How does your nervous system make you stronger at the gym?

What is a motor unit?

Your muscles never move on their own: your brain drives them, through nerves. In your spinal cord, each motor nerve cell, a motor neuron, controls a bundle of fibers in your muscle: together, they make up a motor unit.

To push harder, your nervous system has two dials: it switches on more units, called recruitment, and it sends them signals closer together, called firing rate. Beyond light efforts, it’s mostly this second dial that sets your force (Enoka and Duchateau, 2017).

Why does your strength rise so fast in the first weeks?

In 7 young volunteers, 5 weeks of two-leg leg extensions, 3 times a week, on a flywheel machine: their maximal strength rose by 38.9% and their muscle’s electrical activity by 34.8%, but the cross-section of their quads by only 6.5 to 7.4% (Seynnes et al., 2007).

In 14 volunteers, grids of 128 electrodes tracked the motor units of the muscle that lifts the foot one by one, before and after 4 weeks of contractions without movement, against 14 controls. After training, during contractions at 35, 50 and 70% of their maximal force, their units fired faster and switched on at a lower share of that force: for the authors, the strength gained in those 4 weeks comes from a stronger output sent by the spinal cord (Del Vecchio et al., 2019).

So in the first weeks, your strength comes mostly from your nerves, but not only: neural adaptations may weigh most early on, and muscle growth also starts from the outset (Folland and Williams, 2007). In the 7 volunteers, the thigh had already grown by 3.5 to 5.2% after 20 days, and the authors conclude that muscle growth seems to contribute to strength gains earlier than previously reported (Seynnes et al., 2007).

Does training one leg make the other stronger?

Yes: it’s called cross-education. An analysis of 31 randomized studies, 785 people in all, in which only one side was trained: the untrained side gained 11.9% strength on average, 16.4% in the legs and 9.4% in the arms, with a high risk of bias across the studies (Manca et al., 2017).

And its muscle doesn’t grow: across 31 studies and 1,214 people, training one arm gave the other a little strength without measurably thickening its muscle, with low to very low certainty of evidence (Hernando-Garijo et al., 2026). So it’s your nervous system that learned; exactly where, in the brain or the spinal cord, is still debated, and two hypotheses are put forward (Lee and Carroll, 2007).

Does an arm in a cast have to lose its strength?

Not necessarily. 30 right-handers, in three groups of 10: two groups wore a cast on the left wrist and hand for 3 weeks, the third served as a control. Those in a cast who did nothing lost about 15% of strength in the casted wrist (from 12.2 to 10.4 N·m). Those who trained their right hand, the one without a cast, 5 days a week, with maximal contractions of the wrist toward the little-finger side, without movement, lost none (from 13.9 to 14.2 N·m) (Farthing et al., 2009).

The casted arm’s muscle also thinned a little in those who did nothing, but not in those who trained the other hand; the authors suggest the training may have prevented that loss (Farthing et al., 2009). It’s a small study, in volunteers casted for the experiment, not injured people.

Hurt on one side: should you train the other?

With your doctor’s okay, it can help. In healthy volunteers with an immobilized arm, 8 studies and 189 people: training the other arm limited the loss of strength, and slightly the loss of muscle, results to be read with caution according to the authors (Rodríguez-Coloma et al., 2026). In another analysis of 24 studies, the effect was clear in these immobilized volunteers and in people weakened on one side after a stroke, but not significant in patients with a bone, joint or muscle condition (Valdés et al., 2026).

For a consensus of 32 experts, it could complement treatment, especially for an orthopedic or sports injury on one side, and it takes at least 13 to 18 sessions, or 4 to 6 weeks, to get a useful transfer of strength (Manca et al., 2021).