Energy balance

During single-joint resistance training, non-recruited muscles undergo significant volume loss (atrophy), particularly when energy and protein intake are low, suggesting a localized reallocation of muscle mass from untrained to trained tissues.

If you are doing targeted resistance training (like arm curls or leg extensions) and not eating enough calories or protein, your untrained muscles may actually shrink. To maximize net muscle gain, ensure your energy and protein intake are sufficient to support the metabolic cost of building new tissue in your trained muscles, preventing your body from cannibalizing untrained muscles for energy.