Resistance Exercise

In contrast to endurance exercise, acute whole-body resistance exercise does not alter leucine oxidation.56 In this same study we also did not find an effect of acute resistance exercise on whole-body protein synthesis, either during exercise or for up to 2 h post-exercise.56 We hypothesized that since muscle protein synthesis (MPS) accounted for only 25% of whole-body synthesis,76 changes in MPS either may be not measurable or would be negated by a reciprocal change in the synthesis of another protein, such as one in the gastrointestinal tract.

To measure the acute effect of resistance exercise on muscle-specific protein synthesis, several groups have used the FSR tracer incorporation method described above. We demonstrated that mixed-muscle FSR was elevated for up to 36 h following a single bout of resistance exercise.11,77 Other groups have also shown the increase in muscle protein synthesis after an acute bout of resistance exercise using FSR13,14,59 and arteriovenous balance12 methods. Phillips and colleagues13 demonstrated that FSR was elevated for up to 48 h after a single bout of resistance exercise in relatively untrained men and women, and increases in FSR in older adults have also been found following an acute bout of resistance exercise.78,79 The acute increase in FSR following a bout of resistance exercise is attenuated after a period of training in young men, yet the basal FSR rate is elevated.62

Muscle protein breakdown after resistance exercise can also be studied using the intracellular tracer dilution13 and the arteriovenous balance or tracer12 methods. In parallel with muscle FSR, Phillips and colleagues13 demonstrated that fractional protein breakdown (FBR) was increased after resistance exercise, yet the magnitude of the increase was less than for FSR (i.e., the muscle was in a more positive balance). Furthermore, they showed that FBR returned to baseline values before FSR.13 Biolo and co-workers12 found that muscle synthesis and breakdown were increased following an acute bout of resistance exercise. The net balance (synthesis minus degradation) was negative prior to exercise and was more positive (but still net negative) after exercise, for the subjects were in the fasted state. Taken together, these data indicate that muscle FSR and FBR are increased in the post-exercise period following resistance exercise. In the fasted state, net protein balance is negative, and resistance exercise renders the muscle in a less negative balance. Therefore, the post-exercise period is an important time for the delivery of nutrients, as discussed below.

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