Effects of different after-loads and knee angles on maximal explosive power of the lower limbs in humans

Maximal explosive power during two-leg jumps was measured on four sedentary subjects [mean age 43.0 (SD 10.3) years, mean height 1.74 (SD 0.04) m, mean body mass 73.5 (SD 1.3) kg] using a sledge apparatus with which both force and speed could be directly measured. Different after-loads were obtained by positioning the sledge at five different angles (SA, alpha) in respect to the horizontal so that m x g x sin alpha (where m is the sum of body mass and the mass of the sledge seat, g the acceleration due to gravity) decreased (on average) from 78% body mass at 30 degrees to 27% body mass at 10 degrees, thus simulating conditions of low gravity. The subjects were asked to jump maximally, without counter movement, starting from 70 degrees, 90 degrees, 110 degrees, and 140 degrees of knee angle (KA); the protocol being repeated at 10 degrees, 15 degrees, 20 degrees, 25 degrees and 30 degrees SA. The average (W+(mean)) power output during concentric exercise (CE) was found to decrease when the starting KA was increased, but to be unaffected by SA (i.e. by the after-load, the simulated low g). The higher values of W+(mean) were recorded at 90 degrees KA [15.01 (SD 1.46) W x kg(-1), average for all subjects at all SA]. The subjects were also asked to perform counter movement (CMJ) and rebound jumps (RE) at the same SA as for CE. In CMJ and RE maximal power outputs were also found to be unaffected by the SA; W+(mean) amounted to 16.03 (SD 0.28) W x kg(-1) in CMJ and 16.88 (SD 0.36) W x kg(-1) in RE (average for all subjects at all SA). In CE, CMJ and RE, the instantaneous force at the onset of the positive speed phase (F(i)) was found to increase linearly with SA (i.e. with increasing m x g x sin alpha), and the difference between F(i) in CMJ or RE and F(i) in CE (F(i) in CMJ minus F(i) in CE and F(i) in RE minus F(i) in CE) was unaffected by SA. This indicated that both maximal power and the elastic recoil were unaffected by simulated low g ranging from 1.71 m x s(-2) (at 10 degrees SA) to 4.91 m x s(-2) (at 30 degrees SA).