Background and Purpose. The oxygen demand of breathing exercises and the clinical implications have not been studied in detail. In this study, the oxygen cost of 3 common breathing exercises believed to reduce oxygen cost (ie, work of breathing) was compared with that of spontaneous breathing in patients with chronic obstructive pulmonary disease (COPD). Subjects. Thirty subjects with stable, moderately severe COPD participated. Methods. Oxygen consumption (V o 2 ) and respiratory rate (RR) during spontaneous breathing at rest (SB) were recorded for 10 minutes. Subjects then performed 3 breathing exercises in random order, with a rest between exercises: diaphragmatic breathing (DB), pursed-lip breathing (PLB), and a combination of DB and PLB (CB). Oxygen consumption and RR were measured. Results. Mean V o 2 (ϮSD) was lower during the breathing exercises (165.8Ϯ22.3 mL O 2 /min for DB, 164.8Ϯ20.9 mL O 2 /min for PLB, and 167.7Ϯ20.7 mL O 2 /min for CB) compared with SB (174.5Ϯ25.2 mL O 2 /min). Correspondingly, mean RR (ϮSD) was higher during SB (17.3Ϯ4.23 breaths/min), followed by DB (15.0Ϯ4.32 breaths/min), PLB (12.8Ϯ3.53 breaths/min), and CB (11.2Ϯ2.7 breaths/min). Discussion and Conclusion. Given that patients do not spontaneously adopt the breathing pattern with the least V o 2 and the lowest RR, the results suggest that determinants of the breathing pattern other than metabolic demand warrant being a primary focus in patients with COPD. [Jones AYM, Dean E, Chow CCS. Comparison of the oxygen cost of breathing exercises and spontaneous breathing in patients with stable chronic obstructive pulmonary disease. Phys Ther. 2003;83:424 -431 Due to their increased ventilatory demands, people with chronic obstructive pulmonary disease (COPD) have a higher resting oxygen consumption (V o 2 ) than do people without pulmonary disease. 1,2 This higher resting V o 2 may be explained by increased mechanical work of breathing or reduced ventilatory muscle efficiency, or both, in patients with severe COPD. 3 Mechanical work in biological systems is achieved when a force applied to a structure results in movement. The amount of work produced is the product of the force applied and the distance moved. With respect to spontaneous ventilation, the mean pressure generated by the ventilatory muscles is equivalent to the force, and the tidal volume is equivalent to the distance. Ventilatory muscle work is primarily dependent on minute ventilation, airway resistance, and lung compliance, which, in turn, determine ventilatory muscle efficiency and the pattern of breathing. 4 Efficient ventilatory muscle function is equivalent to the attainment of the requisite minute ventilation with the least energy cost.In COPD, the loss of alveolar tethering and elastic recoil contributes to increased lung compliance and impaired lung perfusion. 5 Typically, these changes result from prolonged inhalation of cigarette smoke, which irritates the airways, thus increasing mucous production and airway resistance. Over time, these pathophysiologic chan...
The effects of muscle fatigue on the temporal neuromuscular control of the vastus medialis (VM) muscle were investigated in 19 young male subjects. The electromyogram (EMG) activities of VM and the force generation capacities of the quadriceps muscle were monitored before and after a fatigue protocol. In response to light signals, which were triggered randomly, the subjects made three maximal isometric knee extensions. This was then followed by the fatigue protocol which consisted of 30 isometric maximal voluntary contractions at a sequence of 5-s on and 5-s off. Immediately after the exercise to fatigue, the subjects performed another three maximal isometric contractions in response to the light signals. The effects of fatigue on the temporal neuromuscular control were then investigated by dividing the total reaction time (TRT) into premotor time (PMT) and electromechanical delay (EMD). The TRT was defined as the time interval between the light signal and the onset of the knee extension force. The PMT was defined as the time from the light signal to the onset of EMG activities of VM, and EMD as the time interval between onset of EMG activities to that of force generation. Following the contractions to fatigue there was a significant decrease in peak force (Fpeak, P = 0.016), an increase in the root mean square (rms)-EMG: Fpeak quotient (P = 0.001) but an insignificant change in the median frequency (P = 0.062) and rms-EMG (P = 0.119). Significant lengthening of mean EMD was found after the fatigue protocol [0.0396 (SD 0.009) vs. 0.0518 (SD 0.016) s P<0.001]. The lengthening of EMD in VM would affect the stabilizing effect of the patella during knee extension. The faster mean PMT [0.2445 (SD 0.093) vs. 0.2075 (SD 0.074) s, P = 0.042] following the fatigue protocol might have compensated for the lengthened EMD and contributed to the insignificant change in the mean TRT [0.284 (SD 0.09) vs. 0.259 (SD 0.073) s, P = 0.164]. This was probably related to the low level of fatigue (15% decrease in force) and the stereotyped nature of the action such that the effects of the fatigue on neuromuscular control were likely to have been attributable to peripheral processes.
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