Influence of training on NIRS muscle oxygen saturation during submaximal exercise

Purpose: Endurance training improves the oxygen delivery and muscle metabolism. Muscle oxygen saturation measured by near infrared spectroscopy (IR-SO2), which is primarily influenced by the local delivery/demand balance, should thus be modified by training. We examined this effect by determining the influence of change in blood lactate and muscle capillary density with training oil IR-SO2 in seven healthy young subjects. Methods: Two submaximal exercise tests at 50% (Ex1) and 80% pretraining (V)over dotO(2max) (Ex2) were performed before and after a 4-wk endurance-training program. Results: (V)over dotO(2max) increased only slightly (+8%, NS) with training but the training effect was confirmed by the increased capillary density (+31%, P < 0.01) and citrate synthase activity (50%, P < 0.01), determined from muscle biopsy samples. Before training, blood lactate increased during the first 5 min of Ex1 and then remained constant (3.8 +/- 0.5 mmol(.)L(-1), P < 0.01), whereas it increased continuously during Ex2 (8.9 +/- 1.8 mmol(.)L(-1), P < 0.001). After training, lactate decreased significantly and remained constant during the two bouts of exercise (2.0 +/- 0.4 and 3.7 +/- 1.2 at the end of Ex1 and Ex2, respectively, both P < 0.001). During Ex1, IR-SO2 dropped initially at the onset of exercise and recovered progressively without reaching the testing level. Training did not change this pattern of IR-SO2. During Ex2, IR-SO2 decreased progressively during the 15 min of exercise (P < 0.05); IR-SO2 kept constant after the initial drop after training. We found a significant relationship (r = 0.42, P = 0.03) between blood lactate and IR-SO2 at the end of both bouts of exercise; this relationship was closer before training. By contrast, IR-SO2 or IR-BV was not related to the capillary density. Conclusion: The training-induced adaptation in blood lactate influences IR-SO2 during mild-to hard-intensity exercise. Thus, NIRS could be used as a noninvasive monitoring of training-induced adaptations.