THE LENGTH DEPENDENCY OF FORCE-FREQUENCY

THE LENGTH DEPENDENCY OF FORCE-FREQUENCY CHARACTERISTICS OF HUMAN TIBIALIS ANTERIOR MUSCLE P. Mela1, P.H. Veltink1, P.A. Huijing1,2 1

Institute for Biomedical Technology (BMTI), Biomedical Signals and Systems, Department of Electrical Engineering, University of Twente P.O. Box 217, 7500 AE Enschede, The Netherlands 2

Faculteit Bewegingswetenschappen, Vrije Universiteit

Amsterdam, van de Boechorststr 9, 1081 BT, The Netherlands ABSTRACT For practical purposes of FES, a better understanding of properties of human muscles, stimulated via surface electrodes, is necessary. With a long term aim of improving control of muscle output, the purpose of this study was to investigate the force-frequency relationships as a function of muscle length in human skeletal muscles under isometric conditions. In order to asses the influence of length, particular attention has been focused on the stability of recruitment in the range of joint angles examined. The tibialis anterior muscle (TA) of healthy subjects was stimulated at different ankle joint angles by means of constant frequency bursts with seven different frequencies (50, 33, 25, 20, 16, 12, 8 Hz). The contractions were isometric and the stimulation time was 2 seconds. The elicited torque output of the muscle was averaged over the final 0.5 second of stimulation. The results of the study show that human TA moment-frequency curves change as a function of ankle angle: with increasing muscle lengths the normalized frequency-moment curves shifted to lower frequencies. This means that, particularly for the lower stimulation frequency range (8, 12, 16, 20 Hz), normalized muscle force is increased substantially. Keywords: electrical stimulation, submaximal activation, stimulation frequency, length dependence, human muscle INTRODUCTION Functional electrical stimulation is a technique to enhance and restore functions in paralyzed muscles in order to be able to use them as actuators of limbs. A good control of these actuators is rather complex because muscle properties are non linear and time dependent. Furthermore it must be considered that muscle force output is the result of complex mechanisms involving many factors which cannot be considered independent of each other. It is well known that muscle length plays an important role in the generation of force (length-force characteristics) and that stimulation frequency has also a great influence in the process, having an effect not only on generation but on maintenance and decline of force as well. However, in most studies the interaction between these two factors is neglected. Experimental evidence gathered on skinned muscle fibers, intact fiber bundles [1, 2, 6] and isolated animal muscles [3, 4] show the effects of the interaction between stimulation frequency and length on force output: submaximal frequencies cause the optimum muscle length to shift towards higher lengths. Therefore length-

force characteristics for submaximal activation are not scaled versions of length-force characteristics for a maximally activated muscle. With this study we focused on the interaction between muscle length and stimulation frequency with respect to the generation of force in human tibialis anterior muscle under isometric conditions. METHODS Tibialis anterior muscles of six healthy subjects with no neuromuscular injury history (2 females, 4 males) were studied during isometric contractions after written informed consent was given. They were seated on a self designed bench with their left foot tightly strapped to an adjustable foot pad connected to a strain gauge force transducer in such a way that the ankle joint axis was aligned to the force sensor axis. The bench was also equipped with a potentiometric goniometer to measure ankle joint angles. A personal computer with a build in AD-converter was used to collect data from the force transducer and the goniometer and to control the stimulation delivered by a current regulated stimulator through surface adhesive electrodes, of which the active one (circular, r =1.5 cm) was placed on the deep peroneal nerve in such a way as to obtained dorsiflexion (i.e. predominantly in the sagittal plane). The indifferent electrode (rectangular, 5x9 cm) was positioned distally on the tibia. Stimulation amplitude was set at a level which resulted in stable recruitment in the three different ankle angles to be tested: the first one towards the extreme of the angle range when dorsiflexing (corresponding to high muscle length), the second close to optimum muscle length and the third one close to maximal plantarflexion (low muscle length). The order in which the ankle positions were tested was from dorsiflexion to plantarflexion. The bursts were two seconds long and the frequency was held constant during the same contraction. Seven different stimulation frequencies were used (50, 33, 25, 20, 16, 12, 8 Hz). For each ankle joint studied, a series of 21 isometric contractions were elicited, three for each of the seven stimulation frequencies, the chronological order of the trials being randomized. Between two consecutive contractions a minute recovery time was allowed. The muscle response to the bursts was recorded and the average value during the last 0.5 seconds of stimulation calculated. For each tested muscle length, the results from the three trials at the same frequency were averaged. The averaged responses were normalized on 0the value corresponding to the frequency yielding the highest force output (always 50 Hz). The normalized moment-frequency curve was thus obtained for all the ankle joint angles. RESULTS Stable recruitment was achieved in all the experiments: on average, a stimulation amplitude increase of 14% during a 20 Hz burst resulted in only maximum 3% increase in joint moment. The investigated range of ankle angles varied from 16o dorsiflexion to 44o plantarflexion. Normalized moment- frequency curves at different lengths for each experiment were obtained and the group average determined (fig. 1).

1 0.9 0.8

dorsiflexion intermediate angle

Normalized moment

plantarflexion 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1

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12

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Frequency [Hz] fig. 1: The influence of joint angle position on normalized moment at different frequencies of stimulation. Normalized moments and std are presented as function of stimulation frequency and ankle angle. Note that the differences in normalized moment among ankle angles decrease with increasing frequency.

Difference in normalized moment

When stimulating submaximally, for a given frequency a higher normalized moment is attained with increasing length. A two way analysis of variance for all the experiments shows that the effects of both stimulation frequency and ankle angle are statistically significant (p