Increased ventilation does not impair maximal voluntary contractions of the elbow flexors

Abstract

Exercise performance is impaired by increased respiratory work, yet the mechanism for this is unclear. This experiment assessed whether neural drive to an exercising muscle was affected by cortically driven increases in ventilation. On each of 5 days, eight subjects completed a 2-min maximal voluntary contraction (MVC) of the elbow flexor muscles, followed by 4 min of recovery, while transcranial magnetic stimulation tested for suboptimal neural drive to the muscle. On 1 day, subjects breathed without instructions under normocapnia. During the 2-min MVC, ventilation was ∼3.5 times that at rest. On another day, subjects breathed without instruction under hypercapnia. During the 2-min MVC, ventilation was ∼1.5 times that on the normocapnic day. On another 2 days under normocapnia, subjects voluntarily matched their breathing to the uninstructed breathing under normocapnia and hypercapnia using target feedback of the rate and inspiratory volume. On a fifth day under normocapnia, the volume feedback was set to each subject's vital capacity. On this day, ventilation during the 2-min MVC was approximately twice that on the uninstructed normocapnic day (or ∼7 times rest). The experimental manipulations succeeded in producing voluntary and involuntary hyperpnea. However, maximal voluntary force, fatigue and voluntary activation of the elbow flexor muscles were unaffected by cortically or chemically driven increases in ventilation. Results suggest that any effects of increased respiratory work on limb exercise performance are not due to a failure to drive both muscle groups optimally.

Feedforward consequences of isometric contractions: effort and ventilation

“…These findings were consistent regardless of whether the simultaneous muscle contractions were performed with contralateral or ipsilateral muscle groups, which suggests that the ventilatory response and perceived effort of contraction at the onset of muscle activation are not related to the total output of the primary motor cortices. The results also show that hypercapnia did not increase the perceived effort of contraction of the limb muscles, and that the ventilatory response to muscle contraction is separate from but summates with the hypercapnia-induced ventilatory response since contraction-induced increases in ventilation were achieved through changes in breathing frequency rather than tidal volume (Smith et al 2008).…”

“…Certainly, during normocapnia, the ventilatory responses for maximal limb contractions did not approach the limit of voluntary ventilation reported by Smith et al. (). Even with the hypercapnia‐induced increase in baseline ventilation at rest, ventilation during maximal limb contractions was only ~50% of the voluntary maximum (c.f.…”

“…The results also show that hypercapnia did not increase the perceived effort of contraction of the limb muscles, and that the ventilatory response to muscle contraction is separate from but summates with the hypercapnia‐induced ventilatory response since contraction‐induced increases in ventilation were achieved through changes in breathing frequency rather than tidal volume (Smith et al. ).…”

“…One possible advantage of having the ventilatory response independent from the total output of the motor pathways is that it negates the need for a large excess capacity. Certainly, during normocapnia, the ventilatory responses for maximal limb contractions did not approach the limit of voluntary ventilation reported by Smith et al (2008). Even with the hypercapnia-induced increase in baseline ventilation at rest, ventilation during maximal limb contractions was only~50% of the voluntary maximum (c.f.…”

“…Smith et al. ). In contrast, if ventilation scaled with the number of muscle groups, the simultaneous activation of more than two muscle groups could potentially drive ventilation toward maximum at the onset of only moderate levels of muscle activity.…”