Fig. 1.

Structural and physiological changes to the aging motor unit (MU) and motor performance outcomes. Shown is a working model of some of the known age-related changes in structures (top) and physiology and function (middle) that are thought to occur at each level of the MU identified as the motor neuron, neuromuscular junction, and muscle fibers (top row). Arrows (left) indicate the direction of change that occurs with advanced age. The supporting evidence varies for those variables identified, so a question mark (?) denotes when there is conflicting or small amounts of evidence. Furthermore, other factors (not shown) such as physical activity, nutrition, genetics, and inflammation among other possible modifiers, can interact with biological aging to alter motor performance in old and very old adults. Supraspinal and spinal inputs are not highlighted here but are influential. Motor performance outcomes (bottom) are each influenced to varying degrees by the variables in the model. The panels are interconnected so that the changes in physiology and function in one column (e.g., the motor neuron) are profoundly affected by age-related changes in the structures and physiology from the other levels (columns) of the motor unit (e.g., the neuromuscular junction and the muscle fibers). Ach, acetylcholine; Ca²⁺, calcium; Endog, endogenous; MHC, myosin heavy chain; NMJ, neuromuscular junction; SR, sarcoplasmic reticulum.