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

study analyzing serum samples from 483 male and 245 female athletes established reference intervals for CK
activity of 82-1,083 U/L for males and 47-513 U/L for females . Levels exceeding these thresholds may
[50]
indicate an increased risk of overexertion or injury. Further, a CK level greater than 2,000 U/L two hours
post-exercise is widely used to diagnose pathological conditions such as myopathy (e.g., statin-induced
myositis) or rhabdomyolysis . As muscle fibers regenerate and the integrity of muscle cell membrane
[51]
structure recovers, the release of CK and Mb significantly decreases . Meanwhile, the body metabolizes and
[26]
clears excessive CK and Mb in the blood through liver and kidney . Therefore, the dynamic change of CK
[52]
and Mb levels can be effective measures to assess muscle injury severity and the repair process, offering
potential guides for modifying the training recovery plan .
[53]

(3) Inflammatory regulation factors

Inflammatory response is the core physiological process during muscle injury and repair, realized by
inflammatory regulation factors mainly represented by the interleukin family (IL-6, IL-1, etc.). Inflammatory
factors are signal molecules secreted by immune cells (macrophages, lymphocytes) and parenchymal cells
(muscle cells, liver cells), widely involved in tissue damage recognition, repair initiation and immune
regulation , and closely related to the occurrence and development of muscle fatigue or injury . Muscle
[55]
[54]
fatigue or minor injuries caused by high-intensity exercise can trigger the release of inflammatory factors
(such as IL-6 and IL-1), which recruit repair cells (such as satellite cells and macrophages) [6,56] at the injury
site, clear necrotic tissue fragments, and create a microenvironment favoring muscle fiber regeneration. At
this point, a moderate increase in inflammatory factors is recognized as a positive signal for initiating the
repair process . However, if the inflammatory response is overly activated or lasts for too long, excessive
[55]
inflammatory factors will inhibit the proliferation and differentiation of muscle satellite cells, hinder the
muscle fiber regeneration and thus delay muscle function restoration . Therefore, inflammatory factor
[57]
monitoring (e.g., from increase to a stable state and a subsequent decrease) can provide insights into the
inflammation regulation state.

Biomechanical indicators
Muscle strength and stiffness are key biomechanical indicators characterizing the functional state of
muscles . The former directly reflects the output capacity of the neuromuscular system, while the latter is
[40]
related to the structural integrity of the muscle-tendon units. These biomechanical cues jointly provide a core
quantitative basis for evaluating the functional status and structural attributes of the muscular system .
[47]

When muscles are fatigued or injured, the maximum force provided by muscle contraction will decrease [56,58] .
Studies have shown that after muscle injury, the decline in muscle strength is significantly positively
correlated with the severity of muscle fiber damage , enabling muscle strength to quantitatively measure the
[59]
degree of injury . Further from a microscopic view, fatigue or injury disrupts the mechanical interaction
[60]
between actin and myosin, resulting in muscle microstructure damage, such as muscle fiber rupture and
Z-disc distortion . Consequently, the force transmission during muscle contraction is impeded, and the
[40]
maximum contractile force is thus decreased . As the activation of muscle satellite cells in healing, muscle
[61]
fiber regeneration and extracellular matrix remodeling gradually advance by restoring the interaction
mechanism between actin and myosin and refining the muscle fiber microstructure. This induces the rising
of force generation and transmission efficiency in muscle contraction towards the pre-injury level . Based
[62]
on this, the rate and extent of muscle strength recovery can serve as the core endpoint indexes for scaling
muscle function recovery, thus guiding the evaluation and optimization of the efficacy of the rehabilitation
plan, especially for professional athletes.