The Physiology of Exercise in Spinal Cord Injury by J. Andrew Taylor

Author:J. Andrew Taylor
Language: eng
Format: epub
Publisher: Springer US, Boston, MA

7.5.1 Upper vs. Lower Body Exercise

Even in able-bodied individuals, there are differences in the thermoregulatory responses to upper vs. lower body exercise (Sawka et al. 1989; Asmussen and Nielsen 1947; Nielsen 1968). Typically, upper body exercise results in greater heat production for a given absolute workrate or metabolic rate/VO2 compared to lower body exercise (Sawka et al. 1984). Due to lower overall muscle mass, the arm muscles must work at a higher percent of maximal intensity in order sustain similar workrates. Efficiency is also reduced at higher intensities (Zoladz et al. 1995), meaning a higher percentage of energy expended is given off as heat rather than used for external work, resulting in greater heat production. Furthermore, due to the reduced muscle mass of the arms compared to the legs, for a given metabolic rate, less afferent input from metaboreceptors and thermoreceptors in the skeletal muscle and venous system will be sent to the brain. This may require more metabolic heat production prior to initiation of heat loss mechanisms. Accordingly, some have suggested that the thermoregulatory set point in the hypothalamus is shifted to a higher core temperature with upper body exercise (Tam et al. 1978a; Asmussen and Nielsen 1947); however, (Sawka et al. 1984) found no difference in threshold or slope of the sweat response.

The cardiovascular system may also limit thermoregulation in upper body exercise. In individuals with SCI, the lack of the skeletal muscle pump limits cardiac output, in turn limiting blood flow that can be routed to the exercising muscles and to the skin for thermoregulation. Due to the smaller muscle mass of the arms, there is less of a need to redistribute blood flow to these muscles during exercise than with leg or whole-body exercise. In individuals with SCI who already have impaired abilities to redistribute blood flow due to loss of sympathetic vasoconstriction, loss of muscle pump does limit the rise in cardiac output. In fact, studies in which the muscle pump has been artificially created using functional electrical stimulation to the legs (Davis et al. 1990) or by wearing an anti-gravity suit (Hopman et al. 1992b) have shown increases in cardiac output and stroke volume in individuals with SCI during submaximal arm-crank exercise. Interestingly, no effects of the interventions were observed in AB subjects in either of these studies. AB individuals also experience greater plasma volume loss and hemoconcentration with upper body exercise for a given VO2 (Miles et al. 1983; Pimental et al. 1984), which will limit sweat loss.

One advantage of upper body exercise is that the arms have a higher surface area-to-mass ratio which should facilitate convective heat transfer. However, this does not make much difference when exercise is performed in air. It may, however, make a difference in water since conduction of heat is so much greater (~24× greater). Toner et al. (1984) demonstrated that subjects exercising in 26 °C and 33 °C water had lower core temperature during upper body exercise compared to lower body exercise at a matched VO2. However,

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