When performing squats, the hamstring muscle on the back of the thigh shows higher electrical activity when lifting 70% of a person's maximum weight compared to lifting 60% with slow reps, and this...
Mechanism
Synthesis from 1 study
Lifting heavier weights makes your muscles work harder because the extra force triggers more nerve signals to fire more muscle fibers. Moving slowly doesn’t do the same—it actually makes your muscles tire out faster due to chemical buildup, which weakens their response over time.
Most probable mechanism
When you lift a heavier weight, your muscles feel more resistance, which triggers more nerve signals to activate muscle fibers. This causes more muscle fibers to fire at the same time, especially the stronger, faster-twitch ones, making the muscle work harder. This effect is stronger than what happens when you just move slowly.
Increased external resistance elevates mechanical tension across muscle fibers during both eccentric and concentric phases of movement
Higher mechanical tension activates Ia sensory afferents from muscle spindles, increasing excitatory input to spinal motor neurons
Spinal motor neurons respond with increased firing rates and recruitment of additional motor units, particularly high-threshold Type II fibers
Greater motor unit recruitment leads to increased summed electrical activity in the biceps femoris, detectable as higher electromyographic amplitude
Less supported by current evidence, but not ruled out
When movements are performed slowly, metabolic byproducts like lactate build up in the muscle, which may interfere with the muscle's ability to sustain strong nerve signals, reducing activation over time.
Prolonged contraction duration increases reliance on anaerobic glycolysis for ATP production
Accumulation of lactate and associated hydrogen ions lowers intracellular pH
Acidosis impairs calcium release from the sarcoplasmic reticulum and reduces cross-bridge cycling efficiency
Reduced contractile efficiency diminishes the ability to maintain high levels of motor unit activation during subsequent sets
Evidence from Studies
Supporting (1)
Community contributions welcome
Acute physiological responses with varying load or time under tension during a squat exercise: A randomized cross-over design.
Contradicting (0)
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