FES INDUCED SURFACE MUSCLE STIFFNESS CAPTURED BY COMPUTER CONTROLLED TONOMETRY

D.Rafolt*, E.Gallasch**, M.Fend**, M.Bijak*, H.Lanmüller*, W.Mayr*

* University of Vienna, Department of Biomedical Engineering and Physics

** University of Graz, Department of Physiology

SUMMARY

A new tonometric test system to assess surface stiffness over relaxed and activated calf muscles was developed. The mechanical arrangement consists of a skin indentor driven by a force controlled galvo-drive which is rigidly connected to an ankle dynamometer. Software routines for cyclic indentation (recording of stiffness curves), static indentation (sensing of twitch responses), and vibration (skin admittance) were implemented. A visual interface is used to capture surface stiffness during defined voluntary calf contraction/ relaxation. For FES-applications the software includes an impulse synthesizer, to generate arbitrary stimulation test patterns. The system’s performance was tested in FES and voluntary contraction procedures.

STATE OF THE ART

Surface stiffness over a muscle belly basically consists of passive and active components. In the relaxed belly passive components such as skin and fiber network viscoelasticity as well as interstitial fluid pressure play a dominant role /1/. During contraction the fiber network shortens which is accompanied by belly shape changes and an increase of intramuscular pressure. If surface stiffness is sensed by local indentation (in vivo) further the shape of the indentor and bone architectural factors have to be considered. Due to these multifactorial influences on surface stiffness, rather reliable relative than absolute results are expected from such measurements.

Various techniques and test schemes for surface stiffness are described in the literature. The first indentation apparatus was described by Schade /2/ to study the creep properties of skin and subcutaneous tissues. Recently Veldi et al /3/ described a hand-held device to produce short force impacts, with an accelerometer to study the excitation response. Here the surface stiffness is expressed by the oscillation frequency estimated from the response. Another approach is based on static preloading and the measurement of muscle belly enlargement. Typically such a system is based on a inductive displacement sensor with an internal spring to provide contact loading during isometric contractions /4/.

The tonometric system described here was projected to assess FES induced surface stiffness over calf muscles. To obtain smooth stiffness curves, the stimulation frequency has to be set high enough (< 30 Hz, fused tetani). In contrast to voluntary contractions, FES causes an inverse recruitment, and therefore big and fast fatigable motor units are activated first. For low stimulation frequencies the contraction responses of these units cause skin surface motions which are also detectable by indentation. To precisely track these muscle responses, a fast responding, force programmable indentor was developed.

MATERIALS AND METHODS

The tonometric system is build around a leg attached isometric ankle dynamometer /5/ consisting of footplate and telescopic stand, see Fig.1. During plantar flexion the produced force against the stand is sensed by a load cell (0-3000 N). The dynamometer further serves as mechanical reference for the indentor. The indentor consists of the rotary drive with lever and an exchangeable skin interface. To achieve a high system dynamics, a force controlled galvo-drive was used for indentation (Type M3, General Scanning Inc, USA). The torque produced by this drive (± 20 Ncm) is linear to the input current. The lever length is 5 cm resulting in a 1N/A indentation force (10N max). Further the drive includes a rotary displacement sensor (± 15°) to measure the

indentation depth. For the skin interface a 7 mm ball element is used.

The hardware to operate the tonometric system is shown in Fig. 2. The rotary drive is powered by voltage controlled current source (drive amplifier) in order to avoid friction from back-EMV. The stimulator with internal galvanic isolation (MYOSTIM) converts an unipolar input signal into biphasic voltage waveform. Both the amplifier and stimulator input signals are generated by realtime DSP processes (DAP1216a/6, Microstar Laboratories, Inc.). Four measurement channels (indentation force and deflection, contraction force and stimulator output) are sampled at 1 kHz. Experiment guidance, visual feedback of contraction force and data storage runs on a standard PC, with the Matlab software package for data analysis. The system is calibrated with elastic specimens.

RESULTS

Three test procedures are implemented until now: (1) cyclic indentation for obtaining stiffness curves during FES induced or voluntary contraction, (2) static indentation for sensing twitch response patterns, and (3) vibration to estimate the skin’s surface impedance. Fig. 3 shows a signal section during cyclic indentation, which was taken from the left medial gastrocnemius. The stimulation frequency was set to 30 Hz and the amplitude was stepwise increased (upper trace). The force signal (second trace) gives a measure of the isometric contraction in the ankle joint, which increases with stimulation time. The indentation input force (third trace) has a constant triangular waveform and the responding indentation displacement magnitude (fourth trace) decreases with contraction level.