The physical characteristics of the current subjects were similar to those noted in other studies of trained female runners (Daniels et al., 1977; Mayhew, Piper & Etheridge, 1979). In addition, the findings of a linear relationship between running speed and oxygen cost agreed with previous studies (Daniels et al., 1977; Mayhew, Piper & Etheridge, 1979), although the magnitude of the correlation was somewhat lower. The current subjects were comparable in efficiency at the lower end of the speed continuum to females from previous studies (Bransford & Howley, 1977; Mayhew, Piper & Etheridge, 1979). At the upper end of the speed range, the current subjects were more efficient than trained runners from the same university measured more than two decades earlier (Mayhew, Piper & Etheridge, 1979). This could indicate an increase in efficiency due to a greater volume of training by the current subjects. Indeed, this concept was supported by the greater efficiency of more highly trained female runners from two decades ago (Daniels et al., 1977). Caloric expenditures relative to body weight and distance (Kcal/kg/mile) were fairly constant across this narrow range of running speeds (Table 2). This lends support to the idea that relative energy expenditure is independent of running speed (Miller and Stamford, 1987). The moderate correlation between body weight and relative caloric expenditure, however, points toward the possibility that heavier runners may expend more relative energy during running than lighter individuals. This fact may not detract from racing ability in female runners since other phenomena such as maximal steady-state lactate threshold and fractional utilization of the aerobic capacity must be considered. The runners in the current study were most efficient while running at 188 m/min. This could be explained partially by the fact that this pace may be more common to the majority of their longer, over-distance training runs. Training at a faster pace might enhance the efficiency of running at that pace, but it could also open the runner to greater possibility of overuse injuries. The runners were least efficient at the slowest speed, which was well below most of their training pace. Most probably, the runners performed greater vertical lift while running at such as slow speed, a fact which could increase energy expenditure (Daniels et al., 1997). In conclusion, relative oxygen cost of running appears independent of speed and somewhat dependent on body weight in females. Runners may become more efficient at frequently used training paces. Efficiency may or may not be a major factor in short-distance racing performance since other factors could play a major role.
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