Retrospective Look at Muscle Fatigue from Sustained Helmet Wear
Colonel Donald J. White, USAF (Ret)
FRAeS, FAsHFA, FAsMA
Today’s flight and ground force helmets incorporate targeting and cueing systems enabling the operator, (flight and ground), to accurately cue onboard and ground weapons against enemy aircraft and ground forces, while flying high-G aircraft and performing strenuous tactical ground and vehicle maneuvers. While these systems undoubtedly increase a warrior’s capabilities, one obvious drawback to putting all this equipment on the helmet is the increase in helmet weight that shifts the combined head and helmet center of gravity (CG) forward and increases moments of inertia on the neck. The incidence of neck pain is widely reported by night vision goggle (NVG) users, including concerns for personal well-being and safety. Operational concerns associated with the heavier helmet may result in decreased performance from muscle fatigue and increase in neck injury during ejection or sudden vehicle deceleration or other ballistic threat. The objective of this retrospective review is to present measurements of an operator’s neck and upper torso fatigue while wearing US Army and Air Force helmets of varied mass properties (weight, CG, moments of inertia) for durations up to eight hours. Results of this retrospective study review found that helmets with a forward CG shift were significantly more uncomfortable on the subject’s neck and back than the helmets with a normal CG fit; and that significant increases in upper neck and upper and lower back discomfort were reported as early as hour two and continued throughout the eight hour session. This review also demonstrated that the 4.5 lb helmet with forward CG shift was significantly more uncomfortable on the subject’s than the 6.0 lb helmet with nominal CG shift. It is likely that it is not the increased weight of the NVG but it is the increased loading in the frontal position that results in neck strain. The Knox Box a criterion is purely physics and should be consider the gold standard. While Electromyography (EMG) shows the studies of real time fatigue over measured time. The Knox criteria are based on formulas that represent an inclusive 5th to 95th percentile anthropometric population. However, the Knox Box is not measuring true analytical performance as EMG quantifies muscle strength and fatigue.
In the 1980’s, United States Navy, United States Army, United States Air Force, and European Air Force surveys documented neck injury rates of 50% or higher ranging from minor neck strain to cervical vertebral fracture. (3, 11-14, 17) Lighter helmets were developed and implemented in hopes of reducing injuries. Although the new generation of head-mounted displays and night vision devices are likely to enable warfighters to improve their effectiveness to complete their mission, the integration of these systems has resulted in increased helmet weight, an alteration of the head and helmet CG, and greater torque on the neck. These head-load changes may lead to greater neck fatigue and susceptibility to neck injury, resulting from the increase in cervical loads during aircraft ejections (catapult, windblast, parachute opening shock), sudden vehicle deceleration, or other ballistic threats as well as integrated headgear systems for mounted (armored vehicle operators) and dismounted (infantry) Soldiers. Research dictates an increase in acute and chronic pain or injury from fatigue associated with prolonged wear (vibration, sustained acceleration, sudden deceleration and ballistic threat) and resulting in compromised effectiveness for long duration missions.
The neck load limits for flexion, extension, and rotation under aerial, mounted or dismounted soldier operational conditions are unknown. Some studies suggest that most in-flight neck injuries occur when pilots move their heads while pulling Gs or when an unsuspecting “back-seater” is subjected to a high-G maneuver. (2-3, 10, 17, 31)
Tests conducted by the Biomechanics Branch of the 711th Human Performance Wing (711 HPW/RHPA) evaluated the effects of variable helmet mass properties on the biodynamic response of male and female human volunteers exposed to vertical (+Gz) accelerations using the vertical deceleration tower. (4, 9, 18, 20-23) A similar study investigated the effects of varied helmet mass properties on human response during lateral +Gy impact on a horizontal impulse accelerator. (24) A continuation of this research was recently conducted evaluating helmet mass property effects during frontal impacts (-Gx). (8) Another study explored the effects of varied helmet weight on human neck response during retraction using the Body Positioning and Restraint Device. (BPRD) (29) Researchers have looked at how pre-existing neck strength and endurance may be correlated to the development neck pain and injury. (14) One may assume that a stronger, well-exercised neck would result in less pain, and injury, but data supporting this are inconsistent. (3, 11-12)
Electromyography (EMG) has been widely used to investigate muscle fatigue. EMG has become an increasingly popular and useful tool used to quantify muscle strength and fatigue. Typically, a muscle group is isolated, then monitored for strength and fatigue by measuring relative changes in EMG root mean square amplitudes and decreases in EMG frequency content. With surface EMG, fatigue is generally accompanied by increases in amplitude (7, 26) and shifts in the EMG spectrum to lower frequencies during prolonged contractions. (5, 6, 16, 26, 30, 34) Amplitude is a function of both the number of motor units recruited and the frequency of their discharge. The frequency components of EMG are a function of the duration of motor units’ action potentials, the geometry of the surface electrodes, the degree of motor unit synchronization, and the conduction velocity of action potentials on the sarcolemma. (25) Well prescribed methods exist for the use of EMG to quantify fatigue, but their efficacy in dynamic environments is uncertain. (10)
In 1983, Phillips and Petrofsky measured Maximum Voluntary Contractions of the upper trapezius and sternocleidomastoid muscle using a fixture called the isometric head dynamometer. (27) Subjects were fitted with flight helmets of various NVG configurations and asked to perform neck rotations for up to Thirty-five minutes. The goal of this study was to quantify the fatigue of neck muscles when loaded by weighted flight helmets. Another study by Phillips and Petrofsky evaluated neck muscle fatigue using different helmet weights and CGs28. A helmet simulator was used to simulate helmet weights up to 9.0 lbs with five different CG locations. Neck muscle fatigue was measured by isometric endurance time.
In 2001 the Naval Air Warfare Center (NAWC) reported an investigation of the dynamic strength capabilities of small-stature female pilots in performing tasks such as aerial combat maneuvers. The tasks were performed under simulated flight conditions in a dynamic flight simulator. Typical testing consisted of a sequence of turns lasting about forty-five minutes. The major concern being evaluated was whether the small females had sufficient upper body and neck strength, and endurance to perform the tasks.
The study showed that the four small subjects tested were able to demonstrate sufficient strength and endurance to safely fly physically strenuous missions. However, the subject sample was small and the authors felt that a larger subject sample was necessary to increase the statistical power of the results. (33) A second study by NAWC, also published in 2001, investigated the dynamic strength capabilities needed to exert the required ejection seat actuation pull force under various conditions including typical flight conditions while wearing helmets with added weight. In this study, all six small females were able to meet the minimum pull forces required to eject. However, some difficulties were noted with tasks that were affected by reach, fit, and accommodation issues. Again, the small subject sample limited generalization of the results to a larger population. (32)
The objective of this retrospective review of biomechanical studies was to present research data on the level of neck and upper torso fatigue while wearing helmets of varied mass properties (weight, CG, moments of inertia) for durations up to eight hours; and to propose an unsolicited request to review the Cervical Counter Weight (CCW) in the same research parameters. This review of upper torso and neck muscle fatigue present data that were quantified by measuring muscle activity, strength, endurance, discomfort, and performance while wearing five helmets with different weights, and mass distributions for up to eight hours. The results have been used to provide information regarding the safety and effectiveness of helmet-mounted systems during long operational missions. The results presented in this retrospective review provide a baseline for future human simulation model development.
While these helmet mounted systems undoubtedly increase a warfighter’s capabilities, one obvious drawback to putting all this equipment on the operator’s helmet is the increase in helmet weight that shifts the combined head and helmet CG forward, while increasing the moments of inertia on the neck. Some of the operational concerns associated with the use of a heavier, and possibly unbalanced system include upper torso, head, and neck muscle fatigue that may result in decreased performance, and the possibility of increasing neck injury risk during egress (ejection and sudden mounted vehicle deceleration or ballistic threat), especially the longer the helmet system is worn. These concerns are realistic and need to be considered due to prolonged helmet wear during long missions, and the expanded warfighter population that now includes small females and large males, (5th to 95th percentile anthropometric population). The most significant contributing factor in chronic neck pain is fatigue caused by the sustaining neck muscle, which is essential to hold the head in various positions. It is also important to study the long term unintended health outcomes related to fatigue including the cost of lost work days and cervical disability determination. It’s imperative that the Department of Defense find real solutions as a replacement for of makeshift alternatives. Upon a supplementary review regarding design features (without solicitation) it would appear that the SERE Industries’ CCW is one such device providing an example of human performance integration. The transferred distribution of weight obliquely uses the surface area of the helmet to counter multi-dimensional forces. The CCW example is using its low profile design to enhance stability closely following the quadratic radius contour of the helmet. This appears to be in direct correlation with previous research (36, 35) that these characteristics will reduce the overall Moment of Inertia with respect to a reduction of injury to neck joints. Furthermore, Perry & Buhrman, 1997; Ivancevic & Beagley, (2004.) recommend through their own independent argument that the head mounted load be chosen that has the smallest mass. Additionally, it should be noted that a measure of this research theory and others contradicts the notion of cautioning additional mass by suggesting a redistribution of load. This observation of redistribution is interesting, accurate, and unmistakable. Sovelius and colleagues (2008) found that the weight of the NVG’s themselves is very important because it shifts the center of gravity of the headborne weight, increasing the loads on the neck structures, and the frontal weight from the NVG’s causes a further increase in the activity of cervical muscles that are already subjected to high strain. From my professional and operational perspective additional advanced and empirical studies need to be conducted to establish a multi-dimensional understanding of the physical, biomechanical and cognitive stresses placed on our warfighter.
1. Albery, C.B., Kaleps, I. (1997) “A Procedure to Measure the Mass Properties of Helmet Systems” NDIA Design and Integration of Helmet Systems Symposium Proceedings,” Framingham, MA.
2. Andersen H. T. (1988) “Neck injury sustained during exposure to high-G forces in the F- 16B” Aviat. Space Environ. Med., 59(4), 356-8.