This was a retrospective cohort study among high school football players who participated in the immersion camp. The investigation was considered public health practice; therefore, review by the institutional review board was not required.

Among 43 players, 22 (51%) met our case definition. All had myalgia referable to the upper arm; 12 (55%) were hospitalized (median length of day 4.5 days). There were no cases of acute kidney injury; the highest creatinine among all players was 1.2 mg/dL. Peak CK for the players with rhabdomyolysis ranged from 2434 to 42 000 U/L. Among the 12 hospitalized cases, each had a peak CK > 10 000 U/L, including 6 with CK ≥ 20 000 U/L. One additional team member who did not meet our case definition for rhabdomyolysis or compartment syndrome was hospitalized overnight for triceps muscle pain and swelling. None of the cases included a diagnosis of heat-related illness. Hospitalized patients were treated with rest, intravenous hydration, and sodium bicarbonate without complications.

Two independently practicing orthopaedic surgeons diagnosed triceps compartment syndrome in 3 players on the basis of significant triceps pain, swelling, and reduced range of motion. None of the 3 players with triceps compartment syndrome had evidence of infection or external trauma. One player had paresthesias and decreased sensation in the musculocutaneous nerve distribution unilaterally. No player had vascular deficits on examination. Compartment pressures, measured on the hospital’s single pressure monitoring device (Stryker Intra-Compartmental Pressure Monitor System, Kalamazoo, Michigan), exceeded 50 mmHg in each of the cases (normal, 0-10 mmHg). These 3 patients had peak CK levels of 17 030, 18 480, and 35 658 U/L; each underwent uncomplicated fasciotomy. Among the 2 other players who had compartment pressures measured, the maximum pressure was 37 mmHg; the remaining players on the team did not have physical findings warranting compartment pressure measurement. Both orthopaedic surgeons had extensive experience with the pressure monitoring device and reported no recent technical problems.

Questionnaires were completed by 40 (93%) of 43 players. Among the 20 teammates with rhabdomyolysis who completed questionnaires, 18 (90%) reported arm pain and 16 (80%) reported arm swelling; arm numbness (n = 3, 15%) and dark urine (n = 1, 5%) were uncommon (Table 1). Among the 18 who reported arm pain on the questionnaire, 10 (56%) had onset on August 17, and 9 (50%) had peak pain on that date (Figure 1). The 2 players with rhabdomyolysis who did not report arm pain on the questionnaire had reported arm pain during medical evaluation at the hospital.

Players with rhabdomyolysis were statistically significantly shorter than those without rhabdomyolysis (mean, 69.7 [177.0 cm] vs 71.6 in. [181.96 cm]; P = 0.05); weight and body mass index were also lower, but differences did not reach statistical significance (Table 2). The frequency of aerobic and resistance exercises during the summer was similar between the 2 groups. Among 19 players receiving at least 1 vote from a teammate as being 1 of the 3 hardest working players, 13 (68%) experienced rhabdomyolysis, compared with 7 (33%) of 21 not considered hardest working (relative risk, 2.1; 95% confidence interval, 1.04-4.0).

Prior use of creatine supplements, reported by 10 (25%) of the 40 players completing questionnaires, was not associated with illness. No genetic or infectious causes of rhabdomyolysis were identified, and no prescription drug, nutritional supplement, or toxin was associated with illness. Players denied use of alcohol, illicit drugs, or performance-enhancing drugs. No positive results were reported from the 4 urine specimens tested for performance-enhancing drugs or the 4 archived serum specimens tested for illicit drugs. The environmental investigation of the high school facilities disclosed no additional findings that might have contributed to rhabdomyolysis; furthermore, levels of carbon monoxide, carbon dioxide, and volatile organic compounds were normal.

We identified 22 (51%) of 43 football players participating in a preseason camp who experienced acute exertional rhabdomyolysis, including 3 with a diagnosis of triceps compartment syndrome. Among the rhabdomyolysis cases, 12 of 22 were hospitalized; each hospitalized player had a CK > 10 000 U/L. We conclude that the extreme triceps exercise on the first day of camp, possibly exacerbated by heat and subclinical dehydration, led to muscle injury and the observed illnesses. Hyperthermia lowers the threshold for muscle injury.⁵ Although the outdoor temperature of 92°F (33.3°C) was not extreme compared with certain parts of the country, it was 13°F (7.2°C) higher than that city’s average for that date.

Clusters of acute exertional rhabdomyolysis after intense, repetitive, short-duration resistance exercises have been reported.^10,18,21 Failure to recognize rhabdomyolysis has caused diagnostic delays and inappropriate training regimens.^23,24,33 In this cluster, players exercised a single muscle compartment to total muscle fatigue; the exercise involved both concentric and eccentric contractions. Eccentric muscle contraction poses substantially greater risk for muscle injury.³¹

Although rhabdomyolysis is variably defined as muscle symptoms and a CK > 5 to 10 times the upper limit of normal, milder cases likely have minimal clinical significance. The degree of CK elevation does not necessarily correlate with muscle fiber damage.¹⁷ Moreover, CK levels are commonly elevated among athletes.^9,25 Ehlers et al reported a mean CK level of 5125 U/L on the fourth day of 2-a-day football practice among 12 college football players.¹⁵ Consequently, CK can best contribute to diagnosis and management when obtained after, or in conjunction with, clinical evaluation. In the absence of dark urine or unusually significant postexertional muscle pain or swelling, CK should not be drawn. In those with elevated CK levels, athletes with CK < 5000 U/L and no evidence of urinary or kidney abnormalities likely only require sufficient rest and hydration for muscle recovery.

Rhabdomyolysis was related to overall physical effort during camp; players considered by peers to have worked harder were twice as likely to experience illness compared with other teammates. The association between physical effort and physical conditioning, however, was unknown; objective individual fitness data were unavailable. Although rhabdomyolysis can occur even among conditioned athletes, physical conditioning reduces the degree of myoglobinuria after strenuous exercise.^28,32,33,36

The 3 cases of exertional triceps compartment syndrome are novel. Exertional compartment syndrome primarily involves the leg and, less commonly, the forearm⁷; the triceps compartment has greater capacity to absorb increased extravascular volume from muscle injury than the leg or forearm.¹⁹ The rare cases of triceps compartment syndrome that have been reported have been secondary to trauma or crush injury.^{4,11,13,27,35} We are unaware of any other reports of exertional nontraumatic triceps compartment syndrome clustered among athletes.

The findings in this study are subject to limitations. First, toxicologic testing was not performed on any cases during hospitalization. Illicit drugs, including cocaine and amphetamines, are known causes of rhabdomyolysis, and case reports have linked creatine and performance-enhancing drugs to rhabdomyolysis.^14,30 Given the magnitude of the attack rate, however, undetected illicit drug or performance-enhancing drug use was unlikely to have had a major role in causing illness. Second, interviews, conducted 11 to 14 days after the camp began, might have been affected by recall bias. Peer perception of teammates’ effort might have been influenced by knowledge of whether illness had developed. In addition, players might have deliberately withheld sensitive incriminating information.

Our investigation highlights that acute exertional rhabdomyolysis and compartment syndrome can complicate intense, short-duration, repetitive resistance exercise among adolescent athletes, particularly exercises to total muscle fatigue that involve eccentric muscle contractions. Despite the known risk for acute exertional rhabdomyolysis from specific resistance exercises, sports medicine guidelines, educating health professionals in recognizing and minimizing this risk, are limited.^1-3 Although physical activity and participation in organized sports is beneficial, recognizing and counteracting specific exercise and heat hazards can minimize the risk for clinically significant muscle injury.