Central and Peripheral Changes in Quadriceps Femoris Function After Different Types of Isometric Strength Training

The purpose of this investigation is to examine the effects of 6-week unilateral isometric training interventions over quadriceps femoris on maximal strength and RFD-SF parameters and cortical excitability. Isometric strength training involved either electromyostimulation, voluntary activation, or the combination of both. The second aim is to investigate the potential cross-over effect on a non-trained leg.

A longitudinal pre-post design with random assignment of the participants to one of three experimental groups or the control group was used. Quadriceps femoris (QF) function and cortical excitability were assessed on four occasions: prior to the intervention (pre-test), after 3 weeks of training (served for the adjustment of exercise intensity), after 6 weeks of training (post-test), and three weeks after the competition of training intervention (detraining) (Figure 1). Isometric strength training involves activation of QF applying EMS, VOLUNTARY, or the combination of both EMS and VOLUNTARY (COMBINED). The pre-test also served to familiarize participants with the training protocols and to determine the intensity of EMS needed to achieve 60%MVC. Quadriceps femoris (QF) function was assessed by MVC and RFD-SF. Cortical excitability was assessed with transcranial magnetic stimulation applied over the M1 region. Prior to the pretest, body mass and percent of body fat were assessed using a bioelectric impedance method (In Body 720; USA) and body height with a standard stadiometer. After the pretest, the participants were randomly assigned to either one of the experimental groups (EMS, VOLUNTARY, or COMBINED) and the control group (CONTROL). Experimental groups performed unilateral isometric strength training three times per week for 6 weeks. All participants were advised to refrain from all resistance training targeting the legs between pre-test and post-test.