No abstract

The exact relationship between the bed rest-induced loss of skeletal muscle and reductions in muscle strength and physical performance in the older individuals is still unclear. Objective. We examined the effect of 10 days of bed rest on changes in regional body composition, muscle strength, and functional status, and the relationship between these variables in older individuals. Design, Participants, and Intervention. Regional body composition was measured using dual energy x-ray absorptiometry. We also determined changes in leg strength and several indices of functional status, including walking speed. Results. Body weight, body mass index, and total and lower extremity lean mass decreased with bed rest. There were also significant reductions in knee extension one repetition maximum, isometric knee extension, knee extension 60° concentric, stair ascent time, stair ascent power, stair descent time, VO 2 max, floor transfer test, 5-minute walk time, and chair stand. The overall change in total and lower extremity lean mass was also directly related to bed rest-induced reductions in one repetition maximum knee extension. Conclusions. Bed rest promoted overall declines in muscle mass, muscle strength, and physical function in older individuals. The changes in lean tissue were closely correlated with the bed rest-induced decline of muscle strength.

We combined the interstitial sampling method of microdialysis with the natural tracer qualities (i.e. non-recyclability) of the amino acid 3-methylhistidine (3MH) to uniquely study in vivo degradation of the two most abundant skeletal muscle proteins, myosin and actin. Interstitial 3MH concentration was measured before and for 24 h following a single bout of resistance exercise in eight young (27 ± 2 years) and eight old (75 ± 4 years) men. The exercise bout consisted of four exercises (3 sets of 8 repetitions at 80% one-repetition maximum (1RM) per exercise) emphasizing the quadriceps. Interstitial 3MH concentration was calculated using the internal reference method from microdialysate samples that were obtained from two microdialysis probes placed in the vastus lateralis. Resting interstitial 3MH concentration was 44% higher (P < 0.05) in the old (6.16 ± 0.56 nmol ml −1 ) as compared with the young (4.28 ± 0.27 nmol ml −1 ). Interstitial 3MH was not different (P > 0.05) from preexercise at any time point within the 24 h following exercise in both the young and the old. Leg arteriovenous exchange measurements in a separate group of young subjects also showed no increase in 3MH release during the 4 h following a resistance exercise bout compared with a non-exercised control leg (control leg: -28 ± 6, exercise leg: -28 ± 11 nmol min −1 ). These results suggest that myosin and actin proteolysis are not increased in the first 24 h following a standard bout of resistance exercise, and this response is not altered with ageing. The higher interstitial 3MH concentration in the old suggests an increased proteolysis of the two main contractile proteins in the rested and fasted state, which is consistent with a decrease in muscle mass with ageing. Microdialysis is an appropriate methodology for use in ageing individuals and is compatible with high-intensity resistance exercise.

In weight-stable subjects, MI resulted in no change in ISGD, and the improvement in ISGD with HI was completely reliant on improvements in nonoxidative glucose disposal.