The way your muscles fire when you do leg presses vs. leg extensions matches up with which muscles end up getting bigger after training.
Scientific Claim
Muscle excitation patterns during leg press and knee extension, as measured by surface electromyography, are associated with the observed differences in muscle hypertrophy after training.
Original Statement
“A follow-up experiment using surface electromyography showed that muscle excitation patterns during KE and LP generally mirrored the between-condition hypertrophic differences and similarities observed after the training intervention.”
Evidence Quality Assessment
Claim Status
overstated
Study Design Support
Design cannot support claim
Appropriate Language Strength
association
Can only show association/correlation
Assessment Explanation
The abstract states that EMG patterns 'mirrored' hypertrophic outcomes, but this is a correlational observation from a small follow-up. No causal link is established. 'Mirrored' implies alignment, not causation.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Randomized Controlled TrialLevel 1bWhether manipulating muscle activation during KE and LP directly alters hypertrophic outcomes.
Whether manipulating muscle activation during KE and LP directly alters hypertrophic outcomes.
What This Would Prove
Whether manipulating muscle activation during KE and LP directly alters hypertrophic outcomes.
Ideal Study Design
A double-blind RCT of 80 untrained adults randomized to perform KE or LP with or without biofeedback to enhance rectus femoris activation during LP or reduce it during KE, measuring 12-week MRI changes in muscle volume and EMG activation patterns.
Limitation: Cannot isolate activation from mechanical tension or metabolic stress as the primary driver of hypertrophy.
Prospective Cohort StudyLevel 2bWhether individuals with higher rectus femoris EMG activation during KE consistently show greater hypertrophy than those with lower activation.
Whether individuals with higher rectus femoris EMG activation during KE consistently show greater hypertrophy than those with lower activation.
What This Would Prove
Whether individuals with higher rectus femoris EMG activation during KE consistently show greater hypertrophy than those with lower activation.
Ideal Study Design
A 12-week prospective cohort of 100 untrained adults performing KE, with pre- and post-training EMG and MRI measurements, analyzing correlation between average rectus femoris activation and volume gain.
Limitation: Cannot control for individual variability in training adherence or recovery.
Cross-Sectional StudyLevel 3Whether long-term KE users show higher rectus femoris EMG activation during the exercise compared to LP users.
Whether long-term KE users show higher rectus femoris EMG activation during the exercise compared to LP users.
What This Would Prove
Whether long-term KE users show higher rectus femoris EMG activation during the exercise compared to LP users.
Ideal Study Design
A cross-sectional comparison of 60 long-term resistance-trained adults (≥2 years) who primarily use KE vs. LP, measuring EMG activation during standardized sets and correlating with muscle volume via MRI.
Limitation: Cannot determine if activation differences caused hypertrophy or resulted from it.
Animal Model StudyLevel 4Whether targeted neuromuscular stimulation of rectus femoris during simulated leg press induces hypertrophy.
Whether targeted neuromuscular stimulation of rectus femoris during simulated leg press induces hypertrophy.
What This Would Prove
Whether targeted neuromuscular stimulation of rectus femoris during simulated leg press induces hypertrophy.
Ideal Study Design
A controlled rat study using implanted electrodes to selectively stimulate rectus femoris during simulated LP movement, comparing hypertrophy to non-stimulated controls over 6 weeks.
Limitation: Rodent neuromuscular control and muscle architecture differ from humans.
Evidence from Studies
Supporting (1)
Hypertrophic Effects of Single- versus Multi-Joint Exercise: A Direct Comparison Between Knee Extension and Leg Press.
The study found that which muscles 'light up' more during leg press vs. knee extension matches which muscles grow more after training—so yes, how your muscles fire during exercise is linked to how much they grow.