Energy cost of backward running at positive gradients

Introduction Backward walking (BW), on level, has a greater (100%) energy cost (C) than forward walking (FW) due to an impaired ability to exchange potential and kinetic energy. On gradients (+15-32%), C of FW increases according to a reduced capacity to use the pendulum-like mechanism, and the difference between BW and FW is reduced to 5-8% (Minetti & Ardigò,2001). As for running on level, C of backward running (BR) is 30% higher than forward running (FR) (Flynn et al, 1994). A greater external work, and an higher stride frequency (SF) (internal work) explain only part of the higher C in BR. A lower efficiency in BR attributable to a reverse landing-take off asymmetry, and therefore a lower elastic recoil, seems to be another possible explanation for this discrepancy (Cavagna et al.,2011). To our knowledge, C during BR on gradient has not yet been determined. Aim of this study was to compare C of BR and FR on positive gradients. Since in FR store and release of elastic energy decrease with gradient, we hypothesized that the difference in C between BR and FR reduces with the gradient. Methods Tests were performed on 10 young (age 26±3 yr) male athletes (V’O2peak 64,5±2,1 ml*kg-1*min-1). Subjects ran both forward and backward several 5-min bouts on an electrically driven treadmill. Gradient was modified from 0% to 20%. Breath-by-breath oxygen consumption (V’O2) was measured and C was calculated using resting and steady-state VO2 values. SF was obtained by analysing the periodicity in the 30-s video recording (240 Hz). Results On level, C was significantly (p<0,01) higher in BR than in FR (5,41±0,67 vs 3,70±0,43 J*kg-1*m-1). A linear relationship between C and slope was found both in BR (r2=0,98, p=0,001) and in FR (r2=0,99, p<0,001). The difference in C between BR and FR did not decrease with the gradient, setting to a value of 35%. SF did not change with the gradient in FR, and it was significantly (p<0,05) higher in BR than in FR on every gradients. Discussion This is the first study showing C of BR on gradient. Differently from walking, gradient did not affect the difference in C between BR and FR. These results confute our hypothesis and lead to suspect that elastic energy utilization is not the main responsible for the higher C of BR. The difference in C between BR and FR could be partially explained by a different internal work, as suggested by a higher SF in BR. Further studies are needed to better clarify the biomechanical aspects of BR at different gradients. References Minetti AE & Ardigò LP (2001) Pflügers Arch-Eur J Physiol 442:542–546 Flynn TW, Connery SM, Smutok MA, Zeballos RJ, Weisman IM (1994) Med Sci Sports Exerc 26:89–94 Cavagna GA, Legramandi MA, La Torre A (2011) Proc R Soc Lond B 278:339-346