Mean data and correlation coefficients between aerobic demand and GRF characteristics are shown in Table 1. The difference between the least and most economical subjects expressed as a percentage of the mean aerobic demand, 26.5%, agrees well with results of previous studies (see Martin & Morgan, 1992 for review). There existed no distinct pattern between subjects' VO2 max and their submaximal aerobic demand as was verified by the poor correlation between these two variables (r = -0.08). The most economical runner used only 59% of his aerobic capacity, whereas the least economical subject used 83%. The magnitudes of GRF characteristics were similar to previously reported values (Holden & Cavanagh, 1991; Munro et al., 1987, Williams, 1980). TVI was the only GRF characteristic to be significantly correlated with aerobic demand. More economical runners exhibited a lower TVI. Positive correlation coefficients, although not significant, were also noted between aerobic demand and Favg, Fz rate, NAPI, and NMLI. In the present study, the combined influence of Fz and the "time course" of that force application, as represented by TVI, explains a significant amount of inter-individual variability in running economy. Kram & Taylor (1990) examined several speeds of running-type gait (e.g., trotting, galloping) in different species of animals and reported a significant relationship between tc and aerobic demand. In contrast, tc was not related to aerobic demand in the present study (see Table 1). Williams (1980) showed no trend between low , medium, and high economy groups and TVI. TVI can be considered an indication of overall muscular support during ground contact. Differences in muscle activation patterns during stance may help explain the significant relationship reported here. A recent study that examined this question found a trend that indicated economical runners exhibited more coactivation between two-joint muscles of the leg during stance (Heise et al., 1996). These neuromuscular differences in runners need to be examined further. Of the impulse measures that reflected momentum changes, NAPI and NMLI exhibited the expected positive correlation coefficients with VO2 , however, the relationships were not statistically significant. It is well documented that the mediolateral component of the GRF is quite variable (e.g., Miller, 1990). The variables describing the free moment applied to the platform historically have not been examined by researchers examining GRF characteristics. Here it was hypothesized that any force application that detracted from forward motion may be metabolically wasteful. The free moment, thought of as a "twisting moment" applied by the foot to the ground does not influence aerobic demand. Its low magnitude prior to normalization (mean NAIMz' = 1.58 N.m.s) may partly explain the lack of a relationship. In conclusion, only one of the GRF characteristics (TVI) exhibited a significant, positive correlation with aerobic demand. Several other correlation coefficients were positive as hypothesized, but tc, NVI, and descriptors of the free moment showed no correlation with VO2.
© Copyright 1996 All rights reserved.