Load carriage is a task undertaken by military and emergency service personnel. Prolonged exercise causes neuromuscular impairment (i.e. a reduction in the force producing capability of a muscle group) due to fatigue and damage to muscle tissue. However, the physiological variables which determine the magnitude of neuromuscular impairment associated with load carriage have not been investigated.
PURPOSE: To investigate the physiological determinants of neuromuscular impairment following prolonged load carriage.
METHODS: Twenty one male participants completed 120 minutes of treadmill walking at 6.5 km·h-1 carrying a 25 kg backpack. To determine the magnitude of neuromuscular impairment, maximal voluntary contraction (MVC) force was measured during isometric knee extensions before and immediately after load carriage, and expressed as a percentage change from the pre-exercise value (%ΔMVC force). Peak torque at two isokinetic velocities of knee (60 and 180 °·s-1) shoulder (60 and 180 °·s-1) and trunk (15 and 60 °·s-1) extensors and flexors were determined. Participants' maximal oxygen uptake and body composition were determined at least seven days before load carriage. Pearson's bivariate correlations were applied to describe relationships between %ΔMVC force and the independent variables. A multivariable model was developed to examine the physiological determinants of %ΔMVC force. Differences between pre and post exercise values were examined using paired t-tests.
RESULTS: MVC force decreased by 17 ± 9 % following load carriage (P<0.001). The %ΔMVC force revealed a moderate relationship with knee flexor torque at 60 °·s-1 (r=0.46, P=0.038) and shoulder flexor torque at 60 °·s-1 (r=0.53, P=0.046). The multivariable model explained 72% of the variance in %ΔMVC force [%ΔMVC Force = -23.60 + 0.13 (Shoulder Flexor Torque 60 °·s-1) + 0.11 (Knee Flexor Torque 60 °·s-1) - 0.78 (Relative VO2max) + 0.1 (Trunk Flexor Torque 60 °·s-1), R= 0.72, R2= 0.51, Adj R2= 0.37, SEE= 7.67 %, P=0.030].
CONCLUSION: Load carriage causes neuromuscular impairment of the knee extensors. The multivariable model indicated neuromuscular impairment was greater for individuals with lower knee, trunk and shoulder flexor strength and higher aerobic fitness relative to body mass.
This work was funded by the University of Chichester.