Human Inspiratory Muscle Fatigue during Strenuous Resistive Loads under Normoxic and Hypoxic Conditions

Abstract

A study involving nine healthy males was conducted to determine the developmental rate of inspiratory muscle fatigue during resistive loads under normoxic and hypoxic conditions. The subjects aged 19 to 38 years performed a bicycle exercise test of increasing power during an exhaustive inspiratory–expiratory resistive load of 40 cm water column/l s–1 inhaling air or oxygen. The volumetric and temporal respiratory parameters, the partial CO2 pressure in the alveolar gas, the total force of inspiratory muscle contractions, the electrical activity of parasternal muscles (EMG), and the initial inspiratory activity were recorded. The degree of inspiratory muscle fatigue was assessed by the “tension–time” index Pm /Pm maxTI /TT as well as by the ratio of the mean amplitudes of the EMG-signal spectrum in the high-frequency (H) range to the mean spectrum amplitudes in the low-frequency (L) range (H/L). It was established that human working capacity during increasing muscular loads against the background of highly resistive breathing was not a function of the oxygen content in the inhalation mixtures within 21 to 100%; i.e., the maximal power of the work done did not significantly differ. It was shown that pulmonary ventilation, the force generated by inspiratory muscles, the breathing effort values, the initial inspiratory activity value, and the “tension–time” index increased in parallel with the intensity of the exercise when both air and oxygen were inhaled. The Pm /Pm maxTI /TT values in the last minutes of the muscular load significantly exceeded the fatigue zone range, attaining 0.25 to 0.45 in different subjects. By the moment of refusal to continue the work, the H/L ratio had decreased by an average of 36% when air and oxygen were inhaled. The limitation of the physical working capacity of a healthy individual during an exhaustive resistive load is considered to be linked to inspiratory muscle fatigue developing at an equal rate under both normoxic and hyperoxic conditions. An impairment of the arterial blood supply to inspiratory muscles due to occlusion of the intramuscular vessels during intense muscular contractions and a considerable shortening of the time of the relaxation of the respiratory muscles in the expiratory phase is supposed to occur in forced respiration with an additional gas flow obstruction. The fact of the alternation of the electrical activity between the parasternal and other inspiratory muscles within individual inspiratory phases combined with resistive and physical loads is established, which may serve as one of the signs of inspiratory muscle fatigue.