Acute Muscle Damage After Eccentric Quasi-Isometric Exercise of the Elbow Flexors

Eccentric resistance exercise is widely used in strength training but is often associated with substantial exercise-induced muscle damage, resulting in strength loss, muscle soreness, swelling, and prolonged recovery. Eccentric quasi-isometric (EQI) exercise is a recently proposed contraction mode that combines a sustained isometric hold with a subsequent extremely slow eccentric lengthening under continuous voluntary resistance. This contraction pattern results in prolonged time under tension while avoiding rapid lengthening or abrupt force spikes.

In this randomized interventional study, thirty healthy young men with no recent resistance training experience were randomly assigned to perform either EQI or traditional eccentric (ECC) exercise of the elbow flexors. Participants performed unilateral dumbbell elbow flexion exercise at 70% of their one-repetition maximum. The EQI group completed five sets of single repetitions consisting of an initial isometric hold at 90 degrees of elbow flexion until task failure, followed by very slow eccentric lengthening to full elbow extension. The ECC group performed five sets of repeated eccentric-only elbow flexion exercises at a controlled tempo until failure. Rest intervals were standardized between sets.

Muscle damage and recovery were evaluated using multiple functional and biochemical indicators, including maximal voluntary isometric contraction torque, elbow joint range of motion, upper arm circumference, pressure pain threshold, and circulating concentrations of creatine kinase and myoglobin. Measurements were obtained before exercise, immediately after exercise, and at 1, 2, 3, and 7 days following the exercise bout.

The primary objective of this study was to compare the acute muscle damage responses and recovery profiles of the elbow flexors following EQI and ECC exercise when performed to voluntary fatigue at the same relative external load.

Inclusion criteria

• Healthy young men
• Age between 20 and 30 years
• No participation in any regular exercise training programs during the previous 12 months
• No history of cardiovascular disease, diabetes mellitus, liver disease, respiratory disorders, neuromuscular disease, or musculoskeletal disorders
• No long-term use of chronic prescription medications
• No history of dizziness, chest discomfort, or falls during physical activity
• Not previously diagnosed by a physician as unsuitable for exercise

Exclusion criteria

• History of upper limb musculoskeletal, joint, or bone injuries
• Inability to independently perform activities of daily living
• Presence of cardiovascular, metabolic, respiratory, neuromuscular, or musculoskeletal disorders
• Requirement for assistive devices for ambulation in daily life
• Previous medical diagnosis indicating unsuitability for exercise participation
• Presence of acute illness at the time of enrollment
• History of major surgery within the past 12 months
• Severe physical injury that could impair exercise performance or daily activities