Effects of vibration exposure on intrinsic muscle properties

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The functions of skeletal muscles include generating forces, facilitating movements, regulation of joint stiffness, and damping of soft tissue vibrations. While the force generation of muscles in relation to movement and joint stiffness has been extensively investigated, there are still open questions regarding the effect of vibrations on the muscles and the mechanisms that might contribute to muscular vibration damping. In this project we investigate the effects of vibrations on muscle force generation. We expect that the intrinsic muscle property of force depression, which has been observed when a contracting muscle is shortened, plays a role when muscles are exposed to vibrations. We speculate that force depression during vibration exposure might either be observed directly as a reduction of the maximal force that the muscle can produce, or it might be observed indirectly as a change in the recruitment of muscle fibers. We will use a custom built exciter to induce longitudinal vibrations into a group of muscles while contracted at maximum voluntary contraction. Muscle force generation will be quantified. Recruitment of muscle fibers during these experiments will be investigated non-invasively by analyzing the muscular activation patterns and by determining the concentration ratio between oxygenated and de-oxygenated hemoglobin in the muscle tissue. Muscle activation patterns will be measured with surface electromyographic sensors and analyzed using a time and frequency resolving wavelet transformation. The ratio between oxygenated and de-oxygenated hemoglobin concentration will be quantified using near infrared spectroscopy. In addition, blood flow within the muscle tissue will be measured using Doppler ultrasound. Damping of soft tissue vibrations is one of the key mechanisms to reduce joint loading and to prevent injuries due to impact shocks. The vibration dampening function of skeletal muscles is therefore essential when coping with the repetitive impact shocks and related vibrations that the body is exposed to during most forms of locomotion. If force depression proves to play a role in how muscles contract during vibration exposure, then it will also affect how the body copes with impact shocks or vibrations.