We investigated how myofibrillar protein synthesis (MPS) and muscle anabolic signalling were affected by resistance exercise at 20-90% of 1 repetition maximum (1 RM) in two groups (25 each) of post-absorptive, healthy, young (24 ± 6 years) and old (70 ± 5 years) men with identical body mass indices (24 ± 2 kg m −2 ). We hypothesized that, in response to exercise, anabolic signalling molecule phosphorylation and MPS would be modified in a dose-dependant fashion, but to a lesser extent in older men. Vastus lateralis muscle was sampled before, immediately after, and 1, 2 and 4 h post-exercise. MPS was measured by incorporation of [1,2-13 C] leucine (gas chromatography-combustion-mass spectrometry using plasma [1,2-13 C]α-ketoisocaparoate as surrogate precursor); the phosphorylation of p70 ribosomal S6 kinase (p70s6K) and eukaryotic initiation factor 4E binding protein 1 (4EBP1) was measured using Western analysis with anti-phosphoantibodies. In each group, there was a sigmoidal dose-response relationship between MPS at 1-2 h post-exercise and exercise intensity, which was blunted (P < 0.05) in the older men. At all intensities, MPS fell in both groups to near-basal values by 2-4 h post-exercise. The phosphorylation of p70s6K and 4EBP1 at 60-90% 1 RM was blunted in older men. At 1 h post-exercise at 60-90% 1 RM, p70s6K phosphorylation predicted the rate of MPS at 1-2 h post-exercise in the young but not in the old. The results suggest that in the post-absorptive state: (i) MPS is dose dependant on intensity rising to a plateau at 60-90% 1 RM; (ii) older men show anabolic resistance of signalling and MPS to resistance exercise.

Skeletal muscle demonstrates extraordinary mutability in its responses to exercise of different modes, intensity, and duration, which must involve alterations of muscle protein turnover, both acutely and chronically. Here, we bring together information on the alterations in the rates of synthesis and degradation of human muscle protein by different types of exercise and the influences of nutrition, age, and sexual dimorphism. Where possible, we summarize the likely changes in activity of signaling proteins associated with control of protein turnover. Exercise of both the resistance and nonresistance types appears to depress muscle protein synthesis (MPS), whereas muscle protein breakdown (MPB) probably remains unchanged during exercise. However, both MPS and MPB are elevated after exercise in the fasted state, when net muscle protein balance remains negative. Positive net balance is achieved only when amino acid availability is increased, thereby raising MPS markedly. However, postexercise-increased amino acid availability is less important for inhibiting MPB than insulin, the secretion of which is stimulated most by glucose availability, without itself stimulating MPS. Exercise training appears to increase basal muscle protein turnover, with differential responses of the myofibrillar and mitochondrial protein fractions to acute exercise in the trained state. Aging reduces the responses of myofibrillar protein and anabolic signaling to resistance exercise. There appear to be few, if any, differences in the response of young women and young men to acute exercise, although there are indications that, in older women, the responses may be blunted more than in older men.

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We explored the relationships between resistance exercise volume/intensity and muscle myofibrillar protein synthetic (MPS) responses in young and older men. In a crossover design, four groups of six young (24±6 years) and older (70±5 years) men performed two volumes of resistance exercise: either 40% one repetition maximum (1RM) (3 × 14, then 6 × 14 repetitions) or 75% 1RM (3 × 8, then 6 × 8 repetitions), such that at the same volume, work was identical between intensities. Muscle biopsies were taken 0, 1, 2, and 4hours after exercise to measure MPS via myofibrillar bound [1,2-(13)C(2)]leucine and indices of mammalian target of rapamycin signaling by immunoblotting. In younger men, doubling exercise volume produced limited added effects, whereas in older men, it resulted in greater MPS and p70S6 kinase (p70S6K(Thr389)) phosphorylation at both intensities, that is, MPS area under the curve: 75% (1× volume: 0.07±0.01 vs 2× volume: 0.14% ± 0.02% protein synthesized/4hours (p < .001). Doubling exercise volume is a valid strategy to maximize postexercise MPS in ageing.