between metabolic and individual leg mechanical power during walking at different slopes and velocities

Abstract

During level-ground walking, mechanical work from each leg is required to redirect and accelerate the center of mass. Previous studies show a linear correlation between net metabolic power and the rate of step-to-step transition work during level-ground walking with changing step lengths. However, correlations between metabolic power and individual leg power during step-to-step transitions while walking on uphill/downhill slopes and at different velocities are not known. This basic understanding of these relationships between metabolic demands and biomechanical tasks can provide important information for design and control of biomimetic assistive devices such as leg prostheses and orthoses. Thus, we compared changes in metabolic power and mechanical power during step-to-step transitions while 19 subjects walked at seven slopes (0°, +/-3°, +/-6°, and +/-9°) and three velocities (1.00, 1.25, and 1.50m/s). A quadratic model explained more of the variance (R(2)=0.58-0.61) than a linear model (R(2)=0.37-0.52) between metabolic power and individual leg mechanical power during step-to-step transitions across all velocities. A quadratic model explained more of the variance (R(2)=0.57-0.76) than a linear model (R(2)=0.52-0.59) between metabolic power and individual leg mechanical power during step-to-step transitions at each velocity for all slopes, and explained more of the variance (R(2)=0.12-0.54) than a linear model (R(2)=0.07-0.49) at each slope for all velocities. Our results suggest that it is important to consider the mechanical function of each leg in the design of biomimetic assistive devices aimed at reducing metabolic costs when walking at different slopes and velocities.

Keywords: Biomechanics; Locomotion; Metabolic; Slope; Step-to-step transition.

Copyright © 2015 Elsevier Ltd. All rights reserved.

Figures

Fig.1

Average (S.D.) metabolic power (Pmet) for seven slopes and three velocities (n=19). In general, Pmet increased at slopes greater or less than −3° and increased with velocity within each slope condition. Specific pairwise comparisons are described in Section 3.

Fig. 2

Average (S.D.) mechanical work of individual legs during step-to-step transitions. (a) Mechanical work of the trailing leg (Ptrail) during the step-to-step transition at each slope and velocity. Ptrail was more positive with increasing slope from −9° to 9° and at faster velocities. (b) Mechanical work of the leading leg (Plead) during the step-to-step transition at each slope and velocity condition. Plead was more positive with increasing slope from −9° to 9° and at slower velocities.

Fig. 3

Relationship between metabolic power (Pmet) and mechanical power of the individual legs during the step-to-step transition phase. For both the trailing leg (Ptrail) (A–C) and leading leg (Plead) (D–F), data are presented for all slopes for all subjects at each velocity tested. We found that a quadratic model best described the correlation between Pmet and Ptrail for all three velocities (R2=0.57 to 0.65) and the correlation between Pmet and Plead for all three velocities (R2=0.71 to 0.76).

Fig. 4

Average (S.D.) individual limb power ratio (ILPR) of the trailing and leading legs during step-to-step transitions at each slope and velocity. ILPR is calculated as the ratio of individual leg mechanical power during the step-to-step transition and overall metabolic power. (a) Trailing leg ILPR was maximized at 0° and decreased at slopes greater or less than 0° at 1.00 m/s and 1.25 m/s. At 1.50 m/s, trailing leg ILPR was maximized at −3°. Regression models were ILPR= −0.0009s2−0.0036s+0.3506 at 1.00 m/s (R2=0.21), ILPR= −0.0031s2+0.0019s+0.4851 at 1.25 m/s (R2=0.72), and ILPR=−0.0046s2− 0.0002s+0.5642 at 1.50 m/s (R2=0.78), where s is slope (deg). (b) Leading leg ILPR increased up to six-fold and decreased by up to 73% at −9° and 9°, respectively, compared to 0°. There were no differences in ILPR due to changes in velocity. Regression models were ILPR=0.2783e−0.219s at 1.00 m/s (R2=0.71), ILPR=0.2511e−0.292s at 1.25 m/s (R2=0.78), and ILPR=0.3458e−0.257s at 1.50 m/s (R2=0.77).