Predictive Contribution of the Superficial Neck Muscles to Short-Latency Rate of Force Development of the Head and Neck.

PURPOSE: To evaluate the contribution of splenius capitis, sternocleidomastoid, and upper fibers of trapezius activation to the gains in rate of force development (RFD) of the head and neck during maximum voluntary ballistic contractions. METHODS: RFD gain was facilitated by a single-session intervention for maximum voluntary ballistic contractions in the anterior direction, oriented at 45° to the midsagittal plane, which require active restraint of axial rotation. Muscle activation for the agonist (sternocleidomastoid) and 2 antagonists (splenius capitis and upper fibers of trapezius) was evaluated. The study sample included 12 physically active men (mean age, 22.6 y). RFD (N·m·s-1; 0-100 ms) and integrated muscle activity (50 ms before and 100 ms after force onset) were measured at 10 minutes, 20 minutes, and 2 days postintervention, relative to baseline. Muscle activation predictive of RFD gains was evaluated by linear regression analysis. RFD reproducibility was evaluated using the coefficient of variation of the typical error. RESULTS: The intervention yielded a 1.95- to 2.39-fold RFD gain (P ≤ .05), with greater RFD gain for participants with a lower peak moment of force (<10.9 N·m) than those with a higher peak moment (≥10.9 N·m) at baseline (P ≤ .002). For the low group, 65% to 74% of the RFD gain was predicted by ipsilateral sternocleidomastoid activation, with ipsilateral splenius capitis activation predicting 77% to 92% of RFD gain for the high group. Absolute peak and impulse of static force were greater for the high than for the low group (P ≤ .04). RFD reproducibility was high (coefficient of variation of the typical error ≤ 14.4%). CONCLUSIONS: The agonist- and antagonist-focused synergies might reflect different functional priorities, higher RFD gain compared with higher head-neck force.

Investigation of impact loading rate effects on the ligamentous cervical spinal load-partitioning using finite element model of functional spinal unit C2-C3.

The cervical spine functions as a complex mechanism that responds to sudden loading in a unique manner, due to intricate structural features and kinematics. The spinal load-sharing under pure compression and sagittal flexion/extension at two different impact rates were compared using a bio-fidelic finite element (FE) model of the ligamentous cervical functional spinal unit (FSU) C2-C3. This model was developed using a comprehensive and realistic geometry of spinal components and material laws that include strain rate dependency, bone fracture, and ligament failure. The range of motion, contact pressure in facet joints, failure forces in ligaments were compared to experimental findings. The model demonstrated that resistance of spinal components to impact load is dependent on loading rate and direction. For the loads applied, stress increased with loading rate in all spinal components, and was concentrated in the outer intervertebral disc (IVD), regions of ligaments to bone attachment, and in the cancellous bone of the facet joints. The highest stress in ligaments was found in capsular ligament (CL) in all cases. Intradiscal pressure (IDP) in the nucleus was affected by loading rate change. It increased under compression/flexion but decreased under extension. Contact pressure in the facet joints showed less variation under compression, but increased significantly under flexion/extension particularly under extension. Cancellous bone of the facet joints region was the only component fractured and fracture occurred under extension at both rates. The cervical ligaments were the primary load-bearing component followed by the IVD, endplates and cancellous bone; however, the latter was the most vulnerable to extension as it fractured at low energy impact.

Effect of the transverse ligament rupture on the biomechanics of the cervical spine under a compressive loading.

BACKGROUND: In order to diagnosis a transverse ligament rupture in the cervical spine, clinicians normally measure the atlas-dens interval by using CT scan images. However, the impact of this tear on the head and neck complex biomechanics is not widely studied. The transverse ligament plays a very important role in stabilizing the joint and its alteration may have a substantial effect on the whole head and neck complex. METHODS: A finite element model consisting of bony structures along with cartilage, intervertebral discs and all ligaments was developed based on CT and MRI images. The effect of head weights (compressive load) of 30 N to 57 N was investigated in the cases of intact and ruptured transverse ligament joints. The model was validated based on experimental studies investigating the response of the cervical spine under the extension-flexion moment. FINDINGS: The predictions indicate a significant alteration of the kinematics and load distribution at the facet joints of the cervical spine with a transverse ligament tear. The vertebrae flexion, the contact force at the facets joints and the atlas-dens interval increase with the rupture of the transverse ligament and are dependent to the head weight. INTERPRETATION: A transverse ligament tear increases the flexion angle of the head and the vertebrae as well as the atlas-dens interval. The atlas-dens interval reaches a critical value when the compressive loading exceeds 40 N. Supporting the head after an injury should be considered to avoid compression of the spinal cord and permanent neurologic damage.