## 3 Results  
Figure 1G,H show the time series of iTOI and itHb reconstructed from the wavelet coefficients related to the frequency component induced by muscle contraction. The gray areas display the exercise phase. The reconstructed iTOI and itHb time series show different trends during the exercise: the oscillations in iTOI increases as the exercise continues while the magnitude of itHb decreases. We divided the whole exercise phase into two segments with the same length and compared the RMS values of the first and the second segment (Figure 3). Result shows that the median RMS value of the second half of iTOI is significantly larger than that of the first half (\(p<0.01\)), while itHb has an opposite trend--the median RMS of the second half is significantly smaller than the first half (\(p<0.05\)). The decreased itHb value demonstrates a lower contraction level of the GL muscle. Considering the decreasing trend of itHb, the increase of iTOI indicates an increased oxygen consumption which may be related to the development of muscle fatigue. Figure 4 shows an example of TOI signals recorded during the pre- and post-exercise phase, as well as the comparison of their mean values. During the pre-exercise phase, TOI remains relatively stable for the whole measurement period, while for the post-exercise phase TOI shows a decreasing trend. The mean TOI value is significantly higher (\(p<0.001\)) in the post-exercise recovery phase (\(65.04\%\pm 2.90\%\)) than that in the pre-exercise rest phase (\(62.35\%\pm 3.05\%\)). This result indicates that oxygen supply plays an important role in the recovery process of muscle after exercise. Figure 4: **(A)** Original TOI signals measured during the 30-min pre-exercise (gray line) and post-exercise (black line) rest phase. **(B)** Comparison of mean TOI values between pre-exercise (gray bar) and post-exercise (black bar) rest phase. Statistical significance: *** \(p<0.001\). Figure 3: Comparison of the root-mean-square (RMS) values between the first half and the second half of the exercise phase of the TOI (**A**) and \(\Delta\)AtHb **(B)** signal. Statistical significance: * \(p<0.05\), ** \(p<0.01\). The comparison of normalized wavelet energy between the pre- and post-exercise rest phases in six frequency intervals are presented in Figure 5 and Table 1. During the post-exercise phase, the normalized wavelet energy values in frequency intervals I (\(p<0.001\)), II (\(p<0.05\)), III (\(p<0.05\)) and IV (\(p<0.01\)) significantly increase by 43.4%, 23.6%, 18.4% and 21.6%, while the value in frequency interval VI (\(p<0.05\)) significantly decrease by 16.6%, compared to that during the pre-exercise phase. The normalized wavelet energy reflects the relative energy contribution of each corresponding physiological activity in the local tissue blood fluctuations. These changes demonstrate modulation of the circulation system in the muscle recovery after exercise. For the resting-state signals, we found that the TOI sustained a relatively high level for at least 30 min during the recovery period. Meanwhile, the relative energy contribution obtained from the \(\Delta\)HHb signal lifted significantly in frequency intervals I, II, III and IV after the exercise, but declined significantly in frequency VI. The \(\Delta\)tHb signal, which reflects blood volume changes in local tissue, is highly sensitive to muscle contraction during exercise [13]. The decrease in itHb oscillations may indicate a decline in muscle contraction force, due to the development of muscle fatigue. Hence, the increasing trend of iTOI fluctuations shows an imbalance between oxygen consumption and delivery. This higher oxygen demand may come from the shift from anaerobic type II fibers to aerobic type I muscle fibers. The imbalance between oxygen consumption and supply may contribute to the accumulation of metabolites such as lactate, hydrogen ions and phosphates in local muscle tissue and accelerate the process of peripheral muscle fatigue [3; 9].