A central tool in the study of the control of breathing during exercise is the exercise test itself: it is important that the subjects are presented with a consistent, reproducible stimulus if the results obtained are to be collated as a group, or if valid comparisons are to be made with results obtained with a different population group or using a different test protocol. The choice of which mode of exercise to choose for a study is subject to a number of constraints, including the availability of equipment, reproducibility of workloads, ease of use, subjects' familiarity with the mode of exercise and the hypothesis to be tested by the study itself. These constraints have resulted in cycle ergometry and treadmill running emerging as the two most commonly used modes of exercise, although other modes of exercise have been used, e.g. rowing ergometer, ski-walking on a treadmill, arm cranking. These, however, involve a greater degree of co-ordination from the subject which may affect the results adversely.
A further consideration when deciding on the appropriate mode of exercise is whether the mode of exercise itself exerts an effect on the particular exercise responses being investigated. An example of this can be seen with one of the more common uses of exercise testing: determination of a subject's Vo2^ . A number of authors have reported that cycle ergometry elicits a lower value of Vo2^ than does treadmill running (e.g. Davis & Kasch,
1975; McArdle et al., 1973; Faulkner et al., 1971), although Hermansen & Saltin (1969) and Âstrand & Rodahl (1986) both report no difference in Vo2^ measured using these
two modes of exercise. This difference in results may be partly due to differing levels of familiarity with cycling between the subject groups. Further evidence for this comes from Withers et al. (1981) who reported that trained cyclists achieved a 4.5% higher value for Vo2^ when exercising on a cycle ergometer than on a treadmill, while in trained runners
this trend was reversed, with treadmill running eliciting a 10.4% higher value for Vo2^
than cycle ergometry.
Exercise responses can also be aflfected by the mode of exercise employed. It is well established that at similar submaximal workloads arm cranking elicits a higher heart rate than cycle ergometry (Âstrand & Rodahl, 1986), while Weiler-Ravell et al. (1983) reported that the initial responses for both heart rate and Ve were attenuated when exercise was
performed in the supine position rather than upright. There is evidence that changes in stride frequency, e.g. when comparing walking and running at equivalent metabolic loads, can affect a subject's ventilatory response (Berry et al., 1989; Caretti et al., 1992 and McMurray & Ahlbom, 1982), although this is not a universal finding (McMurray & Smith, 1985). The change in ventilatory response is associated with an increase in ^ R , with running resulting in a lower Pet,C02 and higher RER than walking (Caretti et al., 1992; McMurray & Ahlbom, 1982; McMurray & Smith, 1985).
One likely reason for this influence of stride frequency over the ventilatory response to exercise is that runners breathe in time with their stride pattern (Bramble & Carrier, 1983; Bechbache & Duflfin, 1977; McMurray & Ahlbom, 1982; McMurray & Smith, 1985). Bramble & Carrier (1983) reported this phenomenon to be more common in highly trained endurance runners than untrained individuals, respiratory-locomotor coupling occurring as early as the fourth stride of running in highly trained subjects. Furthermore, they reported that breathing was coupled to stride pattern, not the other way round.
The ventilatory response to rowing has been investigated by Mahler et al. (1991a, b) and again experienced rowers demonstrate a high level of coupling between respiratory events and certain parts of the rowing stroke, with the incidence of coupling rising with experience. Steinacker et al. (1993) also reported considerable respiratory-locomotor coupling in a group of highly trained oarsmen. They reported that while at low and moderate workloads breathing pattem was constrained by the stroke frequency, at high workloads it was possible for stroke frequency to be driven up by^R .
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Respiratory-locomotor coupling is also common in trained cyclists (Kohl et al., 1981), the incidence falling with decreasing experience. There is also evidence that pedal frequency directly influences a subject's exercise responses, with the metabolic cost of cycling increasing with increasing pedal frequency (Hagan et al., 1992; Casaburi et al., 1978b; Gaesser & Brooks, 1975; Takano, 1988), although Sipple & Gilbert (1966) were unable to find any effect attributable to pedal frequency. Takano also reported pedal frequency as directly influencing VE, although Casaburi et al. (1978) could find no evidence of this.
It would therefore appear that trained subjects phase couple their breathing to their movements within their sport. Whether this coupling is transferable between exercise modes remains to be seen. Berry et al. (1989) was unable to demonstrate any difference in the ventilatory response to exercise in runners cycling at 60 vs. “90 RPM” or in cyclists walking/running at equivalent metabolic loads.
Steinacker et al. (1986) compared rowers' and cyclists' exercise responses on both modes of exercise and reported rowers as achieving a higher Vo2^ than cyclists on the rowing
ergometer and vice versa. Szal et al. (1989) reported rowers as having a higher ventilatory response to both maximal and submaximal exercise when rowing than when cycling.