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 > or = 90% peak O2 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.

Figure 1

Representative within-breath pressure–time traces for one subject ensemble averaged over 1 min for CTRL, IRL and PAV at exercise isotime. P_oe, oesophageal pressure; P_di, transdiaphragmatic pressure. Pressure–time products for the inspiratory muscles (A) and for the diaphragm (B) were calculated by integrating the respective pressure over the period of inspiratory flow and then multiplying the integral by the respiratory frequency.

Figure 2

Group mean change for diaphragm pressure–time product (iP_di×f_R) and inspiratory muscle pressure–time product (P_oe×f_R) at baseline (Pre), during the fifth minute of the warm-up (WU) at 40% peak power output W˙_peak and every minute thereafter at W˙_peak for CTRL versus PAV (A and B) and Inspiratory Resistive Load (IRL) versus IRL-CTRL (C and D). Both measures of respiratory force output were reduced during PAV versus CTRL and increased during IRL versus IRL-CTRL. Values are mean ±s.e.m.n = 8 per group. **P < 0.01, significantly different at the same time.

Figure 3

Group mean peak quadriceps twitch force (Q_tw,peak) and the force of the second twitch (Q_tw,T2) in response to supramaximal stimuli at 1 Hz (single twitch), 10 Hz (100 ms interstimulus duration), 50 Hz (20 ms) and 100 Hz (10 ms) pre- and up to 70 min postexercise for CTRL (A and B) and IRL-CTRL (C and D). Exercise was 292 ± 13 W for 13.2 ± 0.9 min in CTRL and 292 ± 13 W for 7.9 ± 0.6 min in IRL-CTRL. In CTRL, the Q_tw,_peak response was significantly reduced at all frequencies and times after exercise; immediately postexercise the Q_tw,T2 response was significantly reduced at all stimulation frequencies; at 35 min postexercise the Q_tw,T2 response at 10 Hz was less than at 50 and 100 Hz. In IRL-CTRL, Q_tw,T2 (mean for all frequencies) was reduced immediately after versus before exercise. n = 8 per group. Values are mean ±s.e.m.

Figure 4

Group mean change for the second quadriceps twitch amplitude (Q_tw,T2), expressed as percentage change from baseline values, for CTRL versus PAV (A) and IRL versus IRL-CTRL (B) over a range of stimulation frequencies. Data were collected 2.5 min postexercise. n = 8 per group. Values are mean ±s.e.m.*P < 0.05; **P < 0.01, significantly different at the same frequency of stimulation. Note: the IRL and IRL-CTRL values are the mean of two trials performed on different days.

Figure 5

Group mean change across all stimulus frequencies for the second quadriceps twitch amplitude (Q_tw,T2), expressed as percentage change from baseline values, for CTRL versus PAV (A) and IRL versus IRL-CTRL (B). Trial 1 and Trial 2 were performed on different days at identical power outputs and exercise durations for each of the IRL and IRL-CTRL conditions. Data were collected 2.5 min postexercise. n = 8 per group. Values are mean ±s.e.m.

Figure 6

Blood lactate concentration ([La⁻]_B) measured at baseline (Pre), after 5 min of warm-up (WU) at W˙_peak, every 2 min of exhausting exercise at W˙_peak, and after ∼10 min of recovery (Rec) for PAV (A) and IRL (B). n = 8 per group. Values are mean ±s.e.m. Note: the IRL and IRL-CTRL values are the mean of two trials performed on different days.

Figure 7

Ratings of perceived exertion (RPE) for limb discomfort (top panels) and dyspnoea (bottom panels) are shown for PAV versus CTRL (A and B) and IRL versus IRL-CTRL (C and D). n = 8 per group. Values are mean ±s.e.m.*P < 0.05; **P < 0.01, significantly different from corresponding time value. Note: the IRL and IRL-CTRL values are the mean of two trials performed on different days.