Three weeks of respiratory muscle endurance training improve the O2 cost of walking and exercise tolerance in obese adolescents

Abstract

Obese adolescents (OB) have an increased O₂ cost of exercise, attributable in part to an increased O₂ cost of breathing. In a previous work a short (3-week) program of respiratory muscle endurance training (RMET) slightly reduced in OB the O₂ cost of high-intensity cycling and improved exercise tolerance. We hypothesized that during treadmill walking the effects of RMET would be more pronounced than those observed during cycling. Sixteen OB (age 16.0 ± 0.8 years; body mass [BM] 127.7 ± 14.2 kg; body mass index 40.7 ± 4.0 kg/m² ) underwent to 3-week RMET (n = 8) superimposed to a multidisciplinary BM reduction program, or (CTRL, n = 8) only to the latter. Heart rate (HR) and pulmonary O₂ uptake ( $\dot{V}$ O₂ ) were measured during incremental exercise and 12-min constant work rate (CWR) walking at 60% (moderate-intensity, MOD) and 120% (heavy-intensity, HEAVY) of the gas exchange threshold (GET). The O₂ cost of walking (aerobic energy expenditure per unit of covered distance) was calculated as $\dot{V}$ O₂ /velocity. BM decreased (~4-5 kg) both in CTRL and in RMET. $\dot{V}$ O₂ peak and GET were not affected by both interventions; the time to exhaustion increased following RMET. During MOD and HEAVY RMET decreased $\dot{V}$ O_2, the O₂ cost of walking (MOD: 0.130 ± 0.033 mL/kg/m [before] vs. 0.109 ± 0.027 [after], P = 0.03; HEAVY: 0.196 ± 0.031 [before] vs. 0.180 ± 0.025 [after], P = 0.02), HR and rates of perceived exertion; no significant changes were observed in CTRL. In OB a short RMET program lowered the O₂ cost of MOD and HEAVY walking and improved exercise tolerance. RMET could represent a useful adjunct in the control of obesity.

Keywords: O2 cost of breathing; O2 cost of walking; Obesity; respiratory muscle endurance training.

Figure 1

Mean (±SD) values of the O₂ cost of walking (oxidative energy expenditure per unit of covered distance, calculated as Δ$\dot{V}$O₂ per velocity) during the last minute of CWR exercise at ~60% of GET (moderate‐intensity, left panels) and at ~120% of GET (heavy‐intensity, right panels), before and after CTRL and RMET. In the upper panels the O₂ cost of walking is expressed as mL O₂/m, whereas in the lower panels the variable is normalized per unit of BM (mL O₂/kg/m). Dashed horizontal lines are reference values from the literature (Margaria et al. 1963; di Prampero 1986; Ekelund et al. 2004) for a man with a BM of 75 kg. RMET reduced significantly the O₂ cost of walking. See text for further details. *P < 0.05. Bonferroni post‐hoc tests to locate the statistically significant difference (after vs. before RMET). BM, body mass; CWR, constant work rate; GET, gas exchange threshold; RMET, respiratory muscle endurance training.

Figure 2

Mean (±SD) $\dot{V}$O₂ values calculated every 2 min, from the third to the last minute of CWR walking at ~60% of GET (moderate‐intensity, upper panels) and at ~120% of GET (heavy‐intensity, lower panels), before and after RMET and CTRL. During CWR <GET mean values of the individual slopes of the linear regressions of $\dot{V}$O₂ versus time were not significantly different from zero, before and after both interventions. At all‐time points $\dot{V}$O₂ values were significantly lower after versus before RMET, whereas no statistically significant differences were observed after versus before CTRL. During CWR >GET the slopes of the linear regressions of $\dot{V}$O₂ versus time were significantly greater than zero before both RMET and CTRL. The mean values of the individual slopes were significantly lower after versus before RMET, but not after versus before CTRL. See text for further details. *P < 0.05. CWR, constant work rate; GET, gas exchange threshold; RMET, respiratory muscle endurance training.

Figure 3

Mean (±SD) HR values calculated every 2 min, from the third to the last minute of CWR walking at ~60% of GET (moderate‐intensity, upper panels) and at ~120% of GET (heavy‐intensity, lower panels), before and after RMET and CTRL. During CWR <GET (upper panels) the mean values of the individual slopes of the linear regressions of HR versus time were not different from zero, before and after both interventions. At all time‐points values after RMET were significantly lower than those before RMET; no significant differences were observed in after versus before CTRL. During CWR >GET (lower panels) the slopes of the linear regressions of HR versus time were significantly greater than zero before both RMET and CTRL. The mean values of the individual slopes were significantly lower after versus before RMET, but not after versus before CTRL. See text for further details. *P < 0.05. CWR, constant work rate; GET, gas exchange threshold; RMET, respiratory muscle endurance training.

Figure 4

Pattern of breathing in two group of subjects during the incremental test. The relationships between mean values of pulmonary ventilation ($\dot{V}$ _E) and tidal volume (V_T) for RMET (left panel) and CTRL (right panel) are presented, before and after the interventions. Iso‐respiratory frequency (fR) lines (dashed lines, departing from the origin) are also presented. The exponential functions fitting the experimental points are shown. See text for further details. RMET, respiratory muscle endurance training.