Introduction: Wildland firefighters (WLFFs) must undergo a 2-wk critical training (CT) period prior to deployment to the field. This stress may result in clinical risks, including severe muscle damage and rhabdomyolysis. We aimed to document the effects of WLFFs’ CT on physiologic markers of muscle damage and soreness.

Methods: Two interagency hotshot crews (n=25) were followed during spring 2022 for 80 h of training. Activity counts as well as records of upper-body (US) and lower-body (LS) muscle soreness were collected daily. Body weight (BW) and skinfold measurements were recorded on Day 1 (D1) and D11 to estimate body fat (BF) and lean body weight (LBW). Blood was collected on D1 and D11 to measure muscle and liver damage markers.

Results: Critical training resulted in significant elevations in creatine kinase (CK) (216.9±57.4 U/L vs 5166.4±1927.8 U/L, P=0.017) and lactate dehydrogenase (LDH) (175.5±4.0 IU/L vs 340.0±42.6 IU/L, P=0.001) despite no significant changes in BW, BF, LBW, cortisol, or testosterone. Main effects of time were seen in US and LS, peaking on D11 (US: 5.2±0.4 cm, P<0.001; LS: 5.5±0.4 cm, P<0.001). Those who spent the most minutes with activity counts of >1500 counts/min showed the greatest increase in CK and LDH.

Conclusions: These data suggest that WLFFs undergo significant physiologic stressors, resulting in muscle soreness and damage during CT, with 6 of the 25 subjects reaching critical levels of serum CK. It appears that much of the muscle damage and soreness occurred because of unaccustomed WLFF job-specific tasks.