Lower Extremity Injury Risk Is Increased During Load Carriage But Does Not Change In Thermally Stressful Environments

The effect of a loaded ruck march in thermally stressful environments has not been investigated for its impact on lower extremity injury risk. Y-balance test (YBT) composite scores <94% of limb length indicate increased lower extremity injury risk. PURPOSE: To determine if increased strain from a ruck march in thermally stressful environments would increase lower extremity injury risk as measured by the YBT. METHODS: In a randomized crossover design, 15 subjects (8 males; 26 ± 5 y; 173 ± 9 cm; 72.6 ± 13.6 kg) completed a 6.4 km ruck march at 4.8 km·h-1 while loaded to 30% of their body mass (21.8 ± 4.1 kg) in a temperate (TT: 20 °C, 25% RH), warm/humid (WH: 30 °C, 80% RH), and a hot/dry (HD: 40°, 25% RH) environment. Heart rate (HR) and rectal temperature (Tre) were collected every 5 min. Subjects completed the YBT unloaded (UNL) and loaded (LD) before the march and LD after the march. YBT composite scores were calculated for each leg as the sum of the maximal reach in each direction divided by three times the leg length multiplied by 100. UNL and pre-LD YBT composite scores were analyzed using paired t-tests. HR and Tre during the ruck march and LD YBT composite scores were analyzed using repeated measures ANOVA. Post-hoc Tukey’s multiple comparisons tests were used when a main effect was observed. RESULTS: Donning a loaded ruck sack decreased YBT composite scores (right- UNL: 95 ± 0.06%, LD: 91 ± 0.08%, p < 0.01; left- UNL: 96 ± 0.06%, LD: 93 ± 0.07%, p < 0.01). LD YBT composite scores did not differ between trials before the ruck march (right- TT: 90 ± 0.09%, WH: 92 ± 0.06%, HD: 92 ± 0.08%, p = 0.59; left- TT: 93 ± 0.07% ,WH: 93 ± 0.06%, HD: 93 ± 0.07%, p = 0.98). HR was higher in the WH and HD trials at the end of the march compared to TT (HR- TT: 133 ± 12 bpm, WH: 160 ± 12 bpm, HD: 170 ± 12 bpm, p < 0.01 for each). Tre was higher in the WH and HD trials at the end of the march compared to TT (Tre- TT: 37.80 ± 0.22 °C, WH: 38.32 ± 0.39 °C, HD: 38.55 ± 0.39 °C, p < 0.01 for each). LD YBT composite scores did not differ from before to after the ruck march (right p = 0.43; left p = 0.15) or between environmental conditions (right p = 0.45; left p = 0.95). CONCLUSIONS: Donning a loaded ruck sack increased lower extremity injury risk. A ruck march did not worsen lower extremity injury risk compared to the load alone despite increased thermal and cardiovascular strain in thermally stressful environments. Supported by: University at Buffalo Mark Diamond Research Fund