ARS | Publication request: Muscle Fiber Size and Function in Elderly Humans: A Longitudinal Study

Submitted to: Journal of Applied Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 11, 2008
Publication Date: June 12, 2008
Citation: Frontera, W.R., Kieran, R.F., Phillips, E.M., Krivickas, L.S., Hughes, V.A., Roubenoff, R., Fielding, R. 2008. Muscle Fiber Size and Function in Elderly Humans: A Longitudinal Study. Journal of Applied Physiology. 105:637-642.

Interpretive Summary: In older adults, muscle weakness and muscle loss are causally related to the loss of functional independence, higher rate of hospitalization and increases in morbidity and mortality. Examining the change-over-time in both the amount and composition of skeletal muscle mass provides us with important information about the inevitable age-related declines in muscle mass and function. This longitudinal study is particularly novel as it characterizes age-associated changes in muscle mass over time in the same group of elderly subjects. Another important point about this study is that it reports on these alterations not only at the whole muscle level, but also documents the effects of advancing age on the individual muscle fibers that make up the whole muscle tissue. The study revealed the dissimilarity between the physiological changes examined at the whole muscle level versus the changes examined at the single muscle fiber level. Despite the significant and expected decrease in whole muscle size and strength that occurs with age, it was noted that single muscle fiber size and force production are maintained. These data suggest that with the loss in whole muscle size, the remaining individual muscle fibers may compensate and become bigger to counteract overall decreases in muscle size and function.

Technical Abstract: Cross-sectional studies are likely to underestimate age-related changes in skeletal muscle strength and mass. The purpose of this longitudinal study was to assess whole muscle and single muscle fiber alterations in the same cohort of 12 older (mean age: start of study=71.1+/-5.4 yrs and end of study=80+5.3 yrs.) volunteers (5 men) evaluated 8.9 years apart. No significant changes were noted at follow-up in body weight, body mass index, and physical activity. Muscle strength, evaluated using isokinetic dynamometry, and whole muscle specific force of the knee extensors were significantly lower at follow-up. This was accompanied by a significant reduction (5.7%) in cross-sectional area of the total anterior muscle compartment of the thigh as evaluated by computerized tomography. Muscle histochemistry showed no significant changes in fiber type distribution or fiber area. Experiments with chemically skinned single muscle fibers (n=411) demonstrated no change in type I fiber size but an increase in IIA fiber diameter. A trend toward an increase in maximal force in both fiber types was observed. Maximum unloaded shortening velocity did not change. In conclusion, single muscle fiber contractile function may be preserved in older humans in the presence of significant alterations at the whole muscle level. This suggests that surviving fibers compensate to partially correct muscle size deficits in an attempt to maintain optimal force generating capacity.