Is the effect of a countermovement on jump height due to active state development?
OBJECTIVE
To investigate whether the difference in jump height between countermovement jumps (CMJ) and squat jumps (SJ) could be explained by a difference in active state during propulsion.


METHODS
Simulations were performed with a model of the human musculoskeletal system comprising four body segments and six muscles. The model's only input was STIM, the stimulation of muscles, which could be switched "off" or "on." After switching "on," STIM increased to its maximum at a fixed rate of change (dSTIM/dt). For various values of dSTIM/dt, stimulation switch times were optimized to produce a maximum height CMJ. From this CMJ, the configuration at the lowest height of the center of gravity (CG) was selected and used as static starting configuration for simulation of SJ. Next, STIM-switch times were optimized to find the maximum height SJ.


RESULTS
Simulated CMJ and SJ closely resembled jumps of human subjects. Maximum jump height of the model was greater in CMJ than in SJ, with the difference ranging from 0.4 cm at infinitely high dSTIM/dt to about 2.5 cm at the lowest dSTIM/dt investigated. The greater jump height in CMJ was due to a greater work output of the hip extensor muscles. These muscles could produce more force and work over the first 30% of their shortening range in CMJ, due to the fact that they had a higher active state in CMJ than in SJ.


CONCLUSIONS
The greater jump height in CMJ than in SJ could be explained by the fact that in CMJ active state developed during the preparatory countermovement, whereas in SJ it inevitably developed during the propulsion phase, so that the muscles could produce more force and work during shortening in CMJ.
Bobbert
Maarten F
MF
Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Amsterdam, THE NETHERLANDS. M_F_Bobbert@fbw.vu.nl
Casius
L J Richard
LJ
D000465:Algorithms; D001696:Biomechanical Phenomena; D015444:Exercise / Q000502:physiology*; D006112:Gravitation; D006615:Hip / Q000502:physiology; D006801:Humans; D007717:Knee / Q000502:physiology; D008954:Models, Biological; D009043:Motor Activity / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D013647:Task Performance and Analysis