Browse evidence-based analysis of health-related claims and assertions
Compound exercises involving greater total muscle mass do not produce superior hypertrophy in target muscles compared to isolation exercises when training volume and effort are equated.
Assertion
Resistance training to muscular failure induces equivalent hypertrophy in both slow-twitch (type I) and fast-twitch (type II) muscle fibers, regardless of load or repetition range.
When resistance training is performed to volitional muscular failure, hypertrophic outcomes are equivalent across a wide range of loads (from low to high) and repetition ranges.
Individual hypertrophic responses to resistance training are highly variable across muscle groups and are not consistently correlated between upper and lower body musculature.
For small muscle groups, unilateral and bilateral resistance training produce equivalent hypertrophic adaptations when volume and effort are matched.
Unilateral resistance exercises can increase neural drive and muscle fiber recruitment compared to bilateral exercises due to reduced neuromuscular inhibition during single-limb contractions.
Older studies showed bigger strength gains from weight training than newer ones, probably because today’s studies are better designed and less biased.
Correlational
Men tend to get stronger faster than women when lifting very heavy weights, possibly due to differences in how their muscles and nerves respond to intense training.
Causal
If you’ve been lifting for a while, doing more workouts over time will help you build more muscle than just lifting heavier weights.
People who’ve never lifted weights before gain muscle much faster than those who’ve trained before—even if they do the same workout.
Lifting heavier weights (fewer reps) leads to bigger strength gains than lifting lighter weights (more reps), even if both are done until exhaustion.
Whether you lift light weights with lots of reps or heavy weights with few reps—so long as you push until you can’t do another rep—you’ll build muscle about the same amount.
Even people in their 50s and 70s can get much stronger and activate their muscles better after 12 weeks of heavy weight training — age doesn’t stop them from improving.
Descriptive
Whether you train both legs together or one at a time, your thigh muscles grow about the same amount after 12 weeks of heavy lifting.
Quantitative
People who train both legs together show bigger increases in muscle electrical activity during double-leg lifts than those who train one leg at a time, suggesting their nervous systems become more active during those movements.
When people train one leg at a time, that leg gets much stronger than the other leg, even if the other leg didn’t do any training.
Doing leg exercises with both legs at the same time for 12 weeks makes you stronger at lifting with both legs together, while doing them one leg at a time makes you stronger at lifting with each leg separately.
The outer part of your calf and the deeper calf muscle don’t grow better with either straight-knee or bent-knee calf raises — they respond similarly to both.
Not all parts of your thigh and calf muscles grow the same way — some parts grow better with certain exercises, so doing different types of lifts can help you build muscle more evenly.
Doing calf raises with your knees straight (standing on toes) makes the inner part of your calf muscle grow more than doing them with your knees bent (seated), because the muscle is stretched more.
Pushing a heavy leg press machine makes the outer part of your thigh muscle grow more than doing leg extensions, because it puts more overall stress on that part of the muscle.
Doing leg extensions (kicking your leg out while sitting) makes the front thigh muscle right above your knee grow more than doing leg presses (pushing a heavy platform with both legs).
There are only a few small, short studies on this topic, and many didn’t even track whether people stuck to their workout plans — so we can’t trust the results very much.
Even if you lift light or heavy weights, if you push until you can’t do another rep, your muscles end up using nearly all the same fibers — so they grow similarly.
Mechanistic