The purpose of this investigation was to examine the relationships between subconcussive head impact frequency and magnitude and measures of physical exertion and muscle damage.

Subconcussive head impacts, or impacts that do not present with concussion symptoms, are gaining traction as a major public health concern. However, there is a gap in knowledge about the contribution of physiological variables, such as muscle damage and physical exertion, to neurological measures used to assess subconcussive impact-dependent changes. The unknown contribution of physical exertion and strenuous exercise is often listed as a limitation in field studies of subconcussive head impacts. 

Fifteen high school football players wore mouthguards installed with triaxial accelerometers and gyroscopes in order to quantify the linear and rotational accelerations of every head impact sustained throughout one season (practices and games). Additionally, serum samples were collected at twelve time points (pre-season, pre- and post-competition for five in-season games, and post-season) and assayed for CK-MM, the skeletal muscle-specific isoenzyme of creatine kinase. Subjects wore heart rate monitors during the five games, and heart rate data were used to estimate physical exertion in terms of excess post-exercise oxygen consumption (EPOC). 

Mixed-effect regression modeling (MRM) showed significant and positive associations between CK-MM and subconcussive head impact kinematic variables, in addition to a significant and positive association between CK-MM and EPOC. The models were adjusted for cumulative head impact exposures up to each game and the pre-season serum CK-MM levels, when applicable.

When investigating subconcussive head impacts, the effects of muscle damage should be considered when using correlated outcome measures, such as inflammatory biomarkers and vestibular assessments.