Elsevier

Journal of Biomechanics

Volume 41, Issue 10, 19 July 2008, Pages 2097-2103
Journal of Biomechanics

Role of loading on head stability and effective neck stiffness and viscosity

https://doi.org/10.1016/j.jbiomech.2008.05.002Get rights and content

Abstract

This experiment tests the hypothesis that loading the head would increase head stability. In particular, we hypothesized that an arrangement of the head so that muscle activation is required to counteract a load would significantly increase effective neck stiffness and viscosity, which would be associated with lower peak head angular velocity following abrupt force perturbations applied to the head. Seven young healthy subjects had their head loaded (preload) using a weight/pulley apparatus. Then, the head was pulled either forward or backward by dropping an additional weight onto the preload, causing an impulse of force followed by an increase in load. We recorded the applied force and head angular velocity. Neck viscoelastic properties as a function of loading were estimated by fitting experimental data to a second-order mathematical model of the head biomechanics. Across preloads varying from 2.22 to 8.89 N, peak head angular velocity decreased by 18.2% for the backward and by 19.9% for forward perturbations. As preload increased, simulated effective neck stiffness and viscosity significantly increased leading to lower peak angular velocity. These results demonstrated that loading reduces peak head angular velocity and that change in muscle stiffness and viscosity is a feasible explanation for this effect. We propose that reduction in peak head velocity could be caused by modulation of the strength of the vestibulo-collic reflex.

Introduction

Stabilizing the head–neck system is a challenging problem; a large mass is balanced on the top of a multilinked column. Following head perturbation or trunk motion, restoring forces from intrinsic viscoelastic properties of neck muscles, reflex mechanisms, and voluntary intervention stabilize the head and prevent it from falling over (Anderson and Winters, 1990; Viviani and Berthoz, 1975). Stability here is defined as reduction of peak head velocity following the perturbation. An important component of head stabilization is the viscoelastic properties of the neck system. The critical stiffness in the neck needed to keep the head in static equilibrium against the force of gravity is approximately 10 N m/rad (Anderson and Winters, 1990). This value is considerably larger than passive stiffness measured for small angles of flexion or extension, which is roughly 2 N m/rad (McGill et al., 1994; McClure et al., 1998).

In situations where a perturbation of the head is applied, the brain could adjust muscle activation through co-contraction to alter neck stiffness and viscosity. Muscle co-contraction, could be more effective than reflex or voluntary strategies as it is not subject to intrinsic delays. Although, muscle co-contraction may increase stability (Gardner-Morse and Stokes, 1998), it increases tissue loading, compression and metabolic energy consumption and reduces flexibility (Granata and Marras, 2000; Andersen et al., 2004). Although no specific data is available concerning the effect of neck muscle activation on stiffness, it is accepted that in general muscle activation increases stiffness (Weiss et al., 1986; Crisco and Panjabi, 1991; Ma and Zahalak, 1991). In the lumbar region, greater trunk muscle activation increases stiffness (Janevic et al., 1991; Krajcarski et al., 1999; Cholewicki et al., 2000; Gardner-Morse and Stokes, 2001; Andersen et al., 2004). Similarly, ankle stiffness increases with activation of muscles crossing this joint (Weiss et al., 1988). Thus, it seems likely that head loading should increase neck stiffness. It also seems likely that larger stiffness might reduce peak head angular velocity when a perturbation is applied.

Very few measurements, however, have been made on the variation in viscoelastic properties in the neck as a function of head loading. More than 20 years ago, Reid et al. (1981) used an analog computer to fit the results of perturbations to the head. They observed that stiffness was variable over a 4.6-fold range, and that adding a preload caused stiffness to increase. Additionally, they found a 3.3-fold change in viscosity. Unfortunately, the authors did not report if greater neck stiffness and viscosity led to better head stability and they did not report the effect of various preloads.

The purpose of the experiment was twofold: (i) to investigate how neck joint stiffness and viscosity in healthy adult varied as a function of loading; (ii) to determine if changes in neck stiffness and viscosity can be beneficial for head stability (i.e., reduce peak head angular velocity). Neck stiffness can be measured from kinematic response of the head to a force perturbation. The kinematic response of the system depends on the inertia of the head, and the viscosity and stiffness of the neck. We inferred neck joint stiffness and viscosity by fitting experimental data associated with abrupt forward or backward force perturbations applied to the head, in the presence of various initial head loads. We hypothesized that increasing loading prior to the perturbation would significantly increase neck stiffness and viscosity. Finally, we hypothesized that greater neck stiffness and viscosity should reduce peak head angular velocity.

Section snippets

Materials and methods

Seven healthy subjects (four males and three females; mean age 23.5 years; mean height 1.75 m (range 1.65–1.85 m); mean weight 75.9 kg (range 55–114 kg)) with no history of neck and/or back pain volunteered for the experiment. Each participant signed the informed consent form outlining the protocol approved by the Northwestern University Institutional Review Board.

Our experiment was designed to apply a load (preload) to the head–neck system, which we assumed increases neck muscle activation, and

Results

Head stability was significantly improved (lower peak head angular velocity) with greater preload (Fig. 3, upper panel). The ANOVA revealed that forward and backward peak head angular velocity were significantly reduced across preloads (F3,18=30.80, P<0.001). The average decrease of the peak head angular velocity over the range of loading was 18.2% and 19.9% in backward and forward direction, respectively. There was no main effect of perturbation direction (F1,6=0.39, P>0.05) and the

Discussion

We measured the effect of head loading on peak head angular velocity and effective neck stiffness and viscosity following head perturbation in the pitch plane. We assumed that loading would be accompanied by greater neck muscle activation prior to head perturbation, and would increase effective neck stiffness and viscosity. This hypothesis agree with previous investigations (Krajcarski et al., 1999; Cholewicki et al., 2000; Gardner-Morse and Stokes, 2001; Hunter and Kearney, 1982; Weiss et al.,

Conflict of interest

There is no conflict of interest. Authors have not received any payment for conducting this work and are in no conflict of interest.

Acknowledgments

This study was supported in part by grants from NSERC discovery program to MS and the public health service (NIH/NINDS RO1 NS38286) to TCH. The authors thank Wynne A. Lee for her contribution to earlier versions of this manuscript and Geneviève Gagnon and Despina Kotsapouikis for their help during data acquisition and data reduction.

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