An extensive series of experiments has been conducted to quantify the fatigue of neck muscles as measured by isometric endurance time. The neck muscles were dynamically and statically loaded by systematic variation of 15 headgear configurations consisting of 5 different centers-of-gravity (forward-low, center-low, center-high, right-lateral-low and afterward-low) and 3 different weights (3.2 lb, 5.0 lb, 9.0 lb, 1.45, 2.27, 4.09 kg). Each of six subjects would rotate his head laterally (from side-to-side) for 30 min in each of the 15 headgear loading combinations. Immediately thereafter, the subject would position his head in an isometric head dynamometer and exert a sustained right lateral neck contraction at 70% of his maximum strength (MVC), during which endurance time (to fatigue) was recorded. The results indicate that there were no significant differences (p>0.05) in endurance time between the 3.2 lb/forward-low, 3.2 lb/lateral-right-low, and 9.0 lb/afterward-low compared to controls (endurance time for at 70% MVC isometric neck contraction in which there was no prior head loading). All other head loading configurations (weight and center-of-gravity combinations) did result in a significant reduction (p≤0.05) in endurance times (compared to controls).

These results are significant since they provide useful insights into the optimal trade-off between various centers-of-gravity and helmet weight combinations which result in the optimization of neck muscle endurance.