Reductions in Muscle Quantity and Quality in Old and Very Old Men: New Insights into the Progression of Sarcopenia

Healthy adult aging is associated with sarcopenia; a loss of skeletal muscle mass known as. Major contributors to this process include functional and morphological changes in the contractile tissue and within the neuromuscular system. Currently, the relationship between muscle mass, normalized strength, structural integrity, and neuromuscular properties [such as motor unit number estimates (MUNE)], in the tibialis anterior (TA) with aging is largely unknown. Therefore, to examine this relationship we recruited twelve young (25 ± 3 years old), six old (68 ± 5 years old) and six very old (79 ± 3 years old) men. Magnetic resonance (MR) images were obtained from the entire musculature of the leg from the tibial plateau to the malleoli. Muscle cross-sectional areas (CSA) were calculated using image processing software. Strength was measured as maximal isometric voluntary dorsiflexion contraction (MVC) torque; this was then normalized to CSA. Structural integrity of the muscle was evaluated by magnetization transfer ratio (MTR) using magnetic resonance imaging (MRI). Neuromuscular measures were also collected and decomposition-enhanced spike-triggered averaging was used to collect surface and intramuscular electromyography (EMG) signals. From these data, estimates of motor unit numbers were made. Muscle CSA was less only in the very old (11.2cm2), no differences existed between the young (13.4cm2) and old (11.7cm2). Strength was ~26% lower in the old and ~24% in the very old than the young. When strength was normalized to CSA there were no differences between the groups. Very old men had ~8% lower MTR values than the young and old men, with no differences between young and old. Neuromuscular measures, specifically the combination of a decreased compound muscle action potential (CMAP) and increased surface motor unit potential (SMUP), resulted in a decrease in MUNE between young [~147 motor units (MU)] and old (~109 MUs) and also between young and very old (~80 MUs). In conclusion, muscular structure and function appear to be maintained in the older adult due to compensatory motor unit remodeling; however in the very old adult the structural integrity of the muscle becomes compromised as motor unit losses are greater resulting in the acceleration of sarcopenia