Repeated bouts of load carriage alter indirect markers of exercise-induced muscle damage, liver enzymes, and oxygen-carrying capacity in male soldiers

Abstract

Soldiers are often required to carry heavy external loads over multiple days, which may degrade physical performance. We investigated the effects of repeated load carriage bouts on indirect markers of exercise-induced muscle damage, liver enzymes, and oxygen-carrying capacity in active-duty infantrymen. Fourteen male soldiers (age = 24.6 ± 1.1 y; BMI = 25.7 ± 0.7 kg/m²) underwent a 5-day protocol, consisting of baseline/familiarization, 3 load carriage bouts, and a recovery day. There were reductions in maximal voluntary contraction strength (p < 0.05), with the knee flexors and trunk extensors showing the greatest declines. Each load carriage bout produced an inflammatory response, including increases in leukocyte subtypes (neutrophils and monocytes) and monocyte chemoattractant protein-1 (p < 0.05). At the end of the protocol, serum liver enzymes were elevated, and erythrocytes and hematocrit were lower than baseline (p < 0.05). In addition, greater circulating leukocytes at baseline predicted lower knee and trunk torque during recovery. Repeated bouts of load carriage reduce muscle strength and cause inflammation consistent with exercise-induced muscle damage, alter liver function tests, and decrease oxygen-carrying capacity in male soldiers, which could compromise readiness for prolonged and/or intense military operations.

Keywords: immune cells; inflammation; military; performance; skeletal muscle.

FIGURE 1

Overview of the experimental protocol. During each day of the study, participants arrived to the laboratory at 0630 following a ~10‐h overnight fast. Visit 1 included familiarization with the protocol and anthropometric measurements (height, weight, and body composition). Before and after each bout of load carriage exercise (Visits 2–4), a venous blood sample was collected, knee and back/trunk muscle function was measured via isokinetic dynamometry, and perceived muscle soreness was reported. Individualized diet prescriptions were distributed in the morning, and a snack was provided in the afternoon. On BOUT1 (Visit 2) and BOUT3 (Visit 4) of the protocol, participants completed 2 h of loaded walking (50% body mass) at 1.35 m/s and variable grade, including 0% (0–10 min), 3% (10–60 min), −3% (60–110 min), and 0% (110–120 min). On BOUT2, 30 min of loaded walking (50% body mass) was followed by a 4‐mile time trial with reduced load (30% body mass). During each load carriage bout, exercise intensity was monitored via continuous heart rate measurements and rating of perceived exertion was reported every 10 min. During Visit 5, all primary outcomes were measured to characterize recovery following the protocol.

FIGURE 2

Skeletal muscle function in response to repeated bouts of load carriage exercise. Isometric and isokinetic knee (a–c) and trunk (d–f) flexion and extension torque before and after bouts of load carriage and recovery (n = 14). Data represent mean ± SE; *p < 0.05 compared to pre value within the testing day; Percent changes represent difference from pre to post within the testing day; ^† p < 0.05 compared to pre value on BOUT1. n = 13 for isokinetic knee flexion and extension tests at 60 and 120°/s.

FIGURE 3

Systemic inflammation in response to repeated bouts of load carriage exercise. Total circulating leukocyte (a), neutrophil (b), lymphocyte (c), monocyte (d), and monocyte chemoattractant protein‐1 (MCP‐1; e) concentrations, and neutrophil‐to‐lymphocyte ratio (NLR; f) before and after bouts of load carriage and recovery (n = 14). Data represent mean ± SE; *p < 0.05 compared to pre value within testing day; Percent changes represent the difference from pre to post within testing day; ^† p < 0.05 compared to pre value on BOUT1. n = 9 for MCP‐1.

FIGURE 4

Biochemical markers of muscle damage in response to repeated bouts of load carriage exercise. Circulating creatine kinase (a), alanine transaminase (ALT; b), alkaline phosphatase (ALP; c), and aspartate aminotransferase (AST; d) before and after bouts of load carriage and recovery (n = 14). Data represent mean ± SE; *p < 0.05 compared to pre value within testing day; Percent changes represent differences from pre to post within testing day; ^† p < 0.05 compared to pre value on BOUT1. Bottom (e–f): Scatterplots showing relationships between creatine kinase and ALT (e) and AST (f) concentrations following each bout of load carriage exercise. *p < 0.05; **p < 0.01. n = 7 and 6 for creatine kinase on BOUT1/2 and BOUT3, respectively.

FIGURE 5

Red blood cell indices in response to repeated bouts of load carriage exercise. Erythrocyte (a), hemoglobin (b), hematocrit (c), and platelet (d) values before and after bouts of load carriage and recovery (n = 14). Data represent mean ± SE; *p < 0.05 compared to pre value within testing day; Percent changes represent difference from pre to post within testing day; ^† p < 0.05 compared to pre value on BOUT1.

FIGURE 6

Relationships between immune status and skeletal muscle function at baseline and recovery. Scatterplots showing the relationship between total circulating leukocyte count and isometric and isokinetic torque production for knee flexion (a–c) and trunk extension (d–f) at baseline/pre (BOUT1Pre) and recovery/post (RECPost). *p < 0.05.