Movement of the upper body and muscle activity patterns following a rapidly applied load: the influence of pre-load alterations

Abstract

Sudden loading of the spine is not only considered a risk factor for the development of low-back pain but also enables an evaluation of the stability of the spine when conducted under laboratory conditions. In the present study the upper spine was pulled in the anterior direction and the stiffness as well as activity in the erector spinae muscle was measured with different pre-tension in the erector spinae. The results showed that increased activity in the erector spinae prior to loading led to increased stiffness (stiffness coefficients from 297 Nm rad(-1) to 438 Nm rad(-1)) and a decrease in the extra neural signal input to the muscles to maintain the stability. It is therefore clear that increased tension in the erector spinae muscle will create a larger stability of the spine to anterior perturbations. However, contracting the muscles around the spine increases the load on the spinal structures. In 34% of the experiments a silent period in the electromyographic signal was present after loading in the period when the torso was moving in the anterior direction. This phenomenon is discussed.

Trunk stiffness and dynamics during active extension exertions

“…Joint stiffness has been found to increase with increasing muscle activation in the ankle (Agarwal and Gottlieb, 1977;Hunter and Kearney, 1982), elbow (Zhang and Rymer, 1997), and trunk (Cholewicki et al, 2000;Gardner-Morse and Stokes, 2001;Bull Andersen et al, 2004). Therefore, we hypothesized that the method we developed for calculating effective trunk stiffness would show an increase in stiffness with steady-state preload.…”

“…Considering that the major contributing factor to spinal stability is the recruitment and neuromuscular control of active muscle stiffness, research is necessary to investigate the stiffness of the trunk during active exertions. Active trunk dynamics have previously been investigated by applying either transient (Cholewicki et al, 2000;Bull Andersen et al, 2004) or sinusoidal (Gardner-Morse and Stokes, 2001) force inputs to the trunk and fitting a secondorder model to the resulting kinematic data. Cholewicki (2000) utilized a sudden resisted-force release input, while Bull Andersen (2004) used a rapidly applied load.…”

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

“…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;GardnerMorse and Stokes, 2001;Andersen et al, 2004). Similarly, ankle stiffness increases with activation of muscles crossing this joint (Weiss et al, 1988).…”

“…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) 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).…”

Trunk response analysis under sudden forward perturbations using a kinematics-driven model

“…Contribution of the passive subsystems (i.e., disc, ligaments and facets) has been demonstrated to increase at greater trunk flexions (Arjmand andShirazi-Adl, 2005a, 2006;Bazrgari et al, 2008c;Granata and Wilson, 2001) and compression loads (Shirazi-Adl, 2006;Stokes and Gardner-Morse, 2003) whereas it diminishes at near neutral standing postures (El-Rich et al, 2004;Granata and Wilson, 2001). Antagonistic pre-activation of trunk muscles increases the trunk intrinsic stiffness thereby decreasing the perturbation magnitude under sudden loads (Andersen et al, 2004;Brown and McGill, 2008;Cholewicki et al, 2000;Krajcarski et al, 1999;Vera-Garcia et al, 2006). It hence yields higher effective stiffness and a more stable system but at an increased metabolic cost (Franklin and Granata, 2007) and loads on the spine passive tissues (Arjmand et al, 2008;Bazrgari et al, 2008c;Vera-Garcia et al, 2006).…”