The role of inspiratory muscle fatigue and its impact on strength and respiratory function
Authors: Ellis Melissa , Faghy Mark 
Published: July 2024

Inspiratory muscles are essential for breathing and during exercise. However, excessive effort on the respiratory system can cause the inspiratory muscles, such as the external intercostals and the diaphragm, to become fatigued, therefore impairing performance (Wuthrich et al., 2013, Exp Physiol, 12, 1705–1717). Inspiratory muscle fatigue is when there is a loss in the capacity to produce force and/or velocity in a muscle, meaning they become tired and weak due to increased work demands (Tsukamoto et al., 2019, J Phys Ther Sci, 31, 318–325). Inspiratory muscle fatigue impacts performance because the force produced by the inspiratory muscles starts to decline which therefore causes them to fatigue as they cannot work to the maximum capacity no longer. This is vital in aerobic exercises such as rowing, cycling and running as the demand for oxygen is higher to perform the tasks. The diaphragm is the most dominant respiratory muscle involved in inspiration, alongside the external intercostal muscles. During inspiration, the diaphragm contracts and flattens, and the external intercostal muscles contract, pulling the ribcage up and outwards. The main function of these muscles is to allow a maximum amount of oxygen into the lungs by increasing the surface area in the thoracic cavity (Siafakas et al., 1999, Thorax, 54, 458–465). Several studies have explored delaying muscle fatigue by using inspiratory muscle training in healthy individuals. This is because it improves the strength of the respiratory muscles by increasing the endurance and capacity of the diaphragm. Results highlighted that maximum inspiratory pressure increased between 16 and 18% (Segizbaeva et al., 2014, Hum Physiol, 40, 683-689), a 7% increase in inspiratory muscle strength (Smith et al., 2021, Am J Physiol, 312, 1013-1018), a delay in muscle fatigue, and an improvement in performance. There is limited research on cycling, rowing, and running; therefore, this study aims to determine whether any of these exercises delay or increase muscle fatigue and to determine if maximal inspiratory pressure and maximal expiratory pressure improve respiratory function. This study aims to recruit 9–12 participants, which will involve four visits to the laboratory. Exercises include a 10-kilometre cycling session, a 3-kilometre treadmill running session, and a 2-kilometre rowing session, and these will be done in a randomised order. Visit 1 includes familiarisation; visit 2 involves baseline measures being recorded and test 1 being conducted; visit 3 includes test 2 being administered; and visit 4 involves test 3 being conducted. The measurements that will be collected at each kilometre are heart rate, rate of perceived exertion, and dyspnea. Blood lactate, maximal inspiratory pressure, and maximum expiratory pressure, however, will be collected before and after each test. At this present time, results are pending. My hypothesis for this study would be that all the cardio-based exercises will see an improvement in respiratory function and strength, with treadmill running having the biggest improvement as, throughout the 4 weeks, adaptation would have occurred.