ELSEVIER J. E/ecrrom,wgr. Kinesid. Vol. 7, No. 2, pp. I 13-121, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1050-641 l/97 $17.00 + 0.00 PII: SlOSO-6411(96)00023-5 The Effect of Muscle Length on Electrically Elicited Muscle Vibrations in the In-situ Cat Soleus Muscle Marco A. Vaz, Walter Herzog, Yuan-Ting Zhang, Tim R. Leonard and Hoa Nguyen Hlhmnn Pe$ormance Laboratory, The University of Calgary, Alberta, Calgary, Canada Summary: The effects of length changes in the in-situ cat soleus muscleon vibromyographic (VMG) signalswere assessed using electrical stimulation of the soleus nerve in three adult male cats. Force and VMG signals were measured using an E-shaped force transducerand a miniature, unidirectional accelerometer, respectively. In each test, the soleus nerve was stimulated for 6 s at rates ranging from 4 to 35 Hz, and at two to four ankle angles(80- 140”). The force of the soleus muscleincreased with increasing musclelength and stimulation rates.For a given sub-tetanic frequency of stimulation,the root mean square (RMS) values of the VMG signal were larger at intermediate musclelengthsthan at both the longestand the shortest musclelengths.There was a continuous increasein the RMS of the VMG signal with decreasing musclelength when the contraction was tetanic. There was a trend towards an increase in the median frequency (MDF) of the VMG signal with increasing muscle. length. The results of this study support the idea that the amplitude and frequency content of the VMG signal during electrically elicited muscle contractionsare directly affected by changes in the mechanical properties (i.e. the stiffness and the active and passivetension) of muscle caused by length changes. 0 1997 Elsevier ScienceLtd. Key Words: Vibromyography-Electrical stimulation-Muscle length-Motor units-Root meansquare-Median frequency. J. Electromyogr. Kinesiol., Vol. 7, 113-121, June INTRODUCTION Muscle sounds (acoustic myography; AMG) are produced by vibrations which originate from inside the muscle”,‘3. Vibromyography (VMG) is a new technique that has been used to record these vibrations on the surface of muscles during contrac- tion. It has been suggested that muscle Received 12 July 1995. Revised 21 May 1996. Accepted 17 June 1996. Correspondence and reprint requests to Walter Herzog, Human Performance Laboratory. The University of Calgary, 2500 Univer- sity Drive N.W., Calgary, Alberta, Canada T2N lN4. sounds/vibrations originate from the movements pro- duced by the unfused contraction of motor units**.*“,*‘. If so, vibromyography may possibly be used to assess qualitatively the changing behaviour of motor units with increasing muscle forces. A mechanical vibration is the motion of a body which oscillates about a position of equilibrium”. This motion, or oscillation, is usually caused by a forcing function, and depends on the material properties of the system. When this oscillatory motion reaches the surface of the system, areas of high and low pressure occur because of displace- ments of molecules of the medium in which the system is immersed’” (e.g. air, water). This oscillat- 113