## 4 Discussion
Muscle contraction type and exercise intensity affected the oxygen concentration of muscle tissue and blood volume responses. A previous study revealed that regarding isometric contraction, whether the contraction was 20%, 40%, 60% or 80% of MVC, tissue oxygenation and blood volume decreased with contraction time and intensity increases [31]. During the dynamic exercise mode test, oxygen saturation decreased as action cadence increased when participants performed dynamic knee extension exercises with added weights of 2.4 kg. Blood volume was higher at knee extension frequencies of 30 and 50 bpm than blood volume before exercise, but blood volume was lower than before exercise at action frequencies of \(>\)50 bpm [32]. That previous study showed that the oxygen saturation of muscle tissue decreased with an increase in treadmill load, but when speed was increased and load decreased, the oxygen saturation in the muscles increased [33]. According to the aforementioned research, exercise load has marked effects on the oxygen saturation of muscle tissue. Oxygen consumption increases as exercise load increases, and oxygen consumption decreases as exercise load decreases. If exercise load is reduced, blood volume increases even though speed increases, but blood volume decreases if the load is increased alongside a speed increase. In the present study, the pull force during rowing increased with stroke rate (cadence). Therefore, in the present study, the increases in tissue oxygenation and blood volume of the erector spinae during exercise were lower during fast rowing than that during slow rowing because rowing load increased with speed (cadence). Another finding in the present study was the critical influence of cadence on HR and RPE responses to exercise. HR and RPE increased as the rowing cadence increased, which meant that a stronger cardiopulmonary stimulation occurred. To our knowledge, only one study investigated HR responses to rowing on an unstable surface [34]. That study compared the 2000 m rowing performance and HRs measured by a traditional stable ergometer, a transversally compliant ergometer, and a frontally compliant ergometer (FCE) attached to a tilt-board underneath. The study indicated that the FCE resulted in a higher HR response and poorer participant performance in 2000 m rowing racing, and this was potentially due to issues with balance maintenance [34]. In the present study, HR increased by approximately 30 bpm during fast rowing, and the average intensity of exercise (65-67% HRmax) was moderate (64-76% HRmax), as classified by the ACSM [35]. Because rowing exercises involve approximately 70% of the body's muscles [19], the fast rowing demonstrated in the present study can be a favorable means of performing cardiopulmonary exercise when paired with the movement of the whole body's large muscle groups. However, the different seat cushion pressure levels in the present study had no effect on HR and RPE, which suggested that the difficulty related to the use of unstable surfaces did not change the exercise intensity. In the present study, increases in O2 Hb in the recovery period under 80 mmHg during fast rowing were higher than those under 140 mmHg at the same cadence (Figure 3). The increases in O2 Hb during the recovery period were calculated through the subtraction of O2 Hb before exercise, which reflects increases in O2 Hb during the recovery period compared with the rest period. We determined that fast rowing could prevent large increases in tissue oxygenation and blood volume of the erector spinae. Moreover, sitting on cushions with pressure 140 mmHg increased instability and RPE; these increases were obstructive to increases in tissue oxygenation and blood volume of the erector spinae. Therefore, increases in O2 Hb and tHb in the recovery period under 140 mmHg during fast rowing were significantly lower than under 80 mmHg with fast rowing.