The Impact of Load Mass and Distribution on Heart Rate, Perceived Exertion, and Accelerometer Measured Physical Activity During Running.

Many tactical athletes (military and emergency personnel) have fixed load carriage occupational requirements. Understanding the effects of running with common military "fighting loads" (no load to approximately 18 kg) on heart rate (HR), perceived exertion (RPE), and measuring it with accelerometers has implications for training and ensuring physical readiness of recruits. Eleven (7 male and 4 female) civilians (21.5±2.3 years, 1.83±0.09 m, and 80.7±18.5 kg) completed four, 5- minute running (8.05 kph) conditions: no external load, rifle carriage (3.5 kg), pack carriage (13.6 kg), and rifle/pack carriage (17.1 kg) in random order, while HR, RPE, and hip worn ActiGraph accelerometer step cadence, and activity counts per minute (CPM) were collected. One way 4-level analysis of variance or Friedman test explored the condition main effect on HR, RPE, step cadence, and CPM. Pairwise comparisons with Bonferroni correction determined which conditions differed from each other. HR and RPE significantly differed (p < 0.001) by load condition. No external load had the lowest HR and RPE (HR=141.8 bpm, RPE=9.4), while rifle/pack carriage had the highest (HR=160 bpm, RPE=14.8). However, rifle carriage (HR=153.7 bpm, RPE=12.3) elicited similar physiological responses as pack carriage (HR=155.0 bpm, RPE=13.6). Step cadence was equal across conditions, but CPM decreased as loads increased. Because a lighter load carried in the hands caused similar HR and RPE response to wearing a heavier pack, recruits should practice both in preparation for military or tactical occupations. Accelerometers may be used to assess step cadence, but not the increased physiological demands of external load conditions.