Effect of inspiratory muscle work on peripheral fatigue of locomotor muscles in healthy humans

Abstract

The work of breathing required during maximal exercise compromises blood flow to limb locomotor muscles and reduces exercise performance. We asked if force output of the inspiratory muscles affected exercise-induced peripheral fatigue of locomotor muscles. Eight male cyclists exercised at ≥ 90% peak O 2 uptake to exhaustion (CTRL). On a separate occasion, subjects exercised for the same duration and power output as CTRL (13.2 ± 0.9 min, 292 W), but force output of the inspiratory muscles was reduced (−56% versus CTRL) using a proportional assist ventilator (PAV). Subjects also exercised to exhaustion (7.9 ± 0.6 min, 292 W) while force output of the inspiratory muscles was increased (+80% versus CTRL) via inspiratory resistive loads (IRLs), and again for the same duration and power output with breathing unimpeded (IRL-CTRL). Quadriceps twitch force (Q tw ), in response to supramaximal paired magnetic stimuli of the femoral nerve (1-100 Hz), was assessed pre-and at 2.5 through to 70 min postexercise. Immediately after CTRL exercise, Q tw was reduced −28 ± 5% below pre-exercise baseline and this reduction was attenuated following PAV exercise (−20 ± 5%; P < 0.05). Conversely, increasing the force output of the inspiratory muscles (IRL) exacerbated exercise-induced quadriceps muscle fatigue (Q tw = −12 ± 8% IRL-CTRL versus −20 ± 7% IRL; P < 0.05). Repeat studies between days showed that the effects of exercise per se, and of superimposed inspiratory muscle loading on quadriceps fatigue were highly reproducible. In conclusion, peripheral fatigue of locomotor muscles resulting from high-intensity sustained exercise is, in part, due to the accompanying high levels of respiratory muscle work.

Acute respiratory muscle unloading by normoxic helium–O2 breathing reduces the O2 cost of cycling and perceived exertion in obese adolescents

“…Previous investigations demonstrated that respiratory muscle unloading during high-intensity exercise can prevent respiratory muscle fatigue and the reflex vasoconstriction within active locomotor muscles, and can ultimately lead to increased limb blood flow (Babcock et al 2002;Chiappa et al 2009;Harms et al 1997;Romer et al 2006). Although in normal subjects, this phenomenon may not happen during submaximal exercise (Wetter et al 1999), it could occur in obese patient population characterized by a substantially higher work of breathing and increased metabolic demands.…”

“…Improvements in locomotor muscle O 2 delivery would delay/reduce the development of peripheral muscle fatigue (Hogan et al 1999;Romer et al 2006), would increase the metabolic stability in these muscles (Zoladz et al 2008), and would prevent the recruitment of additional muscle fibers (Romer et al 2006) and the loss of muscle efficiency (Cleland et al 2012), thereby explaining, at least in part, the lowered O 2 cost of cycling (−10 % during CWR < GET and −15 % during CWR > GET). A significant reduction in the O 2 cost of cycling, obtained by normoxic helium breathing during constant work rate submaximal exercise, has been described before in normal subjects (Murphy et al 1969;Cross et al 2010b) and in COPD patients (Chiappa et al 2009).…”

Exercise-induced respiratory muscle fatigue: implications for performance

“…The effect of inspiratory muscle unloading on exerciseinduced limb muscle fatigue likely underestimated what might be attributed to the total work of breathing (103). The normal work of breathing was reduced by only about one-half.…”

“…Reductions in limb blood flow and O 2 transport in response to fatiguing respiratory muscle work would be expected to impair limb locomotor muscle function. Indeed, when the inspiratory muscles were partially unloaded during heavy cycle exercise using a mechanical ventilator, the postexercise decrease in stimulated quadriceps force was attenuated by about one-third, and perceptual ratings of limb discomfort were reduced (103). Loading the respiratory muscles using inspiratory resistors exacerbated the severity of quadriceps fatigue by ϳ40% and increased the perceptions of limb discomfort compared with identical exercise with breathing unimpeded (Fig.…”

“…Differences in ⌬Qtw were significant (*P Ͻ 0.01) between control and unload, and control and load. Data are from Romer et al (103). …”