A method to evaluate reflex excitability of the human ankle plantarflexors despite changes in maximal activation capacities.

Stretch reflexes were evoked in the submaximally activated ankle extensors during sinusoidal length perturbations. A mean stretch reflex (SR) amplitude ((-)SRA), i.e., SR area/SR duration was quantified for the soleus muscle and for the gastrocnemii muscles. (-)SRA was also expressed in relative values ((-)SRA(rel)), i.e., SRA was related to the corresponding background electromyogram (EMG). Sinusoidal length perturbations were applied in two ways: (1) at a fixed frequency (16 Hz) and at different levels of voluntary contraction [10% to 70% maximal voluntary contraction (MVC)] and (2) at a constant activation level (50% MVC) and at frequencies ranging from 6 to 16 Hz. Then, new parameters are proposed to characterize muscle reflex excitability. The first parameter, SR(index), consisting in the slope of the (-)SRA-EMG relationship, was considered to be more representative of central influences on the reflex pathway. Secondly, the frequency distribution of the stretch reflex was analyzed and the area under the (-)SRA(rel)-frequency curve gave the second parameter (FD-(-)SRA(rel)). This parameter should account for both central and peripheral mechanisms on the reflex pathway. These parameters were found to be higher for the highly excitable soleus than for the less excitable gastrocnemii muscles. SR(index) and FD-(-)SRA(rel) can be proposed as a tool to analyze changes in reflex excitability, which can accompany a process of neuromuscular plasticity. In order to validate the procedure, the proposed parameters were quantified in a case study before and after a period of plyometric training.