Journal of Biomechanics 34 (2001) 539–543 Technical note Soft-tissue vibrations in the quadriceps measured with skin mounted transducers James M. Wakeling*, Benno M. Nigg Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Accepted 26 October 2000 Abstract The purpose of this study was to develop a method to characterize the frequency and damping of vibrations in the soft tissues of the leg. Vibrations were measured from a surface-mounted accelerometer attached to the skin overlying the quadriceps muscles. The free vibrations in this soft tissue were recorded after impact whilst the muscle was performing isometric contractions at 0, 50, and 100% maximum voluntary force and with the knee held at 20, 40, and 608 angles of flexion. The acceleration signals indicated that the soft tissue oscillated as under-damped vibrations. The frequency and damping coefficients for these vibrations were estimated from a model of sinusoidal oscillations with an exponential decay. This technique resolved the vibration coefficients to 2 and 7% of the mean values for frequency and damping, respectively. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Accelerometer; Damping; Frequency; Soft-tissue; Vibration 1. Introduction Impact forces, which occur at heel strike during running may cause the soft tissues of the leg to vibrate. It has been suggested that the soft tissues are critically damped in order to minimize such vibrations, and that the frequency and damping of the vibrations are controlled by the muscle activity within the tissue (Nigg, 1997). A recent theoretical approach has shown how the response of the body to ground impact could be modified by changes in the coupling between the soft and rigid tissues of the body (Nigg and Liu, 1999); however the actual vibration response of the soft tissues has never been directly measured. The effect of the impact shock on body accelerations has been extensively studied for the rigid, skeletal, tissues. Accelerations due to impact have been quanti- fied using transducers either mounted onto the skin (Derrick et al., 1998; Gross and Nelson, 1988; Hamill et al., 1995; Lafortune et al., 1995; Light et al., 1980; Saha and Lakes, 1977; Valiant et al., 1987), onto bone pins inserted into the tibia (Lafortune et al., 1995; Light et al., 1980), or grasped between the teeth for head movements (Light et al., 1980). Tibial accelerations measured from skin-mounted transducers may show a loss of high-frequency components (Light et al., 1980) due to the movement of the soft tissue between the accelerometer and the tibia. Such movement artefacts can be minimized by using a low-mass accelerometer and preloading the accelerometer onto the skin surface (Valiant et al., 1987). In this study we have developed a method to measure the accelerations occurring in the soft tissues of the quadriceps. Such soft-tissue accelera- tions have previously been considered as artefacts when measuring tibial accelerations but are actually the measurement of interest for this study. The design solutions from the tibial studies are still adhered to with low-mass accelerometers being preloaded onto the soft tissue in order to maximise congruence of motion between the accelerometer and the underlying tissue. However, we record from areas where the underlying tissue is the quadriceps muscle, rather than from the shank where there is little soft tissue covering the tibia. The purposes of this study were (a) to develop a method to determine frequency and damping coeffi- cients for vibrations in the soft tissues of the leg and (b) to indicate the order of magnitude of changes in these characteristics as a function of the intensity of muscle *Corresponding author. Tel.: +1-403-220-7004; fax: +1-403-284- 3553. E-mail address: wakeling@kin.ucalgary.ca (J.M. Wakeling). 0021-9290/01/$-see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII:S0021-9290(00)00203-7