Muscle adapts to the work it is given. Regular loading makes it stronger or more enduring, while bed rest and disuse make it shrink quickly.
Strength training
Heavy loads make fibers thicker by adding contractile proteins, a change called hypertrophy. Early strength gains come mostly from the nervous system learning to recruit more fibers.
Endurance training
Repeated long effort adds mitochondria and capillaries, so the muscle uses oxygen better and tires later. Fiber size changes little.
Atrophy
Without use, muscle loses protein and shrinks. Bed rest, casting, nerve injury and aging all cause atrophy, and older adults can lose strength within days of immobility.
Soreness after exercise
Delayed onset muscle soreness peaks one to two days after unfamiliar exercise and comes from small injuries to the fibers. It fades as the muscle repairs and adapts.
At the bedside
Early mobilization and range of motion exercises limit atrophy in hospitalized patients. Compare limb size and strength on both sides to spot wasting from disuse or nerve damage.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).