Reflex facilitation during the stretch-shortening cycle

Maximal torque during the concentric phase of a movement has been shown to be enhanced by prior eccentric muscle actions, a movement strategy referred to as the stretch-shortening cycle. Although the mechanical basis for this enhancement is well established, the neural component is not. We hypothesi zed that brief high-frequency bursts of spindle afferent discharge during t he eccentric phase of the stretch-shortening cycle could be one mechanism f or facilitating the volitional drive. To test this hypothesis, three sets o f experiments were done. In the first (N=15), we demonstrated that both the peak and mean EMG of the soleus (S) and lateral gastrocnemius (LG) muscles were considerably greater during a reciprocal hopping (RHOP) task than for maximum isometric contractions (MIVCs), In the second experiment, we teste d whether the dynamic nature of the RHOP or the eccentric phase of the RHOP contributed to the EMG potentiation. Peak and mean EMG produced with a con centric hop (CHOP), in which the lengthening phase of the hop was eliminate d, were compared with that produced with the RHOP and MIVCs conditions (N=7 ). The RHOP produced greater peak EMG than either the CHOP or the MIVCs whi le the mean EMG for both hopping conditions was considerably more than the MIVCs. In the final experiment, we attempted to mimic the brief high-freque ncy burst of spindle afferent activity during the lengthening phase of the stretch-shortening cycle in the absence of muscle length changes. High-freq uency (100 Hz) afferent stimulation (HFS) was delivered during MIVCs, At re st, the HFS produced negligible EMG activity but when superimposed over MIV Cs produced a marked potentiation of the S EMG over values obtained during MIVCs alone. Evidence that HFS synchronizes the EMG associated with volitio nal activation is also provided. We conclude that a substantial but brief f acilitation and possible synchronization of the neural drive is provided by the spindle afferents during the eccentric phase of the stretch-shortening cycle. (C) 2000 Elsevier Science Ltd. All rights reserved.