Skeletal muscles have a remarkable plasticity to readily expand their size in response to increased activity and mechanical loading. The adaptive process of muscle hypertrophy is energetically expensive but is retained even in conditions of low dietary energy intake. Our preliminary data now show a surprising new phenomenon, i.e. that this muscle hypertrophy in trained muscles is sometimes accompanied with simultaneous muscle atrophy in untrained muscles. This could hint towards a muscle mass reallocation process, which would make evolutionary sense. Our preliminary data also suggest that this muscle mass redistribution mainly occurs when energy and/or protein availability is limited. The aim of this project is to investigate muscle mass reallocation in different modes of nutritional deficit: energy or protein. This new type of untargeted research is enabled by using a massively accelerated and novel artificial intelligence (AI)-powered methodology to individually segment 150 human muscle volumes from gold standard Magnetic Resonance Imaging (MRI). We will perform two 10-week resistance training studies with isolated exercises and strictly controlled diets in young and elderly people. Project outcomes might reveal that some specific muscles will experience atrophy to ‘feed’ the hypertrophy of recruited muscles. This will initiate a new era for our scientific understanding of muscle mass regulation and is also highly relevant for the sports and (para)medical practitioners.