Effect of neck muscle strength and anticipatory cervical muscle activation on the kinematic response of the head to impulsive loads

Abstract

Background: Greater neck strength and activating the neck muscles to brace for impact are both thought to reduce an athlete's risk of concussion during a collision by attenuating the head's kinematic response after impact. However, the literature reporting the neck's role in controlling postimpact head kinematics is mixed. Furthermore, these relationships have not been examined in the coronal or transverse planes or in pediatric athletes.

Hypotheses: In each anatomic plane, peak linear velocity (ΔV) and peak angular velocity (Δω) of the head are inversely related to maximal isometric cervical muscle strength in the opposing direction (H1). Under impulsive loading, ΔV and Δω will be decreased during anticipatory cervical muscle activation compared with the baseline state (H2).

Study design: Descriptive laboratory study.

Methods: Maximum isometric neck strength was measured in each anatomic plane in 46 male and female contact sport athletes aged 8 to 30 years. A loading apparatus applied impulsive test forces to athletes' heads in flexion, extension, lateral flexion, and axial rotation during baseline and anticipatory cervical muscle activation conditions. Multivariate linear mixed models were used to determine the effects of neck strength and cervical muscle activation on head ΔV and Δω.

Results: Greater isometric neck strength and anticipatory activation were independently associated with decreased head ΔV and Δω after impulsive loading across all planes of motion (all P < .001). Inverse relationships between neck strength and head ΔV and Δω presented moderately strong effect sizes (r = 0.417 to r = 0.657), varying by direction of motion and cervical muscle activation.

Conclusion: In male and female athletes across the age spectrum, greater neck strength and anticipatory cervical muscle activation ("bracing for impact") can reduce the magnitude of the head's kinematic response. Future studies should determine whether neck strength contributes to the observed sex and age group differences in concussion incidence.

Clinical relevance: Neck strength and impact anticipation are 2 potentially modifiable risk factors for concussion. Interventions aimed at increasing athletes' neck strength and reducing unanticipated impacts may decrease the risk of concussion associated with sport participation.

Keywords: head injuries/concussion; head kinematics; injury biomechanics; neck muscle activation; neck strength.

Figures

Figure 1

(Top left) Impulsive loading apparatus configured for forced extension head pulling. Constant force springs (t) apply light, counterbalanced, pretension to maintain the cables taut at any head position. The 1 kg drop weight (W) is released from a height (h) onto the cable clamp (c) such that both the weight and clamp fall from height (k) onto the safety stop applying an impulsive test force to the head, with cable tension recorded by the in-line force transducer (T). (Top right) Close up view of the modified wrestler's headgear illustrating the relative positions of the cable connecting pin and the estimated head center of mass (X), with posteriorly directed cable and in-line force transducer (T) configured for flexion strength testing. (Bottom, from left to right) Differential cable configuration for impulsive loading in sagittal plane extension and flexion, coronal plane right lateral flexion, and axial plane left rotation.

Figure 2

Representative kinematic tracings illustrating typical linear velocity (top row) and angular velocity (bottom row) vs. time plots over the first 150 ms following impulsive loading in each direction of head motion. Motion in the plane of force application is presented in dark blue (sagittal plane for flexion and extension, coronal plane for lateral flexion, axial plane for rotation), while the components of motion in the two non-primary planes are presented in red. For each tracing, positive velocity in the primary plane of motion is associated with motion in the same direction as external force application (forward for sagittal flexion, backward for sagittal extension, etc.). Asterisks at peak linear and angular velocity points represent ΔV and Δω values.

Figure 3

Scatterplots illustrating the inverse relationships between maximum isometric neck strength and the primary kinematic outcomes (ΔV and Δω) in each plane of motion. For each plot, ΔV (left vertical axis scale) is illustrated in solid blue, Δω (right vertical axis scale) is illustrated in open red, the baseline cervical muscle activation condition is depicted by squares, and the anticipatory cervical muscle activation condition is depicted by circles.

Figure 4

Boxplots illustrating the effect of anticipatory cervical muscle activation on ΔV and Δω. Center box line = median value; Center X = mean value; Box Limits = Interquartile range; Whisker Limits = 1.5*Interquartile range (corresponding to approximately 2.7 standard deviations from the mean, truncated to the most extreme data point); Outer plus signs = outlier data points.