The influence of backpack chest straps on physiological and performance variables associated with simulated road marching

Abstract

This study compared physiological and performance variables during heavy load carriage while wearing an armored vest under a standard issue military backpack using three different chest strap conditions (no chest strap, NCS; standard chest strap, SCS; modified chest strap, MCS). Twenty subjects, all right-handed shooters, completed 4 laboratory visits. The subjects filled out paperwork, received consumables for the next visit, and performed a handgrip strength test during the first visit. The following 3 visits were testing visits with 2 treadmill walking trials per visit at a fixed speed (80.5 m/min) and grade (2%). Each testing visit corresponded to 1 of 3 conditions (NCS, SCS, or MCS) and included a baseline trial carrying only a training rifle (3.3 kg) followed by a pack trial wearing an armored vest, pack, and rifle (47 kg total). Heart rate (HR, bpm), relative oxygen consumption (RVO 2, ml/kg/min), minute ventilation (V E, L/min), and breathing frequency (BF, breaths/min) were measured. Circumferences (bicep, C B, and forearm, C F, cm) and blood lactate (BL, mmol/L) were assessed once per trial on the non-trigger finger arm, while two-point discrimination (mm) was assessed once on the back of the trigger finger hand. Rating of perceived discomfort (RPD), fingertip oxygen saturation (SpO 2, %), and stride rate (SR, stride/min) were assessed twice each trial. Maximum handgrip strength was assessed in each hand simultaneously (HG right and left, kg) two times per trial. All variables were assessed using multivariate 2-factor RM ANOVA and Scheffe's post hoc test (alpha = 0.05). Baselines for all variables were similar. Pack trial means for HR, RVO 2, V E, and BF were higher than baseline but no difference between strap conditions. Fingertip SpO 2 was lower during pack trials than baseline. There were no differences in V T, BL, or left HG between conditions. NCS resulted in greater C F and C B than baseline with C F also being greater than MCS. Right HG was greater for MCS than baselines and NCS. Right and left HG increased 2 minutes post walking. MCS and SCS provided the least evidence for negatively affecting physiologic and performance outcomes, whereas NCS provides the most evidence for negatively affecting outcomes.