The results of our study suggest a decrease in inspiratory muscle strength, as well as a decline in functional capacity and muscle oxygen saturation when isolated inspiratory muscle fatigue occurs in older subjects. Furthermore, both inspiratory strength and functionality appear to improve with inspiratory muscle activation.
The aging process has negative effects on respiratory musculature, especially concerning the inspiratory musculature, particularly the diaphragm. Factors such as hyperinflation due to increased residual volume, increased dorsal kyphosis, or the greater rigidity of the thoracic cage induce a loss of strength in the diaphragm of around 30% in older subjects compared to younger individuals, also becoming a marker of sarcopenia (23). Additionally, greater diaphragmatic thickening has been demonstrated in older adults, likely due to the increased need for lumbar stabilization in this population, as well as greater flattening and decreased muscle contraction velocity (3).
Inspiratory muscle fatigue involves the inability of the inspiratory muscles to overcome a given pleural pressure and is considered a limiting factor in both sports performance and exercise tolerance. Such respiratory muscle fatigue induces the appearance of the so-called respiratory metaboreflex or metaboreflex, whereby, through an afferent stimulus from type III and IV fibers, it reaches the supraspinal level, producing reflex vasoconstriction of active peripheral musculature, increasing perceived effort and decreasing performance (7) which is further intensified, with a significant increase in the vasopressor response as subject age increases (12). Various studies throughout current scientific literature have examined the effects of inducing fatigue on respiratory musculature. For example, in the study by Welch et al., it was demonstrated that inducing diaphragmatic fatigue in healthy subjects resulted in decreased exercise tolerance, as well as increased sensation of dyspnea and leg discomfort (24).
Ultrasound constitutes a non-invasive imaging technique that allows for objective evaluation of muscle structure and function, in our case, of respiratory musculature, specifically the diaphragm, and is also a reliable and reproducible assessment method (18). There is evidence regarding the different correlations between diaphragmatic thickness measurements made with the B-mode and diaphragmatic mobility measurements made using the M-mode. Regarding diaphragmatic thickness measurement, its correlation with inspiratory muscle strength and pulmonary function has been demonstrated, while diaphragmatic mobility correlates equally with inspiratory muscle strength, transdiaphragmatic pressure, and esophageal pressure, specifically through deep breathing and sniff maneuvers (25, 26). On the other hand, another determinant of diaphragmatic functionality, in addition to the force it can develop, is diaphragmatic contraction velocity, which is also not influenced by other variables such as thoracic stiffness or pulmonary distensibility, allowing for an accurate reflection of diaphragmatic contraction efficiency(27). Previous studies analyzing diaphragmatic dysfunction using ultrasound demonstrated a decrease in respiratory time, thickening fraction, strength, and diaphragmatic contraction velocity in subjects with sepsis-induced diaphragmatic dysfunction (28).
Muscle oxygen saturation measurements using infrared spectroscopy devices have proven to be an effective measurement method both during activity and at rest, whether for research or training purposes (29). The lower values of muscle oxygen saturation found in the fatigue group may be due to peripheral vasoconstriction and blood flow redistribution resulting from excessive inspiratory muscle work and the appearance of the metaboreflex, leading to reduced blood supply to the area and thus reduced oxygen delivery(30, 31), also occurring in the lower limbs at rest (32). It should be noted that in older adults, there is already dysfunction of the cardiovascular system, leading to reduced vasodilatory capacity during exercise, as well as slower oxygen uptake at the onset of exercise (33, 34). Our results align with another previous study by Dominelli et al., where subjects subjected to increased respiratory work showed increased blood flow in accessory respiratory musculature, as well as a decrease in locomotor musculature (35).
TUG is a commonly used test in clinical practice to assess functionality and fall risk in older adults (36). Additionally, it is widely validated and integrated into the management of respiratory pathology due to its ease of use and its ability to stratify frailty in COPD patients (37). On the other hand, FTSST is a widely validated test in older adults aimed at assessing lower limb strength, as well as postural control and balance (38). Currently, there are no studies investigating the relationship between inspiratory muscle fatigue and functionality, balance, and strength in older adults. Our results demonstrate a significant increase in the execution time of TUG and FTSST in the IMFG. We can hypothesize that, as a decrease in blood flow and oxygen supply to peripheral musculature induced by the metaboreflex occurs (30, 31), ischemia may lead to a loss of muscle contractile capacity and strength (33). This ischemia may present the same issue in respiratory musculature, considering that the diaphragm plays a fundamental role in lumbar stability through control and generation of intra-abdominal pressure (39).
Regarding the improvements obtained by the AG, several studies available in the current scientific literature evidence the various effects produced by activation of respiratory musculature. On one hand, improvements in MIP and various ultrasound variables may be due to improved neuromuscular function of the diaphragm and accessory inspiratory musculature (40). Additionally, moderate loads like those used in our study also allowed for a decrease in dyspnea sensation and increased oxygen supply to active muscles (41) thus improving exercise tolerance (42) with improvements also found in lung function (16).
Among the possible limitations of this study, we found that only pre and post measurements of the different variables were performed, not allowing for the objective assessment of how long the effects of the interventions may last. On the other hand, although other correlated measurements were employed, the application of specific measurements such as phrenic stimulation at the cervical level or transdiaphragmatic pressure could have provided greater objectivity to inspiratory muscle fatigue.