Effects of Inter-Set Rest Intervals on Muscle Damage After Flywheel Exercise in Untrained Men (RCT)

Effects of Different Inter-Set Rest Intervals on Indirect Markers of Muscle Damage and Mechanical Parameters After Flywheel Resistance Training in Untrained Men: A Randomized Parallel-Group Clinical Trial

This randomized parallel-group clinical trial will compare the acute effects of three different inter-set rest intervals during a single lower-limb flywheel resistance exercise session in untrained men. Participants will be randomly allocated to one of three groups: 1-minute, 2-minute, or 3-minute inter-set rest interval. The primary outcome will be the change in maximal voluntary isometric contraction of the knee extensors 48 hours after exercise. Secondary outcomes will include mechanical variables during the exercise session, countermovement jump performance, knee flexor maximal voluntary isometric contraction, muscle soreness, pressure pain threshold, range of motion, ultrasound-derived muscle thickness and echo intensity, perceived exertion, perceived discomfort, perceived recovery status, creatine kinase, and lactate dehydrogenase.

Participants will complete seven laboratory visits. The first two visits will be used for baseline assessments and test-retest reliability. The third and fourth visits will be used for technical familiarization with the flywheel exercises. The fifth visit will include the experimental flywheel resistance exercise session and immediate post-exercise assessments. The sixth and seventh visits will include follow-up assessments 48 and 96 hours after the experimental session, respectively. Blood samples for creatine kinase and lactate dehydrogenase will be collected before the experimental session and 72 hours after exercise in a partner laboratory.

The experimental session will include squat, Romanian deadlift, knee extension, and knee flexion exercises performed using flywheel devices. All groups will perform the same exercises, volume, inertia, and execution instructions. The only difference between groups will be the inter-set rest interval: 1 minute, 2 minutes, or 3 minutes.

Inclusion Criteria:

• Age between 18 and 40 years;
• Apparently healthy and clinically able to perform physical exercise;
• No participation in structured resistance training during the previous 6 months;
• Agreement to comply with all study visits and procedures;
• Signed informed consent form.

Exclusion Criteria:

• History of lower-limb musculoskeletal injury or surgery that may impair performance or safety during the proposed exercise protocol;
• Current pain, injury, or functional limitation in the lower limbs;
• Use of ergogenic aids that may affect physical performance or muscle recovery;
• Use of analgesic or anti-inflammatory medication during the study period;
• Failure to attend the scheduled visits required for the primary outcome assessment.