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Page 4 of 26 Chang et al. Soft Sci. 2026, 6, 29

between muscle injury and fatigue . Although pronounced or prolonged fatigue may increase vulnerability
[24]
to mechanical injury, fatigue and injury remain biologically distinct states within a continuous spectrum of
muscle health: one predominantly functional and reversible, the other structural and often persistent .
[24]

The healing process of muscle fatigue and injury follows a clear physiological mechanism: the body first
eliminates damaged tissue fragments through inflammatory responses , then activates muscle satellite cells
[27]
to initiate muscle fiber regeneration, and precisely regulates cell proliferation and differentiation via
cytokines . During this process, energy reserves must be restored and the extracellular matrix reshaped,
[28]
ultimately achieving comprehensive repair of muscle structural integrity and contraction function .
[29]
Although within a certain range, muscles have the ability to self-repair after injury, this repair process is
regulated by multi-dimensional factors, and the precise regulation of molecular pathways is the core link that
supports these theories and affects the repair efficiency .
[4]

Related physiological indicators
Bioelectrical indicators
During the process of muscle fatigue and injury, sEMG is one of the most relevant and sensitive bioelectric
indicators reflecting the functional state of muscles. Its time-domain and frequency-domain characteristics
showcase regular changes as the injury progresses . In the early stage, time-domain characteristics are
[30]
mainly reflected as a significant increase in signal amplitude. As fatigue or injury intensifies, an increased
number of motor units are recruited to maintain motor function, leading to more muscle fibers contributing
to contraction and thus an overall increase in sEMG amplitude . During the process, changes in core
[31]
time-domain parameters such as integrated EMG (iEMG) and root mean square (RMS) are particularly
significant. Both of them show a continuous increasing trend with the aggravation of fatigue or injury and
can be used to quantify the degree of muscle fatigue or injury .
[32]

As for frequency-domain variations, under the state of muscle fatigue, the conduction of muscle fibers slows
down, resulting in a reduced propagation rate of action potentials among muscle fibers, which can be
witnessed as the frequency components of sEMG signals shift towards the low-frequency range . This
[33]
change can be intuitively reflected by frequency domain parameters such as median frequency (MF) and
mean power frequency (MPF). During the progression of fatigue or injury, both MF and MPF gradually
decrease . While once entering the muscle repair and remodeling period, the characteristic parameters of
[33]
the sEMG signal gradually return to the normal physiological state. Specifically, the total discharge and
discharge intensity of the motor unit tend to stabilize, and the time domain (iEMG and RMS) and frequency
domain (MF and MPF) features gradually recover .
[34]
To further enhance the accuracy of muscle state detection, recent studies have focused on fusion detection of
sEMG and ECG. It has been confirmed that the fatigue recognition model constructed based on the fusion
features of sEMG-ECG can achieve an average recognition rate of 87.83% . Compared with sEMG-only
[35]
detection, this fusion strategy not only significantly improves the accuracy of fatigue state recognition, but
also optimizes the stability and generalization ability of the model, providing more reliable technical support
for the early warning and precise assessment of muscle fatigue or injury [35-38] .

Biochemical indicators
During the process of muscle fatigue, injury and subsequent healing, biochemical indicators related to energy
metabolism, injury and inflammatory response exhibit regular dynamic changes, which can be used as core
biological markers for evaluating injury degree and healing status. The following section systematically
classifies related biochemical markers into three major categories, namely energy metabolism indicators,
muscle injury markers and inflammatory regulation factors. And the changing pattern and physiological