# Changes in Spinal-Reflex Excitability during Static Stretch and/or Explosive Contraction
## 2 Materials and Methods
### Testing Procedures  
All subjects visited the laboratory four times on separate days at the same time of the day, 48 h apart. Subjects were asked to maintain their habitual diets during the experimental period and to refrain from any physical activity for 24 h prior to data collection. For the first session, subjects were screened for measurable H-reflex after providing informed consent. Ambient temperature and relative humidity within the laboratory were set as 25 \({}^{\circ}\)C and 50% during the data collection period. Upon arrival to the laboratory, subjects laid down on the treatment table and rested for 15 min to achieve spinal-reflex and cardiovascular stability prior to the screening. During this rest period, the VM on the dominant limb (i.e., the leg used to kick a ball) were shaved, debrided (with sandpaper), and cleaned with alcohol prep wipes before the placement of self-adhesive surface electromyography (EMG) electrodes (Ag-AgCl; EL 503-10; Biopac System Inc., Goleta, CA, USA). Two EMG electrodes (2 cm apart) were attached to the bulk of the VM. The ground electrode was attached to the medial malleolus of the ipsilateral limb. During the H-reflex and M-response measurements, subjects were asked to place their hands along their sides with their palms kept supinated and to maintain this position as they looked at a spot on the ceiling while listening to white noise through earphones to avoid any possible sound-induced variability in measurements [22]. The stimulating module (Biopac STM 100C), isolation adaptor (Biopac Stimsoc), and a bar stimulation electrode (EL 503, Biopac Systems Inc., Goleta, CA, USA) provided the electrical stimulus over the femoral nerve (just lateral and/or down to the femoral artery). The peak H-reflex and peak M-response were found and recorded through surface EMG (sampling rate: 2000 Hz). Electrical stimulation was gradually increased at 0.1-2 V increments, with a 15 s rest between stimulations [23]. The average intensities (ranges) to elicit the peak H-reflex and the peak M-response were 6.0 (4.0 to 9.0) and 9.3 (7.1 to 10.0) V, respectively. Once peak amplitude was found, the same intensity was applied four times for the pre-condition measurements. After the pre-condition measurements, the researcher who took measurements left, and the other researcher who was blinded to pre-condition measurements guided the intervention conditions in the laboratory. Subjects randomly experienced one of four conditions on each visit (stretch, contraction, stretch and contraction, and control: Figure 1). The order of the conditions for each subject was determined by the opaque envelope method. For the stretch condition, the modified Thomas test [24] was used. Subjects were asked to seat at the end of a treatment table and roll back onto the table while bending and pulling one of the knees to their chest. Once subjects were in the position, the researcher gradually provided manual stretch force (Figure 2). Subjects were asked to stop the researcher when they felt stretch sensation [25] or the point of discomfort [26], then this end position was held for 30 s, which was timed by the researcher. Three repetitions on each leg were alternatively performed (a total duration of 90 s with a rest interval of 30 s). For the contraction condition, subjects performed two-legged countermovement maximal vertical jumps on Vertec (Sports Imports, Hilliard, OH, USA). Subjects' standing arm reach height was measured with a Vertec. Subjects stood (same feet positions for vertical jumps) and maintained their lower-extremity full extension and trunk upright and raised (full scapular upward rotation with abduction) their dominant arm (i.e., the arm throwing a ball) directly overhead as high as possible. Subjects were then asked to vertically jump off of both legs as high as they possibly could and touch the plastic vane of the Vertec (this instruction was given during the standing arm height measurement). A self-selected pre-stretch of the lower-body and trunk and double-arm swinging at take-off were allowed.