Strenuous exercise markedly stimulates the rate of human muscle protein synthesis but the relative importance of intensity of contraction and the associated metabolic changes are unknown. In order to answer this question we arranged to measure muscle protein synthesis in nine subjects (7 men, 2 women, aged 29.7 ± 5.5 years, BMI 23.4 ± 1.6). Subjects were randomly assigned to one of three exercise protocols so that the intensity of isometric contraction of the quadriceps could be varied but the total ATP turnover would be constant. The study had the approval of the local ethics committee. The subjects, all postabsoprtive, exercised as follows: high intensity, 3 repetitions, 90 % 1 RM plus 16.66 further sets, with 1 min rest between sets; medium intensity, 8 repetitions at 75 % 1 RM, plus 7 further sets, with 2 min 21 s rests between; low intensity, 15 repetitions at 60 % 1 RM, plus 4.33 further sets, with 3 min 8 s rests between. A primed constant infusion of [1,2-¹³C]leucine was begun 160 min before exercise and continued for 150 min afterwards. Muscle biopsies were taken under local anaesthesia (lignocaine 1%) immediately after exercise and 90, 120 and 150 min post-exercise. Blood samples were taken intermittently from an antecubital vein. Muscle was separated into myofibrillar and sarcoplasmic components and the fractional protein synthetic rates determined from the incorporation of [¹³C]leucine by our standard methods.

Basal protein synthetic rates were 0.042 ± 0.014 and 0.057 ± 0.011 % h^-1 (myofibrillar and sarcoplasmic, respectively, grand means ± S.D.). In all cases the rate of muscle protein synthesis between 90-150 min post-exercise period was stimulated markedly after a lag phase of ~90 min. The increase in myofibrillar protein synthesis was substantially greater than that in sarcoplasmic, as previously observed (Rennie et al. 1999). However, the extent of the relative increases (as percentage of the resting value) were not significantly different between the three groups (grand means, 3.4 ± 0.5-fold myofibrillar and 2.6 ± 0.8-fold sarcoplasmic), suggesting that in the first 2.5 h after exercise at least, the extent of the stimulation of muscle protein synthesis is not determined by the intensity of the preceding contractile activity. This does not preclude the possibilities that the duration of any intensity-dose effect extends beyond the period studied or that there may be additional effects in the fed state.

This work was supported by the UK MRC.