The relationship between trunk muscle activation and trunk stiffness: examining a non-constant stiffness gain

Abstract

The relationship between muscle activation, force and stiffness needs to be known to interpret the stability state of the spine. To test the relationship between these variables, a quick release approach was used to match quantified torso stiffness with an EMG activation-based estimate of individual muscle stiffnesses. The relationship between activation, force and stiffness was modelled as , where k, F and l are muscle stiffness, force and length, respectively, and q is the dimensionless stiffness gain relating these variables. Under the tested experimental scenario, the ‘stiffness gain’, q, which linked activation with stiffness, demonstrated a decreasing trend with increasing levels of torso muscle activation. This highlights the likelihood that the choice of a single q value may be over simplistic to relate force to stiffness in muscles that control the spine. This has implications for understanding the potential for spine instability in situations requiring high muscular demand.

Keywords:

• spine
• stability
• abdominal muscles
• muscle force
• muscle stiffness

Acknowledgement

The authors would like to thank the continued financial support of the Natural Sciences and Engineering Research Council (Canada).

Notes

1. Present Address: Department of Orthopaedic Surgery, University of California San Diego, 3525 John Hopkins Court, San Diego, CA 92121, USA. [email protected]

Additional information

Notes on contributors

Stephen H.M. Brown

1 1. Present Address: Department of Orthopaedic Surgery, University of California San Diego, 3525 John Hopkins Court, San Diego, CA 92121, USA. [email protected]