Presented at NACOB 98: North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998

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THE IN VIVO DYNAMIC RESPONSE OF THE SPINE TO PERTURBATIONS
CAUSING RAPID FLEXION: EFFECTS OF PRE-LOAD AND STEP INPUT MAGNITUDE

SR. Krajcarski ¹ , JR. Potvin ² , J. Chiang ^1
1 School of Human Biology and Nutritional Sciences, University of Guelph
Guelph, ON, Canada, N1G 2W1
² School of Human Kinetics, University of Windsor, Windsor, ON, Canada, N9B 3P4

INTRODUCTION

In several recent in vivo studies, levels of trunk muscle cocontraction have been observed prior to the application of unexpected or sudden loads. To date, little work has been done to evaluate the impact of this muscle pre-activation on the response of the trunk to different loading conditions. This study examines the effect of different levels of muscle pre-activation and load magnitudes on the response of the trunk to unexpected loading conditions causing rapid flexion of the trunk

REVIEW AND THEORY

Albeit mechanically and energetically costly, cocontraction of the trunk musculature is required to maintain stability during normal physiological loading (Lavender et al. 1992). The magnitude of this cocontraction has been shown to increase with trunk flexion angle, asymmetry, extension velocity, acceleration and exertion level (Granata et al., 1995). During unexpected loading trials, the activation of both antagonists and agonists has been shown to over-react by a factor of two when compared to the same loads under expected conditions (Marras et al. 1987). Other studies have also demonstrated an increase in muscle cocontraction prior to the application of sudden loads (Cholewicki et al. 1996; Lavender et al., 1993). The effect of this muscle "preactivation" or "pretensionning" on the trunk response to unexpected loading remains unclear. Insight into this relationship may provide a better understanding of the dynamic stabilizing system of the spine in vivo.

The purpose of this study was to determine the responses of the trunk to combinations of initial static loads and step inputted sagittal plane loads. Of specific interest was the differences in the response of the trunk musculature between two loading conditions that yielded a common final load. It was hypothesized that the condition with the smaller pre-load and larger added load would result in greater flexion displacements of the trunk, trunk extension moments and EMG amplitudes when compared to the loading condition with a larger pre-load and initial trunk stiffness.

PROCEDURES

Eight healthy male subjects were asked to maintain an upright standing posture within a specially designed test apparatus while resisting the application of forward flexion moments produced by 4 different loading conditions. Each loading condition consisted of a combination of a pre-load (4 or 16% of subject's maximum isometric extensor moment (IEM_max)) and an added load (12 or 24% IEM_max) resulting in final loads of 16% [4+12], 40% [16+24] and two conditions of 28% IEM_max [16+12] and [24+4]. Approximately 10 trials were collected for each trial.

Measurements were made of the trunk extensor moments, angular displacement of the trunk and unilateral surface EMG amplitudes of three abdominal (rectus abdominus (RA), internal (IO) and external (EO) obliques, and three trunk extensor muscles (latissimus dorsi (LD), thoracic (TES) and lumbar (LES) erector spinae. Values were recorded during the isometric pre-load and for the maximum magnitude of each variable in response to the added load.

The data was analyzed with a 2-way within subjects multivariate analysis of variance (MANOVA) for repeated measures and orthogonal means comparisons were used to determine which pairs of means were significantly different (alpha = 0.05)

RESULTS AND DISCUSSION

A summary of the results obtained from this study are presented in Table 1. Higher pre-loads resulted in lower flexion rotations of the spine and higher added loads caused larger rotations (Table 1, Figure 1). With increasing magnitudes of final loads, a corresponding increase in trunk extensor moments and trunk muscle cocontraction were observed (Figure 2). The largest activations were observed in the LES and TES muscles, while smaller yet substantial EMG activity was observed in the IO and EO, particularly in response to perturbations at low levels of initial spine stiffness.

Table 1. Summary of post hoc findings comparing maximum observed variables in response to added loads. Conditions are ranked from 1 (highest) to 4 (lowest), with cells having the same number being not statistically significant at p<0.05.

Between the conditions with the final common load, the condition with the lower pre-load resulted in larger angular displacements, larger maximum moments and higher increases in EMG amplitude for both extensor and flexor muscles. Presumably, the higher levels of pre-activation can serve to enhance trunk stiffness and attenuate the flexion displacements caused by rapid loading. The results of this study provided insight into several mechanisms involved in the dynamic stability of the spine. Although further studies of the reflex response to rapid loading are needed, the results from this study suggest that pre-activation during trunk loading and trunk muscle cocontraction may play and important role in enhancing spine stability. Such studies may be critical to better understanding the mechanisms causing lumbar tissue injuries during load bearing.

REFERENCES

Lavender, S. et al. J Orthop Res 10:691-700, 1992.

Granata, K.P. et al. Spine, 20(8):913-919, 1995.

Marras, W.S. Ergonomics, 30(3):551-562, 1987.

Cholewicki, J. et al. Clin. Biomech. 11(1):1-15., 1996.

Lavender, S. et al. Spine, 18(14):2097-2105, 1993.