Adaptations in Muscle Contractility
The fascination that modem exercise physiologists have had for the theory that oxygen alone determines exercise performance may have blinded us to other possibilities (Noakes, 1988b). One possibility is that muscle power or contractility may determine performance (see post 2, p. 21) and that training may effect important changes in muscle contractility (Noakes, 1988b). These changes would likely result from increased skeletal muscle myosin ATPase activity and enhanced calcium handling by the sarcoplasmic reticulum.
The Time Course of the Mitochondrial Adaptations: The Effects of Detraining
The adaptations of V02max to training occur very quickly, so that it is possible, for example, to show an increase in V02max within a week of beginning an intensive training program (Hickson et al, 1977). Changes in heart rate, blood pressure, the lactate tumpoint, and glycogen storage can be detected even earlier (H.J. Green et al, 1989; M.A. Rogers et al, 1988). Unless the intensity of the training program is increased progressively, the total increase in V02max will be achieved within 3 weeks (Hickson et al, 1981). Detraining brings about a
Rapid fall in V02max in the first 2 to 3 weeks with a more gradual decrease thereafter (Coyle et al, 1984).
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The time course of muscle enzyme changes with training has been less clearly defined, but in studies lasting up to 12 weeks, enzyme levels appear to show a gradual and progressive increase. At present, it seems that the rate and magnitude of these changes are functions, within limits,, of the total amount of muscle contractile activity. The rate can be increased either by performing more contractions in a given time period (i.e, increasing exercise intensity) or by maintaining the same frequency of contraction for a longer period (i.e, increasing exercise duration; Holloszy & Coyle, 1984).
In a study of rats, Dudley et al. (1982) found that at any exercise intensity, 60 minutes of training 5 days a week produced maximum adaptation in mitochondrial enzyme content. Longer daily training periods produced no further increase in mitochondrial oxidative enzyme changes. The authors found that high-intensity interval training at approximately 116% V02max for relatively short periods (15 min/day) produced as great an increase in mitochondrial enzyme content as did exercise for 90 min/day at 83 to 94% V02max.