Standardized assessment of static neck strength capacity for representative in-flight head postures of CAF helicopter aircrew

Introduction: The capacity of the human neck to generate static moments of force is key in maintaining the stability of the cervical spine (c-spine) under various conditions of dynamic external load application. Compared to the lower cspine (T \u2081), the upper c-spine (C \u2080-C \u2081) has a decreased capacity to generate moments, and is therefore more susceptible to injury, as forces that exceed this maximal capacity will be borne by passive tissues (ligaments and vertebrae). Deviations in head posture from neutral will modify the balance of muscle moments, increasing the risk of upper cspine injury. The aim of this study was to quantify the effects of seven postures, typically held by helicopter aircrew during in-flight, on the static strength capacity of the upper and lower cspine of Canadian Armed Forces (CAF) helicopter aircrew. Methods: Seven postures were identified by measuring head-neck-torso angles, relative to self-selected neutral, of a CAF helicopter pilot seated in a Bell-412 helicopter: 1) Left 20\u2da rotation, 20\u2da flexion, 2) 0\u2da rotation, 0\u2da flexion; 3 and 4) left and right 45\u2da rotation, respectively, 0\u2da flexion; 5) left 45\u2da rotation, 20\u2da flexion; 6) 0\u2da rotation, 10\u2da extension; 7) right 45\u2da rotation, 10\u2da extension. Two male CAF CH-146 helicopter pilots (36 and 49 years old) performed one maximal voluntary isometric muscle effort (MVIE) in the seven head-on-neck postures. MVIEs were measured using a custom-modified Multi-Cervical Unit (2048Hz; BTE Technologies). Absolute peak external force (N) was measured using a thin-beam strain gauge load cell. Anatomical landmarks were digitized to establish reference points at C \u2080-C \u2081 and T \u2081; (30Hz; Optotrak). Resultant peak moments (Mpeak, Nm) were calculated as the cross product of the external force and the distance from the load cell to the anatomical reference points. Results: Normalized T \u2081 Mpeak relative to neutral (posture 2) was 0.85, 1.00, 0.87, 0.92, 0.53, 0.63, and 0.40 (postures 1-7, respectively). Magnitude of Mpeak generated at C \u2080-C \u2081 was 41% compared to T \u2081. Head postures 3 and 5 elicited differential C \u2080-C \u2081 vs. T \u2081 Mpeak ratios of 47% vs. 34% and 44% vs. 26% (pilot 1 vs. 2, respectively). Conclusions: Head postures that combine sagittal head flexion or extension with axial rotation decrease the moment generating capacity and increase the vulnerability of the upper c-spine to injury. Static neck strength assessment in representative in-flight postures may help to identify pilots who may be at risk of developing flight-related neck pain, and identify postures that may increase the risk of injuryPeer reviewed: YesNRC publication: Ye