Skeletal Muscle Adaptations with Age, Inactivity, and Therapeutic Exercise

Abstract

One of the remarkable features of skeletal muscle is its adaptability. Skeletal muscle adaptations are characterized by modifications of morphological, biochemical, and molecular variables that alter the functional attributes of specific skeletal muscle fiber types. Skeletal muscle adaptation is diverse and the magnitude of change is dependent on many factors, such as activity pattern, age, and muscle fiber type composition. The adaptation of skeletal muscle in the adult population is well described. In contrast, the adaptation of skeletal muscle in the older population is less documented, especially in the area of inactivity-induced alterations. Age-related changes in skeletal muscle may play a significant role in the magnitude of change with inactivity and influence the rehabilitation process for the older adult.A consistent feature of age and inactivity is limb muscle atrophy and the loss of peak force and power. Differences exist in the rate and mechanisms of muscle wasting and in the susceptibility of a given fiber type to atrophy. Most likely, the rapid muscle wasting might be in part due to a decrease in protein synthesis coupled with an increased degradation. Besides the quantitative change in muscle mass, age and inactivity induce important qualitative changes in the structure of key skeletal muscle proteins that are manifested in alterations in contractile properties.Therefore, the purpose of this clinical commentary is to identify the major effects of age and inactivity on skeletal muscle structure and function, and discuss potential therapeutic interventions. Special emphasis will be placed on how alterations in muscle structure affect function and on the cellular and molecular mechanisms of the age-related and inactivity-induced muscle changes.

Rehabilitative Ultrasound Imaging of the Lower Trapezius Muscle: Technical Description and Reliability

“…Alternatively, the probe could have been moved laterally, at a proportionate distance between the vertebra and the scapula. However, as scapular position can vary among subjects, particularly where there are abnormalities of muscle function (weakness, causing lengthening of fibers, or overactivity, causing shortening), 39 this was not considered to be a reliable option. Another option, when measuring the thickness of irregularly shaped muscles, may be to measure the thickness at the location offering the maximum thickness.…”

Age-related muscle dysfunction

“…Age-associated muscle atrophy is the result of a combination of individual-fiber atrophy, along with a decrease in the total number of fibers, with a preferential loss of type II (fast-twitch, glycolytic, fibers with myosin heavy chain type II isoforms) muscle fibers (Thompson, 1994). In addition to the loss in muscle mass and consequent loss in muscle force, muscles of old animals show a deficit of ~20% in specific force (force generation normalized for muscle cross-sectional area), suggesting qualitative (e.g., dysfunctional proteins) as well as quantitative deficiencies associated with aging (Brooks and Faulkner, 1994;Thompson, 1999Thompson, , 2002.Age-associated qualitative-and quantitative-deterioration of muscle contractility is observed at the whole muscle level and at the single fiber level. Two experimental approaches are available to evaluate single skeletal muscle fiber contractility, the intact and the permeabilized muscle fiber preparations.…”

Age-Associated Changes in Atrophy of the Extensor Digitorum Longus Muscle in Hindlimb-Suspended Rats

“…There is supporting evidence to show that the extensor digitorum longus (EDL), a non-postural muscle, becomes less atrophied compared to the soleus muscle, a postural muscle, during prolonged unweighting 2) . In addition, type II muscle fibers have been found to be less affected by prolonged unweighting than those of type I 3,4) .…”