Facilitation of quadriceps activation following a concentrically controlled knee flexion movement: The influence of transition rate

Study Design: Single group repeated measures design. Objective: To determine ii the rate of transition between knee flexion and extension influences the subsequent concentric activation of the quadriceps and knee extension torque during reciprocal movements. Background: Preloading a muscle by stretching, a prior isometric or eccentr ic muscle action, or a prior movement controlled concentrically by the anta gonist muscle group increases the maximal torque-generating capability of t he agonist. We hypothesized that the rate of transition from the prior move ment may be the critical factor that influences the degree of muscle facili tation and torque potentiation. Rapid reversal of antagonistic movements ha s been postulated as a potential facilitatory mechanism. Methods: Knee extension torque and electromyographic (EMG) amplitude (depen dent variables) from 2 of the vasti muscles were recorded while subjects (N = 20; 12 men, 8 women, mean age, 28.5 +/- 2.68 years) maximally activated their quadriceps at 3 constant angular velocities, 100 degrees /s, 200 degr ees /s, and 300 degrees /s, and 2 preload conditions, SLOW and RAPID (indep endent variables). In the SLOW transition condition, subjects actively flex ed their knee to 110 degrees from an extended position, paused in this posi tion for 3 seconds, and then extended to 0 degrees. In the RAPID transition condition, the same movement from knee flexion to extension was performed without a pause. Results: Peak torque, the root-mean-square (RMS) average, peak (peak rectif ied and smoothed), and initial (100 milliseconds prior to torque onset) EMC amplitudes were all significantly greater during the RAPID transition cond ition. Peak torque decreased with increasing movement velocity. There were no interactions between the preload conditions and angular velocity on peak torque or the EMG amplitude variables. There was also no influence of velo city on the EMC amplitude variables. Conclusions: The effect of preloading the quadriceps by prior concentric ac tivation of the hamstrings is dependent on the rate of transition between t he flexion and extension movements and is due primarily to neural facilitat ion.