Sensitivity of Load-Velocity Measures for Detecting Fatigue During Smith-Machine Squats

This study investigates how different levels of fatigue influence the parameters of the load-velocity (L-V) relationship during the Smith-machine squat exercise. After an initial familiarization and baseline testing session used to establish 1-repetition maximum (1RM) and to perform a repetition-to-failure test at 70% 1RM, participants completed three separate experimental sessions. Each session implemented a distinct fatigue protocol, and all protocols were performed between two incremental loading tests to determine pre-session and post-session L-V profiles.

The moderate-fatigue condition consisted of five sets at 70% 1RM, with participants performing half of their maximal possible repetitions per set. The high-fatigue condition required five sets performed to volitional failure using the same relative load. A control condition with no exercise was also included to establish non-fatigue reference values. L-V variables of interest included L0, v0, and Aline (calculated as L0 × v0 / 2). These variables were compared across conditions to determine their responsiveness to increasing levels of fatigue and their association with changes in neuromuscular performance, particularly reductions in 1RM strength.

Inclusion criteria

Healthy males aged 18-40 years.

Resistance-trained (minimum 2 years of regular lower-body resistance training and ≥2 sessions/week).

Able to perform a proper smith-machine back squat with correct technique.

1-RM smith-machine squat measurable and ≥ bodyweight (or specify threshold used in study).

No musculoskeletal injury of lower limbs or back in the past 6 months.

Not using performance-enhancing drugs or anabolic steroids for the past 12 months.

Willing and able to attend all testing and training sessions and provide written informed consent.

Exclusion criteria

Any cardiovascular, pulmonary, metabolic, or neurological disease contraindicating intense exercise.

Recent (≤6 months) lower-extremity or spinal surgery or acute injury.

Current musculoskeletal pain or injury that limits squat performance.

Use of medications that affect neuromuscular performance (e.g., systemic corticosteroids) or stimulant drugs.

Regular participation in structured lower-body rehabilitation programs.

Failure to complete familiarization or preliminary 1-RM testing.

Inability or unwillingness to provide informed consent or follow study procedures.