Consequences of exercise-induced respiratory muscle work☆
Section snippets
Hypothesis
Our hypothesis is shown in the flow chart in Fig. 1. It explains our view that the demand placed on the respiratory muscles because of the ventilation required by heavy intensity, sustained exercise in healthy subjects will contribute significantly to exercise performance limitations through its effects on limb vascular conductances and blood flow and locomotor muscle fatigue.
Near-ideal characteristics/adaptations of the respiratory muscles
We start with the important qualification that the diaphragm and to a lesser extent the expiratory abdominal muscles possess unique characteristics that make them the most fatigue-resistant of all the skeletal muscles. The diaphragm has a very high aerobic enzymatic capacity, multiple sources of blood supply, a unique resistance to vasoconstrictor influences on vascular diameter and therefore a substantial fatigue resistance. Furthermore, with progressively increasing exercise expiratory muscle
Exercise-induced diaphragmatic fatigue
Despite the substantial aerobic capacity of the diaphragm this muscle will show significant fatigue during exercise sustained to exhaustion at intensities greater than 80% of . This has been documented using the bilateral phrenic nerve electrical stimulation technique (1–20 Hz). The trans-diaphragmatic pressure evoked by supra-maximal stimulation immediately following exercise is reduced 25–50% below pre-exercise baseline and takes about 2 h to recover (Johnson et al., 1993). This
Exercise-induced diaphragmatic fatigue: cardiovascular effects
In prolonged high intensity exercise, hyperventilation usually prevails throughout, demonstrating that fatigue of the diaphragm does not commonly impede the ventilatory response. Indirect evidence from pressure recordings suggests that the fatiguing diaphragm actually reduces its output as a force generator during the latter stages of sustained endurance exercise and accessory inspiratory and expiratory muscles become the dominant effectors of a time-dependant hyperventilation (Johnson et al.,
Exercise-induced diaphragm fatigue: effects on endurance performance
Time to exhaustion during cycling at high intensity was determined by having subjects cycle at a fixed work rate to exhaustion with randomized trials of three types, repeated three times each; control, inspiratory muscle loading (via inspiratory resistors) and respiratory muscle unloading (via PAV). Loading decreased and unloading increased time to exhaustion, averaging ±14–15% (Harms et al., 2000). These alterations in exercise performance occurred coincidentally with increases or decreases in
Exercise-induced diaphragmatic fatigue: effects on locomotor muscle fatigue
We quantified the effects of inspiratory muscle work on exercise-induced quadriceps muscle fatigue using supra-maximal stimulation of the femoral nerve with a double twitch stimulation technique (Yan et al., 1993, Polkey et al., 1996). Variations in the inter-stimulus interval created force: frequency curves for the quadriceps muscle. Within-subject random variability of twitch force, obtained upon repeat testing both within and between days under resting conditions, was less than 6% (Romer et
Conditions under which the effects of respiratory muscle work on limb flow and fatigue might be significant
We would only expect these effects of respiratory muscle work on limb blood flow and fatigue to occur during sustained heavy intensity exercise with high ventilatory requirements, sufficient to elicit significant levels of fatigue in the diaphragm and/or accessory respiratory muscles. Environmental hypoxia and especially chronic hypoxic exposure potentiates the hyperventilatory response to exercise and markedly increases ventilatory work (Thoden et al., 1969). Thus, we would expect respiratory
Fatigue: “central/peripheral”?
In summary, relieving much of the respiratory muscle work had a positive effect on endurance exercise performance coincident with a significant increase in muscle blood flow and oxygen transport and a reduction in locomotor muscle fatigue and reduced perceptions of effort—both dyspnea and limb discomfort. We interpret these data to mean that peripheral muscle fatigue played a pivotal role in determining the observed increase in exercise performance both though its direct effect on limiting
Several important unknowns remain!
While we believe the essential features of our original hypothesis (see Fig. 1, Fig. 6) are supported by the available evidence, there is much left unexplained in this proposed respiratory: cardiovascular linkage leading to limb fatigue with a significant impairment to exercise performance.
- •
Exactly how do respiratory and limb locomotor muscle vasculatures compete for the available cardiac output during heavy exercise intensity? When respiratory muscle work is increased and limb flow reduced, we
Acknowledgements
The original work cited in this review was supported by NHLBI and the AHA. We are indebted to Elizabeth Aaron, Bruce Johnson, Mark Babcock, Craig Harms, Claudette St. Croix, Barbra Morgan, William Sheel and Alex Derchak who each contributed significantly to this series of original studies.
References (35)
Discharge of group IV phrenic afferent fibers increases during diaphragmatic fatigue.
Brain Res.
(2000)- et al.
Quadriceps fatigue after cycle exercise in patients with COPD compared with healthy control subjects
Chest
(2003) - et al.
Diaphragm arterioles are less responsive to alpha1-adrenergic constriction than gastrocnemius arterioles
J. Appl. Physiol.
(2002) - et al.
Differential adenosine sensitivity of diaphragm and skeletal muscle arterioles
J. Appl. Physiol.
(2002) - et al.
Oxygen cost of exercise hyperpnea: implications for performance
J. Appl. Physiol.
(1992) - et al.
Effects of respiratory muscle unloading on exercise-induced diaphragm fatigue
J. Appl. Physiol.
(2002) - et al.
Contribution of diaphragmatic power output to exercise-induced diaphragm fatigue
J. Appl. Physiol.
(1995) - et al.
Effects of expiratory muscle work on muscle sympathetic nerve activity
J. Appl. Physiol.
(2002) Spinal and supraspinal factors in human muscle fatigue
Physiol. Rev.
(2001)- et al.
Respiratory muscle work compromises leg blood flow during maximal exercise
J. Appl. Physiol.
(1997)
Effects of respiratory muscle work on cardiac output and its distribution during maximal exercise
J. Appl. Physiol.
Effects of respiratory muscle work on exercise performance
J. Appl. Physiol.
Exercise-induced diaphragmatic fatigue in healthy humans
J. Physiol.
Mechanical constraints on exercise hyperpnea in endurance athletes
J. Appl. Physiol.
Exercise starts and ends in the brain
Eur. J. Appl. Physiol.
Blood flow to the respiratory and limb muscles and to abdominal organs during maximal exertion in ponies
J. Physiol.
Both central command and exercise pressor reflex reset carotid sinus baroreflex
Am. J. Physiol. Heart Circ. Physiol.
Cited by (278)
Relationship between disturbances of CO<inf>2</inf> homeostasis and force output characteristics during isometric knee extension
2023, Respiratory Physiology and NeurobiologyAcute psychophysiological responses during exercise while using resistive respiratory devices: A systematic review.
2022, Physiology and BehaviorCitation Excerpt :The underlying mechanisms postulated to explain improved exercise performance after a period of RMT are varied. Firstly, respiratory muscle work at maximal intensities requires an average of the 14 – 21% of the total cardiac output [12,13], so the respiratory muscle fatigue has been identified as a limiting factor in performance due to competition between respiratory muscles and exercising muscles for blood flow [13,14] at intensities greater than 80% of VO2max [15]. This so-called metaboreflex is initiated by fatigue of the respiratory muscles.
Load carriage physiology in normoxia and hypoxia
2024, European Journal of Applied PhysiologyThe Effect of Body Armor on Pulmonary Function Using Plethysmography
2024, Military Medicine
- ☆
This paper is part of a special issue entitled “ New Directions in Exercise Physiology,” Guest Edited by Susan R. Hopkins and Peter D. Wagner.