Phase 3: neuromuscular function
twitch
SEPARATION, CONTRACTION, CREST
Neuromuscular function was assessed using knee extensor MVC’s with peripheral nerve stimulation on a custom-built dynamometer chair. Participants sat on the chair with ninety-degree hip and knee angles. A thinly padded cuff, connected to a force transducer, was tightly secured above the lateral malleolus. Force was sampled at 4000 Hz (Powerlab; AD Instruments, Sydney, Australia). EMG electrodes from the TMS procedure remained on the VM and VL for the neuromuscular function testing. EMG signals were sampled at 4000 Hz (Powerlab; AD Instruments, Sydney, Australia) and band pass filtered (20–500 Hz).For femoral nerve stimulation, anode and cathode gel electrodes (Red Dot; 3 M, Minnesota, USA) were placed on the gluteus maximus at the midpoint between the iliac crest and the femoral head, and within the femoral triangle, respectively. The optimal position of the cathode was determined by stimulating at 30 mA using a portable cathode probe in various positions within the femoral triangle. The cathode electrode was then placed in the position that elicited the greatest knee extensor contraction. Maximum stimulation intensity was then determined by incrementally (20 mA) increasing the current until the resultant resting twitch force plateaued. Test stimulation intensity during the experimental trial was set at 130% of the maximum intensity, corresponding to a current of 187 ± 78 mA. Electrical stimulations were initiated during, and 2–3 s after each MVC, as 0.2 ms square waves applied as 10 Hz doublets. We chose 10 Hz doublets to reduce participant discomfort and to allow clear separation of the M-waves. 10 Hz doublets allow summation of the quadricep twitch force and previous research has found no difference in potentiated twitch force or voluntary activation trends between 10 and 100 Hz doublet stimulation (Marshall et al. After a self-selected warm-up, participants were familiarized with the procedure by performing 2–3 MVCs without stimulation and then 2–3 MVCs with stimulation. Each MVC was 2–3 s long, and participants were instructed to contract as hard and as fast as possible. For each experimental condition, the solution was mouth rinsed (10 s) and ingested followed immediately by an MVC. To determine the reliability of the procedure in our laboratory, three baseline measurements were taken prior to introducing the experimental conditions. Most neuromuscular function variables had a CV of ≤ 10% which is comparable to previous reliability work (Place et al. Measures of maximum voluntary force, potentiated twitch force, voluntary activation, and impulse were taken for each MVC. Maximum voluntary force was defined as the highest force throughout the MVC excluding the interpolated twitch. Voluntary activation (Merton For each MVC, the EMG signal was processed to calculate EMG Max and peak to peak M-wave amplitude for the VM and VL. EMG Max was calculated as the highest 0.25 s averaged root-mean-square EMG between contraction onset and the interpolated twitch.