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Chang et al. Soft Sci. 2026, 6, 29 Page 5 of 26

relevance of each indicator are clarified in detail.

(1) Energy metabolism indicators

The balance between energy consumption and synthesis is a core parameter to reflect the degree of muscle
fatigue, injury and healing. The energy metabolism indicators mainly include reserved glycogen, lactic acid
and lactate dehydrogenase, as well as urea nitrogen.

Lactic acid is a characteristic product of anaerobic glycolysis, and the lactate dehydrogenase (LDH) family is
a key enzyme system regulating the conversion of lactic acid to pyruvate. In the early stage of fatigue or
injury during high-intensity exercise, the muscle oxygen supply is relatively insufficient, activating the
anaerobic glycolytic pathway to induce excessive lactic acid production and accumulation in the blood .
[39]
Such an elevating level of lactic acid within a short period decreases the pH of body fluids, thereby inhibiting
muscle fiber contraction function and further impairing athletic performance . It deserves to note that the
[40]
proportion of LDH isoenzymes undergoes various adjustments facing enhanced anaerobic metabolism,
presenting suppressed activities of LDH1 and LDH2 (biased towards aerobic metabolism), while stimulated
activities of LDH3, LDH4 and LDH5 (biased towards anaerobic metabolism) . As exercise ceases and
[41]
oxygen supply resumes in the repair and remodeling period, blood lactic acid is gradually metabolized and
cleared through pathways such as oxidative decomposition and gluconeogenesis. The proportion of LDH
isoenzymes simultaneously reverses towards aerobic metabolism, with the activities of LDH1 and LDH2
rebounding and the activities of LDH3, LDH4 and LDH5 decreasing . In this way, through monitoring the
[42]
lactic acid and LDH isoenzymes dynamics, the transformation of muscle metabolic patterns (anaerobic
compensation and aerobic homeostasis) can be accurately determined, and the progress of injury repair can
be indirectly evaluated. When the proportion of isoenzymes returns to normal, it indicates that the muscle
metabolic function has basically recovered .
[43]
Blood urea nitrogen (BUN) is the final product of protein and amino acid metabolism. Its level changes
reflect the energy metabolism pattern and nitrogen balance status insider the body. The normal reference
range for BUN is 2.86-7.14 mmol/L . In prolonged and high-intensity exercises, the body will initiate
[44]
protein (amino acid) catabolism for energy supply, while nucleotide metabolism intensifies, once the
breakdown of sugar and fat fails to meet the energy demands. The bi-phasic effect induces increased BUN
level with the extended exercise time and the higher intensity [45,46] , making it an important signal to indicate
insufficient body energy reserve or fatigue. In the repair stage, protein catabolism and BUN production
weaken, as energy supply recovers (glycogen reserves rise). At the same time, the kidney’s excretory function
is enhanced, accelerating the clearance of BUN in the blood and gradually bringing its level back to the
normal range. Therefore, the downward trend of BUN can serve as an important indicator of the restoration
of metabolic balance in the body and the relief of fatigue.

(2) Muscle injury markers

Muscle injury can disrupt the integrity of muscle cell membranes, leading to the release of specific substances
from cells into the bloodstream. At the same time, imbalance in protein metabolism can also cause changes
in characteristic indicators, mainly including creatine kinase (CK) and myoglobin (Mb) .
[47]
CK is a key enzyme regulating energy conversion within muscle cells, and Mb is an oxygen-carrying protein
within muscle cells. Both exist in the cytoplasm of muscle cells, and their concentration changes in blood
directly reflect the integrity of the muscle cell membrane and the degree of damage . CK concentration can
[48]
quantitatively indicate the severity of injury, where > 5,000 U/L is regarded as severe muscle injury . A
[49]