Muscle
size is the strongest single predictor of strength, as your prison officer friend suggests, it isn’t the only one. Even with motor unit recruitment equal, several factors can let a leaner person out-lift a bigger one.
For example myofibrillar density since muscle’s size can come from sarcoplasmic fluid, glycogen, and other non-contractile material. Two muscles of the same size can have different amounts of force-producing protein. Type II (fast-twitch) fibres produce more force per unit of cross-sectional area than type I. A person with a higher proportion of them can be stronger at the same muscle size. And something you’re born with is favourable Muscle architecturefor example
pennation angle and fascicle length affect how much force a given muscle volume can produce and how quickly it produces it.
Then there’s mechanical properties such as leverage. Tendon insertion points, limb lengths, and joint angles vary between people. A slightly longer moment arm for a given muscle gives a real advantage in some lifts, and a shorter range of motion (shorter arms in a bench press, for example) means less work to do. Stiffer tendons transmit force more efficiently-think elastic and steel ropes
Even neural factors can influence strength. If the number of motor units recruited is equal, strength can still differ through firing rate, synchronisation, and coordination between agonists, antagonists, and stabilisers. The last of these, along with skill in a specific movement is of more significance than you might expect.
But, bigger is stronger is generally a
good rule of thumb. Across the population, and especially at the same level of training, more muscle usually does mean more strength. Leaner people who out-lift bigger ones are typically exploiting some combination of the above, particularly leverage, neural efficiency, and the fact that some bulk is not very force-productive. Powerlifters and gymnasts in lower weight classes are good examples, with high strength relative to body size.
It’s fascination, eh?