Differentiating thermal from non-thermal eccrine sweating during exercise and heat stress

This project investigated the non-thermal factors which influence the control of eccrine sweating during exercise, with particular emphasis upon mechanoreceptor feedback and feedforward regulation. The aim of this project was to attempt to differentiate between these two neural pathways using three experimental treatments (active exercise; passive exercise and passive heating), with core temperature clamped among treatments and two pedal frequencies used for both the active and passive exercise conditions. It was hypothesised that during active (dynamic) exercise, sweat rates (m_(subscript SW)) and sweat expulsion frequencies (f(subscript SW)) would exceed those of the passive exercise and passive heating trials. It was expected that, when the pedal force was doubled during the active exercise trials, both the m_(subscript SW) and f(subscript SW) would exceed those values observed at the lower pedal force. Ten male subjects participated in five experimental trials: (a) two active (dynamic) exercise trials, in which the subjects voluntarily cycled at two different pedal frequencies; (b) two passive exercise trials, in which subjects were driven at the same two pedal frequencies, but did not actively recruite muscles to either track or resist the pedal motion; and (c) a seated resting trial, with subjects passively heated to track core temperature (T(subscript C)) changes in the other conditions. The combination of a water-perfusion garment and a climate chamber was used to increase and clamp T(subscript C) at similar rates across the five trials. During these trials, the following variables were measured: core temperatures at the oesophagus (T(subscript es)), auditory canal (T(subscript ac)), and the rectum (T(subscript re)); skin temperatures at eight sites; m_(subscript sw) were measured simultaneously at six locations; f(subscript sw) were identified using sweat data from the forehead and forearm sites; cardiac frequency (f(subscript c) ; ventricular depolarisation); thermal sensation; and ratings of perceived exertion. Of particular interest for this project were the variables of m_(subscript sw) and f(subscript sw) , and how they were affected by differences in pedal frequency (active versus passive exercise) and passive heating. The primary observation for these trials was that, when comparing the active and passive trials at the same pedal frequency, m_(subscript sw) and f(subscript sw) were very similar for each of the pedal frequencies, in the period from 15 to 25 minutes. However, the initial comparisons between m_(subscript sw)and f(subscript sw)of the active and passive trials were significantly different. When comparing trials at different pedal frequencies, but within the active exercise mode, a consistent trend in the m_(subscript sw) and f(subscript sw) was observed, with both being at 80 rev.min-1, relative to 40 rev.min-1 trials, though this was not statistically significant. For the same comparison in the passive exercise mode, the principal difference was the thermal load which was imposed on the subjects, with the data from the seated resting trials being greater than both the passive and active exercise trials. These observations may be interpreted in the following manner. First, the role of joint and muscle mechanoreceptors feedback may have been an influencing factor in the similarities of m_(subscript sw) and f(subscript sw) in the period from 15-25 minutes. Second, in the active exercise trials, the initiation of sweating seemed to be more related to central feedforward command, a non-thermal influence, while the passive and seated resting trials, were related more to feedback control, created from the differences in thermal gradient of T(subscript c) and T(subscript sk). Third, it would seem that thermal and non-thermal influences both play a role in the control of sweating, but their relative contribution may be modified by internal temperature and skin temperature changes