Strength in breath: respiratory metaboreflex response to training and detraining

Intense muscular effort increases demand for blood flow to fuel mechanical work and displace the waste products of metabolism. Muscle metaboreflexes arise from the accumulation of metabolites within contracting muscles and drive afferent feedback to the central nervous system, leading to increased sympathetic activity and cardiovascular adjustments facilitatingmuscle performance. Increased respiratory muscle work elicits the respiratory metaboreflex, which is especially powerful in prioritising allocation of cardiac output to the muscles of breathing over other working muscles, including large locomotor muscles. In this issue of Experimental Physiology, Chan et al. (2023) investigated the effect of inspiratory muscle training and a subsequent equivalent detraining period on the respiratory metaboreflex in young healthy participants. Inspiratory muscle training consisted of two sets of 30 loaded inspiratory efforts (50% of maximal inspiratory pressure) performed 5 days per week for 5weeks. Training increased respiratory muscle strength, evidenced by increased maximum inspiratory pressure, and it blunted the respiratory muscle metaboreflex. The observations confirm the findings of others (Chiappa et al., 2008; Witt et al., 2007), pointing to potential clinical applications of inspiratory muscle training. Interestingly, improvements in respiratory muscle strength were maintained following the detraining period of 5 weeks, echoing the findings of others (Romer & McConnell, 2003). Moreover, after 5 weeks of discontinued training, heart rate and pressure responses to a loaded breathing task to failure were lower than pre-training, demonstrating preserved attenuation of the respiratory

Intense muscular effort increases demand for blood flow to fuel mechanical work and displace the waste products of metabolism.
Muscle metaboreflexes arise from the accumulation of metabolites within contracting muscles and drive afferent feedback to the central nervous system, leading to increased sympathetic activity and cardiovascular adjustments facilitating muscle performance. Increased respiratory muscle work elicits the respiratory metaboreflex, which is especially powerful in prioritising allocation of cardiac output to the muscles of breathing over other working muscles, including large locomotor muscles.
In this issue of Experimental Physiology, Chan et al. (2023) investigated the effect of inspiratory muscle training and a subsequent equivalent detraining period on the respiratory metaboreflex in young healthy participants. Inspiratory muscle training consisted of two sets of 30 loaded inspiratory efforts (50% of maximal inspiratory pressure) performed 5 days per week for 5 weeks. Training increased respiratory muscle strength, evidenced by increased maximum inspiratory pressure, and it blunted the respiratory muscle metaboreflex. The observations confirm the findings of others (Chiappa et al., 2008;Witt et al., 2007), pointing to potential clinical applications of inspiratory muscle training. Interestingly, improvements in respiratory muscle strength were maintained following the detraining period of 5 weeks, echoing the findings of others (Romer & McConnell, 2003). Moreover, after 5 weeks of discontinued training, heart rate and pressure responses to a loaded breathing task to failure were lower than pre-training, demonstrating preserved attenuation of the respiratory muscle metaboreflex for at least 5 weeks following termination of the training paradigm.
The authors report a strong inverse relationship between blood pressure and maximum inspiratory pressure; the greater the increase in respiratory muscle strength, the greater the attenuation of the respiratory muscle metaboreflex. A greater capacity for inspiratory muscle force production per se is unlikely to be the sole determinant of the training-induced attenuation of the respiratory metaboreflex, particularly since training thresholds were adjusted in accordance with strength gains during the 5-week training period. Indeed, the authors consider the potential for enhanced anaerobic capacity, speculating that the threshold for considerable accumulation of fatiguing metabolites may have been raised. However, the time frame of the training block was likely too short for such metabolic adaptation considering the type of training employed. Moreover, it is unlikely that metabolic adaptations would be so resilient to detraining as observed in the study, since it is generally accepted that decay of such adaptation occurs at a rate equivalent to accumulation. Neurological changes such as improved synchronisation and firing frequency of motor units may have increased the efficiency of respiratory muscle activation. It is well-established that such adaptations present promptly and robustly in untrained populations. It would be very interesting to investigate the effects of inspiratory muscle training paradigms on the respiratory metaboreflex in well-trained participants.
Further exploration of the underlying causal mechanism(s) should also consider the fundamental principle of specificity of exercise Experimental Physiology. 2023;108:541-542.
wileyonlinelibrary.com/journal/eph training. It is possible that multiple physiological adaptations act to attenuate the respiratory metaboreflex, as a function of stimulus.
To pull at this thread would require comparative studies of training type, which might offer further insight into the mechanism of the attenuated respiratory metaboreflex and preferred modes for implementation of inspiratory training. Indeed, differential outcomes of respiratory muscle training for athletic performance appear to relate to differences in the paradigms employed. It is widely acknowledged that effective training strategies require focus on the principles of frequency, intensity, type and timing.
The authors allude to an important unknown, whether the strong linear relationship between maximum inspiratory pressure and blood pressure would be maintained over a greater duration of inspiratory muscle training. Inspiratory muscle strength was shown to increase by 41% over 9 weeks of inspiratory muscle training (Romer & McConnell, 2003

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