Students and military personnel often carry heavy torso loads (e.g. backpacks) which can lead to pain in the shoulders and back. To minimize pain associated with torso loads, load redistribution devices have been developed to off-load weight from the shoulders to the hip. Previous studies have used the cervical flexion relaxation ratio (cFRR), a measure of muscle activity between the extension and flexion phase of forward head motion, to correlate backpack loads with the potential of developing neck pain.
PURPOSE: Using the cFRR, assess the impact of redistributing torso-borne loads and whether changes in cFRR are observed following prolonged load carriage.
METHODS: Twelve volunteers walked at 1.34 m·s-1 at 0% grade for 40 min while wearing a torso-borne baseline load of 27 kg and two load distribution conditions to offload shoulder pressure. Flexible pressure sensors monitored pressure distribution at the shoulders and hips. To capture cFRR, bilateral EMG sensors with integrated tri-axial accelerometers were placed on the cervical erector spinae muscle bellies. EMG data were sampled at 2 kHz, demeaned, band pass filtered from 30 to 500 Hz, and full wave rectified. cFRR (Table 1) was measured pre-walk and post-walk and outcomes were submitted to repeated measures ANOVA based on load conditions, side of body and time.
RESULTS: The average pressure measured at the shoulders for the baseline load (3.76 ± .90 kPa) was significantly higher than the two load distribution conditions (1.55± 1.68 kPa, p = .001; 1.72± 1.12 kPa, p < .0005, respectively). There were no main effects due to load condition or body side (p > .074). However, there was a significant main effect of time (p = .033).
CONCLUSION: The load distribution equipment successfully shifted pressure from the shoulders to the hips, however this did not correlate with an improvement in cFRR. Changes in cFRR were observed pre- vs. post-walk indicating that the cFRR measure is sensitive to load carriage over time.