Takuya Osawa

Attenuation of Muscle Deoxygenation Precedes EMG Threshold in Normoxia and Hypoxia

PURPOSE The aim of this study was to investigate the relationship between the attenuation point of muscle deoxygenation (APMD) and the EMG threshold (EMGT) during incremental cycling exercise under different fractions of inspired O2 (FIO2). METHODS Nine male subjects performed ramp cycling exercise tests (20 W·min(-1)) to exhaustion under normoxic and hypoxic conditions (FIO2 = 0.12). Pulmonary O2 uptake (VO2), muscle deoxygenation, and EMG activity in the vastus lateralis muscle were simultaneously measured during the tests, and both APMD and EMGT were calculated. RESULTS Hypoxia significantly reduced peak VO2 (VO2peak). At the same absolute exercise intensity and at VO2peak, muscle deoxygenation, but not EMG activity, was significantly greater in hypoxia. VO2 at APMD was significantly decreased in hypoxia (P < 0.01). Similarly, VO2 at EMGT was significantly lower in hypoxia than in normoxia (P < 0.01). In addition, VO2 was lower at APMD than at EMGT under both conditions (P < 0.01). However, the relationships between APMD and EMGT were significant under both normoxic (r = 0.95, P < 0.01) and hypoxic (r = 0.89, P < 0.01) conditions. CONCLUSIONS These results suggest that the attenuation of muscle deoxygenation near VO2peak is related to and precedes changes in neuromuscular activity under normoxic and hypoxic conditions.

Unchanged Muscle Deoxygenation Heterogeneity During Bicycle Exercise After 6 Weeks of Endurance Training

The purpose of this study was to examine the changes in muscle oxygen saturation (SmO(2)) level and its heterogeneity after 6 weeks of endurance training using multi-channel near infrared spatially resolved spectroscopy (NIR(SRS)). Nine healthy subjects participated in this study (Male = 6, Female = 3, age: 27 +/- 5 years, height: 168.7 +/- 7.4 cm, weight: 62.4 +/- 12.4 kg). The subjects performed a 30 W ramp incremental bicycle exercise test until exhaustion before and after endurance training. The NIR(SRS) probe was attached to the left vastus lateralis muscle along the direction of the long axis. The subjects performed bicycle exercise for 30 min/day, 3 days/week for 6 weeks. The work rate during training was set at 60%V(O)(2peak) and increased every 5%V(O)(2peak) when the subjects could maintain the work rate three times consecutively. After training, V(O)(2peak) was significantly increased (Pre: 42.7 +/- 9.9 ml/kg/min, Post: 52.3 +/- 7.2 ml/kg/min, p < 0.001) and the mean SmO(2) within measurement sites at was significantly decreased (Pre: 56.1 +/- 1.1 %, Post: 53.3 +/- 2.2 %, p < 0.05). Conversely, the heterogeneity of the SmO(2) during exercise was not changed by training. These results suggest that the functional heterogeneity of O(2) balance did not change due to endurance training, and the O(2) balance heterogeneity may not interfere with O(2) exchange in the activating muscle in healthy individuals.

Which Is the best indicator of muscle oxygen extraction during exercise using NIRS?: Evidence that HHb is not the candidate

Recently, deoxygenated hemoglobin (HHb) has been used as one of the most popular indicators of muscle O2 extraction during exercise in the field of exercise physiology. However, HHb may not sufficiently represent muscle O2 extraction, as total hemoglobin (tHb) is not stable during exercise. The purpose of this study was to measure various muscle oxygenation signals during cycle exercise and clarify which is the best indicator of muscle O2 extraction during exercise using NIRS. Ten healthy men performed 6-min cycle exercise at both moderate and heavy work rates. Oxygenated hemoglobin (O2-Hb), HHb, tHb, and muscle tissue oxygen saturation (SmO2) were measured with near-infrared spatial resolved spectroscopy from the vastus lateralis muscle. Skin blood flow (sBF) was also monitored at a site close to the NIRS probe. During moderate exercise, tHb, O2-Hb, and SmO2 displayed progressive increases until the end of exercise. In contrast, HHb remained stable during moderate work rate. sBF remained stable during moderate exercise but showed a progressive decrease at heavy work rate. These results provide evidence that HHb may not sufficiently represent muscle O2 extraction since tHb is not stable during exercise and HHb is insensitive to exercise-induced hyperaemia.

Effects of Acute Hypoxia on the Inflection Point of Muscle Oxygenation

The purpose of this study was to investigate the effects of acute hypoxia on the inflection point of muscle oxygenation (IPMO) using near-infrared spectroscopy during incremental bicycle exercise. Eight male subjects (age: 22 +/- 2 years) performed two ramp incremental bicycle exercise tests until exhaustion under each normoxic and hypoxic (F(I)O(2): 0.12) condition. Pulmonary gas exchange and locomotor muscle oxygenation were continuously measured. IPMO was determined as the attenuated point of muscle deoxygenation. Muscle oxygenation level was significantly lower in hypoxia than normoxia throughout the tests. IPMO was found in both conditions, and V(O)(2) at IPMO was significantly decreased in hypoxia. However the percent rate of V(O)(2peak) between normoxia and hypoxia was not significantly different. These results suggest that IPMO was not associated with absolute exercise intensity, but relative exercise intensity.

Comparison of Muscle O2 Dynamics at Different Sites of the Forearm Flexor Muscles During Incremental Handgrip Exercise

This study investigated heterogeneity of muscle O(2) consumption (diffusive m-VO(2)) and muscle oxygenation difference (m-O(2) difference) within the forearm flexor muscles using multi-optical fibers near-infrared continuous wave spectroscopy (NIRcws) during incremental exercise. Nine healthy male subjects performed incremental dynamic handgrip exercise until exhaustion. The workload was increased by 5% maximal voluntary contraction (MVC) every 1 min, starting at 10% MVC. The NIRcws probes (10 channels) were placed on the right forearm flexor muscles to monitor muscle oxygenation. The diffusive m-VO(2) and the m-O(2) difference were evaluated at each exercise stage. The diffusive m-VO(2) at the medial site was significantly greater than the lateral site at 25% MVC (p < 0.05). Similarly, m-O(2) difference at the medial site increased significantly over the lateral site (p < 0.05), whereas there were no significant differences in diffusive m-VO(2) or m-O(2) difference between the proximal and distal sites. These results in forearm muscle were different from the previous study which found that there were longitudinal differences in muscle VO(2) in the femoral muscle.

Do Two Tissue Blood Volume Parameters Measured by Different Near-Infrared Spectroscopy Methods Show the Same Dynamics During Incremental Running?

UNLABELLED Both the change in total hemoglobin concentration (cHb), assessed by near-infrared continuous-wave spectroscopy (NIR-CWS), and the normalized tissue hemoglobin index (nTHI), assessed by NIR spatially resolved spectroscopy (NIR-SRS), were used to quantify changes in tissue blood volume. However, it is possible that these parameters may show different changes because of the different measurement systems. The present study aimed to compare changes in cHb and nTHI in working muscles, which were selected for measurement because the parameters changed dynamically. METHODS After a standing rest, seven male runners (age 24±3 years, mean±S.D.) performed an incremental running exercise test on a treadmill (inclination=1%) from 180 to 300 m min(-1). During the tests, cHb and nTHI were monitored from the vastus lateralis (VL) and medial gastrocnemius (GM) muscles. These parameters were relatively evaluated from the minimal to maximal values through the test. RESULTS When the exercise began, cHb and nTHI quickly decreased and then gradually increased during running. In comparison with both VL and GM, there was significant interaction between cHb and nTHI. CONCLUSIONS The present results suggest that cHb and nTHI in working muscles are not always synchronized, particularly at the onset of exercise and at high intensities. Although cHb was previously used as the change of tissue blood volume, it is implied that tissue blood volume assessed by cHb is overestimated.

O2 Saturation in the Intercostal Space During Moderate and Heavy Constant-Load Exercise

To examine the hypothesis that the relationship between minute ventilation (VE) and deoxygenation from the intercostal space (IC) would be steady regardless of exercise protocols, if an increase in O2 consumption of the accessory respiratory muscles with an increase of VE brings about deoxygenation in IC, we measured the relationship between VE and O2 saturation in IC (SO2IC) during a constant-load exercise test (CET), and the relationship was compared with that during a ramp incremental exercise test (RIET). Six male subjects performed RIET. On a different day, the subjects performed a moderate and heavy CET (CET_MOD and CET_HVY, respectively). SO2IC decreased from the start of both CET_MOD and CET_HVY and changed little from 2 min. Moreover, SO2IC was significantly lower during CET_HVY than during CET_MOD. In comparison between RIET and CET_HVY at the similar VE level, SO2IC was significantly higher during CET_HVY than RIET. These results suggest that the decrease in SO2IC was caused not only by an increase in O2 consumption in IC with an increase in VE but also by a decrease in O2 supply.