## 3 Results
Results of a previous study showed that the oscillation magnitude in the \(\Delta\)tHb signal caused by repeated contractions decreased as the exercise continued [13]. However, no study has extracted the specific fluctuations related to muscle contraction from the original hemodynamic signal. The present study reconstructed the iTOI and itHb signals, which presented components mainly induced by muscle contractions, by assigning an exercise-specific frequency range to the inverse wavelet transform. This novel method could exclude irrelevant interferences from the hemodynamic signals and provide a clearer understanding between muscle fatigue development and hemodynamic responses. It has been widely reported that the tissue oxygen saturation will sustain a relative higher level after exercise [11]. Our study also demonstrated that the higher oxygen supply might last for at least 30 min after exercise and may advance the recovery of muscle fibers. Spontaneous oscillations in blood flow reflect the effects of both vasomotion and flow motion and play a key role in the automatic regulation of the circulatory system, especially in the peripheral tissues [35]. Numerous studies have demonstrated that spontaneous fluctuations in hemodynamic signal can be separated into six frequency components, with each component corresponding to one physiological origin: I: 0.6-2 Hz, cardiac activity; II: 0.145-0.6 Hz, respiration; III: 0.052-0.145 Hz, myogenic activity; IV: 0.021-0.052 Hz, neurogenic activity; V: 0.0095-0.021 Hz, NO-related endothelial metabolic activity and VI: 0.005-0.0095 Hz, endothelial activity [25; 26]. This method has been used to investigate lumbar muscle fatigue under whole-body vibration with different vibration frequencies. Results showed that vibration at 4.5 Hz could induce a significant decrease in the wavelet amplitude in frequency intervals I, II and III, which might indicate a reduced efficiency of oxygen supply and contribute to muscle fatigue [21]. The periodic oscillation in frequency intervals I and II reflect the effects of heart beat and respiration on blood flow, which are global influences [25; 30]. The pressure difference generated by cardiac activity and respiration between arteries and veins is the fundamental source of the blood flow in the circulatory system [35]. It has been reported that the heart rate and respiration would increase significantly during exercise and recovery periods, which contribute to blood flow and oxygen supply to muscle tissues [36; 37]. Increased energy contribution in frequency intervals I and II in post-exercise recovery indicates higher activity intensities in the heart and lungs as a natural reaction to exercise. Enhanced cardiac and respiratory activities have also been reported in a previous study which analyzed cutaneous blood flow after exercise in the frequency domain [30]. Since the cardiac and respiratory responses have global influence on blood flow oscillations, it is reasonable that this phenomenon is found in both skin and muscle tissues. Increases in heart rate and cardiac contractility, as well as rate and depth of respiration, are also responsible for the higher tissue oxygen saturation, which can accelerate the recovery of muscle from fatigue [35]. The fluctuation in frequency interval III (0.052-0.145 Hz) originates from intrinsic myogenic activity of vascular smooth muscles (VSM) [26; 38]. Myogenic activity refers to automatic contraction or relaxation of VSM in response to the increased or decreased transmural pressure (the difference between intravascular and extravascular pressure) [39; 40]. This mechanism provides a background vasomotor tone against which vasodilators and vasoconstretrics can work to change vessel caliber in response to the increased oxygen consumption associated with exercise [39]. The increase in the normalized spectral amplitude in this interval suggests greater contribution of myogenic activity to the local blood circulation after exercise.