Letter about the recent paper by Lam et al

I congratulate Lam, Greenhough, Nazari, White, & Bruce (2019) for their nice work about muscle metaboreflex activation during dynamic exercise. The authors found that increased muscle metaboreflex activity, generated by postexercise circulatory occlusion (PECO) of one leg, augmented the ventilatory and heart rate (HR) responses to effort of the contralateral leg. Thus, they demonstrated that metaboreflex activation can contribute to both hyperpnoea and tachycardia during dynamic exercise. This was an elegant approach to demonstrate that only during exercise (i.e. when the metaboreflex is operating in combination with central command), the stimulation of the muscle metaboreflex increases HR and ventilation. Moreover, the protocol they used (leg exercise) was different from those of most of the other studies in humans (handgrip). These results are similar to what found by our group in 2011 using a different experimental approach. Specifically, we demonstrated that when PECO was applied during dynamic exercise (handgrip), substantial ventilation and HR responses occurred. On the contrary, when PECO was applied after dynamic handgrip, neither ventilation nor HR took part in the response (Crisafulli et al., 2011). In the last 20 years, hemodynamics and cardiovascular regulation during PECO have been investigated by several studies conducted in humans, which are summarised in a recent review (Crisafulli, 2017). One possible weak point of the paper by Lam et al. (2019) is that in their interpretation the authors almost overlooked the role played by the arterial baroreflex in the HR response during PECO. Specifically, during PECO after exercise parasympathetic activity increases and obscures the effect of the metaboreflex-induced sustained sympathetic activity upon the sinus node, thus explaining the lack of influence of PECO upon HR (Iellamo et al., 1999; Nishiyasu et al., 1994). A different HR behaviour occurs when the metaboreflex is evoked by means of PECO during exercise. In this situation, HR increases because the central command is still operating and decreases the parasympathetic tone, which, in turn, cannot counteract the enhanced sympathetic activity towards the sinus node. In short, from these previous papers it appears that the HR behaviour during the metaboreflex is a complex biological phenomenon that is under the control of several mechanisms. In this context, the role of the baroreflex should not be ruled out. In my opinion, all these previous findings support the authors’ hypothesis and would help in interpreting their results.