Passive Cervical Spine Ligaments Provide Stability during Head Impacts in Vivo

TL;DR

This study quantifies how passive cervical spine ligaments and neck muscles contribute to head stability during impacts, revealing ligaments' significant role at high impact rates, which limits muscle strengthening effectiveness.

Contribution

It introduces a musculoskeletal model to differentiate passive ligament and active muscle contributions to head stabilization during impacts.

Findings

Ligaments provide greater stabilizing impulses at high impact rates.

Active muscles contribute significantly during impact acceleration and deceleration.

Ligament stiffness increases with impact rate, reducing reliance on muscle strength.

Abstract

It has long been suggested that neck muscle strength and anticipatory cocontraction can decrease head motions during head impacts. Here, we quantify the relative angular impulse contributions of neck soft tissue to head stabilization using a musculoskeletal model with Hill-type muscles and rate-dependent ligaments. We simulated sagittal extension and lateral flexion mild experimental head impacts performed on 10 subjects with relaxed or cocontracted muscles, and median American football head impacts. We estimated angular impulses from active muscle, passive muscle, and ligaments during head impact acceleration and deceleration phases. During the acceleration phase, active musculature produced resistive angular impulses that were 30% of the impact angular impulse in experimental impacts with cocontracted muscles. This was reduced below 20% in football impacts. During the deceleration phase, active musculature stabilized the head with 50% of the impact angular impulse in experimental impacts with cocontracted muscles. However, passive ligaments provided greater stabilizing angular impulses in football impacts. The redistribution of stabilizing angular impulses results from ligament and muscle dependence on lengthening rate, where ligaments stiffen substantially compared to active muscle at high lengthening rates. Thus, ligaments provide relatively greater deceleration impulses in these impacts, which limits the effectiveness of muscle strengthening or anticipated activations.