Energy cost and heart rate response to static and dynamic leg exercise

The energy cost (VO2) and heart rate (HR) response to variable-load static (isometric) and dynamic (isotonic) leg exercise were determined in 5 young women who performed 5-minute bouts of leg extension while in a sitting position on a weight-loaded dynamometer using 4 loads for each type of contraction. Results indicated that VO2 and HR increased as apparently linear functions of workload for both types of exercise, although a difference in the amount of change was observed. When the 2 types of exercise were compared using the same resistance load, the dynamic bouts required a significantly greater net VO2 and HR. At a given VO2 level, HR was higher during static exercise. Net HR also appeared to increase more steeply as a function of VO2 during static than during dynamic exercise. It was concluded that bouts of static and dynamic exercise which employ equal loads for an equal period of time are not physiologically equal or directly comparable since the VO2 and HR response are not the same for the 2 types of exercise. For this reason, VO2 and HR response should not be used interchangeably for relating static to dynamic workloads, or when prescribing exercise for the purpose of patient conditioning or rehabilitation.