High-Intensity Interval Training: Impacts on Function, Neuromuscular Control, and Muscle Architecture

Change in Evoked Muscle Function Assessments: Evoked Peak Force from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Evoked muscle function will be assessed using electrical stimulation delivered to the superficial quadriceps femoris via a cathode-anode arrangement, with the cathode probe pressed into the femoral triangle over the femoral nerve and the anode fixed over the greater trochanter. Manual stepwise increases in stimulation will be completed until max contraction is achieved. Once max contraction is achieved, a maximal, single and doublet stimulus will be administered. The single stimulation will be used to elicit a max M-wave amplitude (defined as the max M-wave peak-to-peak amplitude). A doublet stimulus will be used to drive the muscle to maximal contraction capability to investigate the muscle contractile capability. To examine maximal contractile capacity, torque produced from the quadriceps femoris from the dynamometer during the first 50 ms, evoked peak force.

Change in Voluntary Electromechanical Delay from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

To examine muscle activation, surface electromyographic (sEMG) signals collected during the voluntary and evoked muscle contractions will be collected from the quadriceps femoris (vastus lateralis, rectus femoris, and vastus medialis muscles), hamstrings (biceps femoris muscle), biceps brachii, and triceps brachii muscles. Peak muscle activation will be assessed as the peak sEMG amplitude. Voluntary electromechanical delay will be calculated from the time difference from the onset of sEMG to the onset of force.

Change in Rate of Muscle Activation from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

To examine muscle activation, surface electromyographic (sEMG) signals collected during the voluntary and evoked muscle contractions will be collected from the quadriceps femoris (vastus lateralis, rectus femoris, and vastus medialis muscles), hamstrings (biceps femoris muscle), biceps brachii, and triceps brachii muscles. Peak muscle activation will be assessed as the peak sEMG amplitude. Rate of muscle activation will be assessed in the first 50 ms following muscle activation onset.

Change in Voluntary Muscle Strength Tests (Isokinetics) from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Isokinetic strength testing on the hamstrings, quadriceps femoris, biceps brachii, and triceps brachii muscles at slow (60° per second), medium (180° per second), fast (300° per second) and fast unweighted (500° per second) speeds with three maximal repetitions performed for each speed to determine voluntary peak torque production capabilities at each speed using a dynamometer. During the strength testing assessments, each subject will be asked to push or pull "as hard as possible" against a lever arm on the dynamometer until the limb reaches the end of the range of motion.

Change in Evoked Muscle Function Assessments: Evoked Peak Rate of Force Development from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Evoked muscle function will be assessed using electrical stimulation delivered to the superficial quadriceps femoris via a cathode-anode arrangement, with the cathode probe pressed into the femoral triangle over the femoral nerve and the anode fixed over the greater trochanter. Manual stepwise increases in stimulation will be completed until max contraction is achieved. Once max contraction is achieved, a maximal, single and doublet stimulus will be administered. The single stimulation will be used to elicit a max M-wave amplitude (defined as the max M-wave peak-to-peak amplitude). A doublet stimulus will be used to drive the muscle to maximal contraction capability to investigate the muscle contractile capability. To examine maximal contractile capacity, evoked peak rate of force development 0-50ms will be recorded resulting from a maximal doublet stimulus from the quadriceps femoris.

Change in Muscle Biopsy-based Muscle Architecture from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Muscle biopsies will be collected to determine myofiber size from the quadriceps femoris (i.e., vastus lateralis). After necessary biopsy preparations, the distribution of myofiber type and size will be determined within each sample.

Change in 6-Min Walk Test performance from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

The 6MWT will be assessed by tallying the number of laps a participant is able walk on a 100 ft straight path. Partially completed laps will be measured and added to the total distance. Participants will be allowed to take standing rest breaks, if necessary, but the time will continue during all rest breaks. Participants will be instructed to "cover as much ground as you can in 6 minutes".

Change in Voluntary Muscle Strength Tests (Isometrics) from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Isometric strength of the hamstrings, quadriceps femoris, biceps brachii, and triceps brachii muscles will be assessed using a dynamometer. For maximal voluntary isometric contraction assessments, participants will complete two, 3- to 4-second MVICs of the hamstrings, quadriceps femoris, biceps brachii, and triceps brachii. Participants will be instructed to "push or pull as hard and fast as possible" and hold against the dynamometer to achieve max voluntary torque and rate of torque development.

Change in Ultrasound-based Muscle Architecture: Cross-sectional Area from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Ultrasound images of the vastus lateralis, rectus femoris, biceps femoris, vastus medialis, biceps brachii, and triceps brachii will be obtained. Muscle cross-sectional area for each muscle will be assessed.

Change in Ultrasound-based Muscle Architecture: Echo Intensity from baseline to following HIIT intervention

Time Frame: change from baseline to immediately after the HIIT intervention

Ultrasound images of the vastus lateralis, rectus femoris, biceps femoris, vastus medialis, biceps brachii, and triceps brachii will be obtained. Echo intensity for each muscle will be assessed.