Trunk stiffness increases with steady-state effort

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Abstract

Trunk stiffness was measured in healthy human subjects as a function of steady-state preload efforts in different horizontal loading directions. Since muscle stiffness increases with increased muscle activation associated with increasing effort, it is believed that coactivation of muscles helps to stiffen and stabilize the trunk. This paper tested whether increased steady-state preload effort increases trunk stiffness. Fourteen young healthy subjects each stood in an apparatus with the pelvis immobilized. They were loaded horizontally at directions of 0, 45, 90, 135 and 180° to the forward direction via a thoracic harness. Subjects first equilibrated with a steady-state load of 20 or 40% of their maximum extension effort. Then a sine-wave force perturbation of nominal amplitude of 7.5 or 15% of maximum effort and nominal period of 250 ms was applied. Both the applied force and subsequent motion were recorded. Effective trunk mass and trunk-driving point stiffness were estimated by fitting the experimental data to a second-order differential equation of the trunk dynamic behavior. The mean effective trunk mass was 14.1 kg (s.d.=4.7). The trunk-driving point stiffness increased on average 36.8% (from 14.5 to 19.8 N/mm) with an increase in the nominal steady-state preload effort from 20 to 40% (F1,13=204.96, p<0.001). There was a smaller, but significant variation in trunk stiffness with loading direction. The measured increase in trunk stiffness probably results from increased muscle stiffness with increased muscle activation at higher steady-state efforts.

Introduction

Trunk stiffness is important not only in the elastic behavior of the upper body, but also in contributing to trunk stability. Stability is defined as the ability of a system to return to equilibrium after a small perturbation. The ligamentous spine is unstable at compressive loads of only 88 N (Crisco et al., 1992) while in vivo the compressive force acting on the spine can exceed 2600 N (Nachemson, 1966). Among the forces that return the trunk to an equilibrium position after a small displacement are those generated by elastic forces due to spine stiffness, augmented by activated muscle stiffness. It has been shown analytically that spine stiffness alone is insufficient, and that activated muscle stiffness is necessary for trunk stability (Bergmark, 1989; Cholewicki et al., 1997; Crisco and Panjabi, 1991; Gardner-Morse et al., 1995; Gardner-Morse and Stokes, 1998). Muscle stiffness increases with muscle activation as a result of the increased number of activated cross-bridges (Crisco and Panjabi, 1991; Ma and Zahalak, 1991). Therefore, theoretical considerations suggest that muscle stiffness contributes to trunk stability and that this mechanism could be controlled through modulation of muscle activation.

Theoretically, spine stiffness can be increased while maintaining equilibrium by increasing the coactivation of antagonistic muscles (Cholewicki et al., 1997; Gardner-Morse and Stokes, 1998) as has been shown experimentally in other joints (Baratta et al., 1988; Hunter and Kearney, 1982; Zhang et al., 1998). Disadvantages of coactivation are increases in tissue loading and metabolic energy consumption. While additional stiffness helps to stabilize the trunk, coactivation also paradoxically increases the compressive load that tends to destabilize the spine (Gardner-Morse and Stokes, 1998; Granata and Marras, 1995; Thelen et al., 1995). These qualitative concepts require quantitative experimental evidence to support them.

Trunk stiffness can be measured from the dynamic response of the trunk to a force or displacement perturbation. The dynamic behavior depends on the inertial, damping, and stiffness properties of the trunk and these variables can be extracted from the measured response to perturbations. The dynamic behavior of other joints have been studied either by measuring the displacements produced at the joint under controlled force inputs, or by measuring the forces produced at the joint under controlled displacement inputs (Kearney and Hunter, 1990). The inputs may be sinusoidal, pseudo-random, impulse, or a step function. For small displacements of a joint from a set position, a linear second-order differential equation adequately represents the dynamic behavior under a wide range of muscle activation (Kearney and Hunter, 1990).

Previous investigations of the trunk have reported changes in measured trunk stiffness with breath holding (increased intra-abdominal pressure) and belt wearing (Cholewicki et al., 1998; McGill et al., 1994), and decrease in the amount of trunk motion resulting from a perturbation with a increased flexion preload (Krajcarski et al., 1999). These reports suggest that the degree of muscle activation has an effect on trunk stiffness, but the exact relationship between the loading state, muscle activation and trunk stiffness is poorly understood. The purpose of this study was to measure the trunk driving point stiffness as a function of steady-state preload effort in different loading directions in the horizontal plane. These experiments were designed to test the hypothesis that increasing steady-state preload efforts increases trunk stiffness at all loading directions in the horizontal plane.

Section snippets

Methods

Fourteen young healthy human subjects were tested after they had signed a consent form approved by the institutional human research committee. There were 8 males and 6 females. The mean age was 25.7 years (range 20.7–33.2, s.d.=3.9); mean height was 1.76 m (range 1.59–1.90, s.d.=0.10); and mean body mass was 73.8 kg (range 52.6–102.1, s.d.=12.5).

Each subject stood in an apparatus that effectively immobilized the pelvis and they wore a harness around the thorax, attached via a cable and pulley to

Results

The measured trunk stiffness varied with steady-state effort (F1,13=204.96, p<0.001) and with loading direction (F4,47=2.81, p=0.036) (Fig. 4). There was an average 36.8% increase (from 14.5 to 19.8 N/mm) in the values of trunk stiffness with increased steady-state effort (pooled across loading directions and perturbation amplitudes). The stiffness at the 45° loading direction was significantly higher than the stiffness at 0, 90 and 180°, but not significantly different from the stiffness at

Discussion

Since muscle stiffness depends on muscle activation, we hypothesized that increasing steady-state trunk efforts would increase trunk stiffness. This experiment demonstrates that increasing muscle activation by increasing the steady-state effort from a nominal 20 to 40% of the maximum extension effort produced a 36.8% increase in measured trunk stiffness (Fig. 4). While the trunk stiffness varies with the loading direction, the trunk stiffness increased significantly with steady-state effort at

Acknowledgements

This work supported by National Institutes of Health grant R01 AR 44119. The authors thank Dr. Jack Winters for helpful initial discussions, Mr. David F. Norton for assembling the testing apparatus and conducting the experiments, and Mr. Gary Badger for his assistance with the statistical analyses.

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