Passive neck stiffness and range of motion for males and females from early to late adulthood.

Background context: Understanding human passive neck range of motion (ROM) and stiffness, and their association with age and sex, can be beneficial for clinical assessment, and surrogate and computational modelling.

Purpose: To assess passive head-neck ROM and stiffness, and to investigate association with age and sex, in flexion, extension, left and right lateral bending, and axial rotation.

Study design/setting: In-vivo human participant testing.

Methods: Eighty participants aged 20 to 79 years (nearly even distribution), who self-reported no history of significant health conditions and with no neck pain, were recruited. Two custom apparatus were used to support participants in relaxed lying. Their head was rotated to maximum ROM; applied moment and head-torso motion were recorded. Muscle activation was monitored in real-time to ensure electromyographic signals from agonist muscles remained below a passive threshold. Stiffness was determined from the moment-angle data within each of three zones, with zone boundaries delineated to maximize moment-angle linearity within each zone. The age and sex effects on passive stiffness and ROM were assessed using generalized linear models for flexion and extension, and linear mixed models for lateral bending and axial rotation.

Results: Passive neck ROM decreased by 0.2° per year of age in lateral bending and axial rotation for males and females, and extension ROM for males was 5.8° lower than for females. Passive stiffness in lateral bending (zone 1 and 2: 0.9 and 3.5 Nmm/°/year; zone 3: 3%), axial rotation (zone 1 and 2: 1%; zone 3 for males and females: 1.9 and 0.9 Nmm/°/year) and some zones in extension (zone 2: 0.8 Nmm/°/year; males in zone 3: 2.7 Nmm/°/year) increased with age, and males had higher stiffness than females in lateral bending (zone 1 and 2: 22.3 and 43.9 Nmm/°; zone 3: 35%) and axial rotation (zone 1 and 2: 49% and 35%).

Conclusions: Passive neck ROM decreased with age in lateral bending and axial rotation, while passive neck stiffness tended to increase with age in all motions but flexion. Extension ROM was higher for females, and lateral bending and axial rotation stiffness at lower angles were higher for males.

Clinical significance: The neck ROM, stiffness, and moment-angle corridors developed in this study provide benchmarks for clinical assessment of cervical spine function, and can assist the development of surrogate and computational models incorporating minimal muscle activation, for injury simulation and clinical skill training.