Backpack Load Carriage Affects Motor and Sensory Responses of the Median Nerve

Use of the upper limbs is often necessary for military and firefighter personnel who carry backpacks. Backpack straps can compress the brachial plexus nerves of the upper limb. It is known that carrying a backpack can lead to rucksack palsy, but it is unknown if effects of upper limb nerve compression from carrying a heavy backpack can be demonstrated after a single session of backpack use. Our study aimed to investigate the short-term effects of backpack carriage on upper limb nerve conduction.

Thirty-six participants including 18 female (mean ± SD: age 24.3 ± 7.6 years; height 168.8 ± 9.4 cm; mass 73.1 ± 16.6 kg; BMI 25.4 ± 4.5 kg/m2) and 18 male (24.1 ± 5.8 years; 178.2 ± 9.4 cm; 80.2 ± 11.7 kg; BMI 25.0 ± 4.3 kg/m2) were recruited as a convenience sample and assessed by nerve conduction study (NCS) of the median nerve on the dominant upper limb. Skeletal muscle mass (SMM) and body mass index (BMI) were evaluated using bioelectric impedance. Nerve conduction study measurements were taken before (PRE), after 20 minutes of walking and while still wearing a military-style large frame backpack with hip belt loaded to 30% bodyweight (POST), and immediately after removing the backpack (DOFF). We analyzed comparisons statistically using mixed factor analysis of variance (ANOVA) with significance level of P < .05.

Motor nerve action potential amplitude significantly decreased when stimulation was at the axilla from PRE to POST (P = .025) and PRE to DOFF (P = .012). Motor nerve action potential latency was significantly increased PRE to POST and PRE to DOFF with stimulation at the elbow (P = .029 and P = .030, respectively). Latency was significantly longer for males as compared to females (P ≤ .008). Sensory nerve action potential amplitude decreased significantly between PRE and POST (P = .007). Significant correlation was determined between amount of SMM and percent difference PRE to POST in motor nerve action potential amplitude (r = 0.438, P < .01). Participants with lower SMM demonstrated greater difference in motor nerve action potential after backpack carriage with POST measurements lower than PRE measurements. Body mass index was significantly correlated with sensory baseline-to-peak amplitude percent difference PRE to POST (r = 0.428, P < .01) indicating that those with lower BMI had a larger negative impact on sensory nerve response after backpack carriage.

The results of this study reveal that walking for 20 minutes while carrying a 30% bodyweight backpack resulted in an increase in upper limb motor nerve latency and decrease in sensory and motor nerve action potential amplitude. The NCS findings indicate that SMM may have a protective effect and was therefore beneficial to maintaining upper limb nerve conduction after backpack carriage while lower BMI was a risk factor for reduced sensory nerve conduction. The demonstrated deficits in nerve conduction after backpack carriage could have implications for use of hands, especially the thumb, index, and middle finger, as they are innervated by the median nerve.