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.

Study Overview

• Status: Not yet recruiting
• Conditions: Exercise-induced Muscle Damage
• Intervention / Treatment: Behavioral: Flywheel resistance exercise with 1-minute inter-set rest; Behavioral: Flywheel resistance exercise with 2-minute inter-set rest; Behavioral: Flywheel resistance exercise with 3-minute inter-set rest

Detailed Description

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.

• Study Type: Interventional
• Enrollment (Estimated): 30
• Phase: Not Applicable

Contacts and Locations

Study Locations

• Brazil
  • Minas Gerais
    • Viçosa, Minas Gerais, Brazil, 36570-900
      • Federal University of Viçosa
      • Contact: Study Coordinator; Phone Number: (31) 98359-5541; Email: wanderson.machado@ufv.br

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: Yes

Description

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.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

3

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: 1-Minute Inter-set Rest Interval; Participants allocated to this group will perform a single lower-limb flywheel resistance exercise session using a 1-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises.	Behavioral: Flywheel resistance exercise with 1-minute inter-set rest; Participants will perform a single lower-limb flywheel resistance exercise session using a 1-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises. Each exercise will be performed for 4 sets of 2 preparatory repetitions followed by 8 valid repetitions, using the same flywheel inertia, exercise order, and execution instructions across all groups. Participants will be instructed to perform the concentric phase as fast as possible and to brake the eccentric phase during the final portion of the movement. The interval between exercises will be standardized at 3 minutes.
Experimental: 2-Minute Inter-set Rest Interval; Participants allocated to this group will perform a single lower-limb flywheel resistance exercise session using a 1-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises.	Behavioral: Flywheel resistance exercise with 2-minute inter-set rest; Participants will perform a single lower-limb flywheel resistance exercise session using a 2-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises. Each exercise will be performed for 4 sets of 2 preparatory repetitions followed by 8 valid repetitions, using the same flywheel inertia, exercise order, and execution instructions across all groups. Participants will be instructed to perform the concentric phase as fast as possible and to brake the eccentric phase during the final portion of the movement. The interval between exercises will be standardized at 3 minutes.
Experimental: 3-Minute Inter-set Rest Interval; Participants allocated to this group will perform a single lower-limb flywheel resistance exercise session using a 1-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises.	Behavioral: Flywheel resistance exercise with 3-minute inter-set rest; Participants will perform a single lower-limb flywheel resistance exercise session using a 3-minute inter-set rest interval. The session will include squat, Romanian deadlift, knee extension, and knee flexion exercises. Each exercise will be performed for 4 sets of 2 preparatory repetitions followed by 8 valid repetitions, using the same flywheel inertia, exercise order, and execution instructions across all groups. Participants will be instructed to perform the concentric phase as fast as possible and to brake the eccentric phase during the final portion of the movement. The interval between exercises will be standardized at 3 minutes.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in maximal voluntary isometric contraction of the knee extensors	Maximal voluntary isometric contraction of the knee extensors will be assessed using a load cell with the knee positioned at 90 degrees of flexion. The primary outcome will be the change in force between baseline and immediately, 48 hours, and 96 hours after the experimental exercise session using a flywheel.	Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in maximal voluntary isometric contraction of the knee flexors	The maximum voluntary isometric contraction of the knee flexors will be assessed using a load cell with the knee positioned at 45 degrees of flexion. The primary outcome will be the change in force between baseline and immediately, 48 hours, and 96 hours after the experimental exercise session using a flywheel.	Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in vastus lateralis and biceps femoris long head muscle thickness		Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in vastus lateralis and biceps femoris long head echo intensity		Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in countermovement jump performance		Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in active and passive knee flexion and extension range of motion		Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in lower-limb muscle soreness score assessed by the Visual Analogue Scale	Lower-limb muscle soreness will be assessed using a 100-mm Visual Analogue Scale. The scale ranges from 0 to 100, where 0 indicates no pain and 100 indicates the worst possible pain. Higher scores indicate greater muscle soreness. The outcome will be reported as the change in muscle soreness score from baseline to the post-exercise time points.	Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in pressure pain threshold of the thigh muscles		Baseline, 10 minutes after the experimental exercise session, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session
Change in serum creatine kinase and lactate dehydrogenase concentrations		Pre-exercise and 72 hours after the experimental exercise session
Peak concentric and eccentric power output during flywheel exercise	Peak concentric and eccentric power output will be recorded during each valid repetition of the flywheel exercise session using a load cell connected to the flywheel device. Data will be collected for all sets and exercises, excluding the initial preparatory repetitions. The outcome will be reported as peak concentric power, peak eccentric power, and eccentric/concentric power ratio across the experimental exercise session.	During all 16 sets performed in the experimental exercise session
Rating of perceived exertion assessed by the OMNI-Resistance Exercise Scale	Rating of perceived exertion will be assessed using the OMNI-Resistance Exercise Scale. The scale ranges from 0 to 10, where 0 indicates extremely easy and 10 indicates extremely hard. Higher scores indicate greater perceived exertion. The outcome will be reported after each set of the experimental flywheel exercise session and 10 minutes after the end of the session.	After each of the 16 sets performed during the experimental exercise session and 10 minutes after the experimental exercise session
Perceived discomfort assessed by the Discomfort Scale	Perceived discomfort will be assessed using a 0-to-100 Discomfort Scale. The scale ranges from 0 to 100, where 0 indicates no discomfort and 100 indicates maximal discomfort. Higher scores indicate greater perceived discomfort. The outcome will be reported after each set of the experimental flywheel exercise session and 10 minutes after the end of the session.	After each of the 16 sets performed during the experimental exercise session and 10 minutes after the experimental exercise session
Perceived recovery status assessed by the Perceived Recovery Status Scale	Perceived recovery status will be assessed using the Perceived Recovery Status Scale. The scale ranges from 0 to 10, where 0 indicates very poor recovery and 10 indicates full recovery. Higher scores indicate better perceived recovery. The outcome will be reported as the perceived recovery status score at baseline and at the post-exercise follow-up time points.	Baseline, 48 hours after the experimental exercise session, and 96 hours after the experimental exercise session

Collaborators and Investigators

• Sponsor: Federal University of Vicosa

Study record dates

Study Major Dates

• Study Start (Estimated): June 1, 2026
• Primary Completion (Estimated): July 1, 2026
• Study Completion (Estimated): July 1, 2026

Study Registration Dates

• First Submitted: May 29, 2026
• First Submitted That Met QC Criteria: June 25, 2026
• First Posted (Actual): July 1, 2026

Study Record Updates

• Last Update Posted (Actual): July 1, 2026
• Last Update Submitted That Met QC Criteria: June 25, 2026
• Last Verified: May 1, 2026

More Information

Terms related to this study

• Keywords: Resistance training; Muscle damage; Maximal voluntary isometric contraction; Flywheel exercise; Untrained men
• Other Study ID Numbers: 7.151.525 (Other Identifier: Ethics Committee Approval Number); 81740024.5.0000.5153 (Other Identifier: CAAE)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No