Stretch load influences on stretch reflex characteristics of the triceps surae muscle in drop jump exercises

Abstract

STRETCH LOAD INFLUENCES ON STRETCH REFLEX CHARACTERISTICS OF THE TRICEPS SURAE MUSCLE IN DROP JUMP EXERCISES Janne Avela, Pedro M. Santos, Paavo V. Komi and Heikki Kyrgliiinen Department of Biology of Physical Activity University of JyvtlskylP, DF-40100 Jyvilskylg, FINLAND Preactivation and stretch reflexes seem to play an important role in regulating muscle force. The present study was designed to examine how the different stretch-loads influence the neuromuscular control. Nlme males performed drop jumps in five different stretch-load conditions. A special Lifting Block System with possibilities for increased or reduced loads was utilized to obtain different accelerations while the velocity at impact was kept constant at 3.0m*s'l in all the conditions. The extra loads were blocked off electrically at the moment of touchdown. The main performance parameters demonstrated the superiority of the zero load (body weight) over the other load conditions. This supports the idea of the adaptation of the body to the body weight (a=9.81 m*Sm2) through prior learning processes. The importance of the pre-programming was demonstrated by the relationship between the pre-activation of the gastrocnemius and the soleus muscles and the peak dorsiflexion velocity at impact (~(0.01). The expected high muscle stretch resulted in high pre-activation level. This prepared the whole tendomuscular system to become adequately stiff even prior to the ground contact, but expecially during the eccentric phase to resist high impact forces.