Reply

We appreciate the interesting comments and perspective offered by Dr Marino on our article ‘Cardiovascular strain impairs prolonged self‐paced exercise in the heat’ (Périard et al. 2011) and are thankful for this opportunity to respond. Firstly, the notion that cardiovascular strain cannot be a limiting factor was questioned because power output at time trial completion either surpassed or returned to values similar to those observed at time trial commencement. According to Dr Marino, these increases should not occur over an ‘extended period’ if cardiovascular strain was truly limiting (Marino, 2011). We agree, but contend that a power output increase in both conditions, for less than 4 min from the penultimate data point, does not constitute an extended period. Moreover, the issue was raised as to why power output was not maximal at the start of exercise and increased significantly at trial completion. Naturally, a pacing strategy is involved at the onset of self‐paced exercise. In a 40 km time trial, this behavioural response involves refraining from recruiting and maximally activating all motor units, as this can increase power output beyond 1000 W. Such an output would result in the rapid accumulation of fatigue‐related metabolites and task failure. Optimal pacing therefore requires balancing the relative contribution of aerobic and anaerobic metabolism to exercise at the physiological limit, or critical power (i.e. fatigue threshold). In the heat, power output sustained at this effort decreases because of the decline in peak oxygen uptake inline image and increase in relative intensity. Conversely, if a pacing strategy was formulated in anticipation to mediate the rate of heat storage by integrating prior experience, environmental factors and thermal strain (Tucker et al. 2006), would it not limit muscle recruitment at the onset of exercise so that power output and heat production were not identical to that of an environment 15°C lower? A truly anticipatory mechanism, a feedforward system that should act before external disturbances have a chance to influence it (Williamson, 2010), would delay the rapid increase in heat storage at the start of exercise and allow for power output to be more evenly maintained throughout the performance (i.e. even pacing). However, as demonstrated by our data this is not the case, as the decrease in power output is associated with the development of cardiovascular strain.