Sarcopenia: etiology, clinical consequences, intervention, and assessment

Abstract

The aging process is associated with loss of muscle mass and strength and decline in physical functioning. The term sarcopenia is primarily defined as low level of muscle mass resulting from age-related muscle loss, but its definition is often broadened to include the underlying cellular processes involved in skeletal muscle loss as well as their clinical manifestations. The underlying cellular changes involve weakening of factors promoting muscle anabolism and increased expression of inflammatory factors and other agents which contribute to skeletal muscle catabolism. At the cellular level, these molecular processes are manifested in a loss of muscle fiber cross-sectional area, loss of innervation, and adaptive changes in the proportions of slow and fast motor units in muscle tissue. Ultimately, these alterations translate to bulk changes in muscle mass, strength, and function which lead to reduced physical performance, disability, increased risk of fall-related injury, and, often, frailty. In this review, we summarize current understanding of the mechanisms underlying sarcopenia and age-related changes in muscle tissue morphology and function. We also discuss the resulting long-term outcomes in terms of loss of function, which causes increased risk of musculoskeletal injuries and other morbidities, leading to frailty and loss of independence.

Fig. 1

Hierarchical depiction of skeletal muscle structure, depicting the skeletal muscle fibers within the muscle bundle, a motor unit branching out to two muscle fibers and the detailed structure of myofibrils

Fig. 2

Effect of age on the motor unit, depicting, young, aged, and aged sarcopenic fibers. This drawing depicts the pronounced denervation of type II fibers and the recruitment of type I fibers into surviving motor units in older subjects, with impairment of recruitment in sarcopenic subjects

Fig. 3

Age effects on systemic factors influencing synthesis and degradation of skeletal muscle proteins

Fig. 4

CT acquisition through midthigh. Location of axial section is shown on localizer image at the left, with corresponding axial image in the middle and segmentation into distinct tissue compartments at the right. Green: subcutaneous fat. Olive: quadriceps muscle. Yellow: hamstrings muscle. Red: adductor muscles. Orange: sartorius muscle

Fig. 5

MRI image of calf at the right, with green and yellow boxes indicating locations of spectroscopic acquisitions of the tibialis anterior and soleus muscles, respectively. Proton spectroscopy studies may be used to assess the relative amounts of intramyocellular and extramyocellular lipid. At the right, a proton spectrum corresponding to the soleus muscle shows ¹H resonances associated with creatinine (CR2 and CR3), water, extramyocellular lipid (EMCL), intramyocellular lipid (IMCL), and trimethylamines (TMA)