ELECTRONIC BRACE FOR THE MEASUREMENTS AND ELICITING OF MUSCLE CONTRACTIONS IN A DOG'S ANKLE Matjaž Bunc and ^ Janez Rozman School of Medicine, Institute of Pathophysiology, ^ITIS d. o. o. Centre for Implantable Technology and Sensors, Ljubljana, Republic of Slovenia Keywords: medicine, physiology, physiologic measurements, dogs, electronic braces, ankle rotation, muscle contraction, spontaneous muscle contraction, stimulated muscle contraction, isometric muscle contraction, isotonic muscle contraction Abstract: An experimental electronic brace, which is able to evaluate torque in the ankle joint of a dog elicited by spontaneous or stimulated muscle contraction, has been developed. The brace is also able to impose electrically controlled passive movements on the dog leg. Precise-passive movements, as passive external, electrically-controlled flexion or extension of the ankle of a dog leg, are defined in as speed and angle of rotation/movement. On the other hand, switching in a certain working mode, the brace, equipped with force transducers and a goniometer, could serve for measurements of isometric [locked mode) or isotonic contractions {active mode and passive mode) of a dog leg. A range of the rotation around the ankle joint is limited between -40 and +55 degrees according to the neutral position. The calculated endurance moment of the brace is 2.41 x10 kg m^ while the speed of electronically controlled movement of the brace in the passive mode is up to 78 degrees/second respectively In the active mode the brace is able to rotate synchronously with the dog ankle joint with a speed of up to 400 degrees/second. The maximum frequency on activation of the f/b/a//s anfer/o/-muscle current, when the amplitude of flexion was 50 degrees, was 7/min. In the/octed mode the brace is able to measure the amplitude of force of a dog leg isometric contraction elicited by electrical stimulation. The force transducer with a natural frequency of 8 Hz and compliance of 0.4 |.im/g represents a very linear dependence of the output voltage upon the load with a transducer sensibility of 0,5 mV/mN at a bridge excitation voltage of 57. The nominal range of each transducer is 0-70 N. Elektronska opornica za pasivno gibanje pasje noge in meritev kontrakcije v pasjem kolenskem sklepu Ključne besede: medicina, fiziologija, merjenja fiziološka, psi, opornice elektronske, rotacija gležnja, krčenje mišic, krčenje mišic spontano,krčenje mišic stimulirano, krčenje mišic izometrično, krčenje mišic izotonično Povzetek: Izdelali smo elektronsko opornico za pasjo nogo, s katere je moč meriti momente v pasjem gležnju, ki ji izzovejo mišice ob spontanem ali stimuliranem krčenju. Poleg tega je mogoče z opornico izzvati v naprej predvidene pasivne gibe pasje noge z natančno določeno hitrostjo krčenja ali raztegovanja in kotom premika opornice ter s tem na njo pritrjene pasje okončine. Razen tega lahko opornico priredimo za meritve izometrične kontrakcije (locked mode) ali pa izotonične kontrakcije (active mode) ter za prej omenjena programirana gibanja opornice (passive mode). Kot za katerega se lahko zavrti opornica opornice glede na nevtralno lego, določeno z nevtralno lego pasjega gležnja, je od -40 do 55 stopinj. Izračunani vztrajnostni moment opornice je 2.41x10"" kg m^s^'. Pri pasivnem, programiranem gibanju opornice je moč nastaviti hitrost rotacije v območju od nekaj stopinj/sek do največ 78 stopinj/sek. V aktivnem načinu pa se opornica lahko zavrti, skupaj z stimulirano pasjo nogo, s hitrostjo tudi do 400 stopinj/sek. Največja frekvenca draženja mišice tibialis anterior, pri kateri Je bilo moč izvajati meritve v aktivnem načinu brez popačenja (amplitudi zasuka opornice 50 stopinj) je bila 7/min. Na opornico je bil vgrajen tudi senzor sile, katerega resonančna frekvenca je bila BHz, podajnost (compliance) 0.4 (,im/g in občutljivos 0.5 mV/mN (napajanje 5V). Občutljivostjo bila pri napajanju s 5V v pričakovanem področju merjenih sil (0-70 N), povsem linearna. Introduction In physiological studies of muscle contraction and contemporary nerve activity it is suitable to have special equipment for eliciting controlled mechanical contractions of different muscles. The aim of this work w/as to develop a mechanical system that would be able either to measure or to impose movements of a dog ankle. Therefore, the aim of our work was to develop a special electronic brace forthe dog leg. The brace should be able to elicit precisely defined (angle and speed) passive movements of a dog leg. On the other hand, the characteristics of both, isometric and isotonic contractions of a dog ankle muscle, caused by electrical stimulation, should be measured. Materials and Methods The brace The brace consists of a mechanical joint that could be attached to the ankle of a dog. Such a fixed mechanical joint (ankle) turns around together with the ankle of a dog continuously. The artificial mechanical joint is a construction of one fixed part, artificial ankle and a rotate-able fine bearing, which, fixed on a dog leg, forms common axes with the joint of a dog (Fig, 1). The joint is connected to the actuator by mechanical transmission with a hysteresis angle of ±0.5 degree. The system measures the angle of rotation and the torque induced by the ankle of the dog either due to electrically powered passive rotation of the mechanical joint or electrical stimulation of the dog muscles. The rotate-able artificial ankle has the function of a force transducer at the same time. The brace is construct- ed in such a way that it could be used for experiments either on the left or on the right leg. It just has to be turned around on the white or black plate (see Fig. 1). tions divided by angles of rotation and is equal to 2.66:1. The mechanical transmission was selected on the basis of the specified requirements of acceleration and velocity in fmm^^^mwrn Fig 1. The brace. The bear brace is shown on the left of the picture, the positioning of the dog ieg into the brace is shown on the right. A) position of force transducer, B) DC motor/gear, C) goniometer Description of the sensors Mechano-electric transducer The force transducers were made up of a full Wheatstone bridge composed of four semi-conductor strain gauges /1 / bonded on the artificial ankle (Fig.1.). The voltage signal, produced by a deformation of the semi-conductor strain gauges, is amplified by a precision strain gauge amplifier (Linear Technology, LT 1101). Sensor of angle rotation - a custom designed goniometer In order to measure and control an angle in the joint a custom designed goniometer manufactured from a precision potentiometer with a resolution of 0.1 degree {iTiS d.o.o., Ljubljana) is mounted at common axes with the rotate-able artificial ankle (Fig. 1). Mechanical part of the brace Passive movements of a dog ankle are elicited electrically by powered motor movements of the artificial ankle brace transmission (Fig. 1). Actuator system The complete actuator system is mounted on an aluminum plate, the base of the brace. It is possible to regulate the motor speed and number of revolutions by the PC controller. The ratio of transmission is defined by motor revolu- the passive mode with respect to the friction of transmission. The chosen system comprises a direct current (DC) motor within grounded iron cage to minimize electromagnetic artifacts. Motor control An actuator system involving the aforementioned DC motor/gear system (Fig.1) is mechanically connected to the mechanical joint, and joint thus transferring the torque to the dog joint. By position feedback obtained through measuring an angle in the mechanical joint, the motor is regulated in such a way that it rotates at a chosen speed for any angle according to the neutral position of the ankle. The mechanical system is able to operate in three modes: passive, iociced and active mode. In the passive mode the brace is able to rotate the ankle by a predefined angle at a different predefined speed. Therefore, in this mode a rotation from the actuator is transferred to the ankle joint, thus imposing a stretch of a dog ankle extensors or flexors. The common friction, expressed as the certain amount of torque in the passive mode, is composed of friction of the potentiometer, four fine bearings and the transmission. In the /oc/ced mode, the position of the motor and artificial ankle is locked at a desired angle in order to measure the isometric torque elicited by electrical stimulation of the muscles or muscle group under investigation. The force transducers, described above, measure the torque of contrac- tion through deformation of the sensors. In order to achieve a dynamic range of measurement in the active mode, the system is able to follow the ankle joint rotation with fast cadence elicited by electrical stimulation of a nerve or muscle. Measurements of passive and dynamic characteristics of the brace Passive characteristics of the brace The maximum speed of the ankle movement was determined by goniometric measurement of an angle speed of the spare brace rotation at the highest DC motor performance. Brace friction The common friction of the brace was defined by feeding a known DC to the motor and measuring the mechanical energy output. The difference of the input and output energies reveals the friction of the system. Dynamic characteristic of the brace The dog muscle contraction was elicited by electrical stimulation of the sciatic nerve using stimuli with frequencies ranging from 0 to 20 min"''. Since contractions of the leg were detected by the brace, we could determine the frequencies where the response of the brace is linear. This means that the ratio of stimulus/contraction detected without artifacts due to the brace friction or endurance is 1. Endurance moment of the artificial ankle The endurance moment of the brace was calculated considering the dimensions (14 x 3.3x 1 cm), shape and material (aluminium) of the artificial ankle. Selective stimulation of fibers in the sciatic nerve of a dog with a 33-electrode stimulating and recording spiral cuff The cuff was made by bonding two 0.1 mm thick silicone sheets together /2-5/. One sheet stretched and fixed in that position was covered by a layer of adhesive material (NuSil, MED-1511). A second unstretched one was placed on the adhesive and the composite was compressed to a thickness of 0.3 mm. When released, the composite curled into a spiral tube as the stretched sheet contracted to its natural length. 33 electrodes (0.6 x 1.5) mm made of 0.05 mm thick platinum ribbon connected to lead wires were mounted on the third silicone sheet. They were arranged in three parallel spiral groups each containing 11 electrodes at a distance of 0.5 mm. The distance between the spiral groups was 6 mm. Electrodes of the central group were connected to lead wires individually, while the corresponding outer electrodes were shunted to each other and then connected to lead wires. The silicone sheet with electrodes was bonded on the inner side of the cuff. The cuff with an inner diameter of 2.5 mm was trimmed to a length of 20 mm. The lead wires were connected to the connector to be implanted within the lateral subcutaneous tissue for the time between stimulation. Rectangular, bi-phasic, charge balanced, current pulses with a frequency of 20 Hz and amplitude of up to 1 mA were delivered on the central electrode of each GTE within the cuff. As a neutral electrode a hypodermic needle was inserted in the subcutaneous tissue of the thigh, slightly proximal to the cuff. Selective recording of electro-neurogram (ENG) from the sciatic nerve of a dog The cuff already described above was used also for the selective recording of the ENG from a dog nerve after passive or active dog leg movements. ENGs are recorded differentially and selectively with the spiral cuff (see above) from two superficial regions of the sciatic nerve innervating mostly the aforementioned muscles /6-7/. Since the motor system has to operate simultaneously with noise-sensitive ENG measurements, the electromagnetic noise of the motor system was reduced by using a RFI filter and ferrite cores on the supply connections. Shielded wires and the motor and proper ground connection were implemented throughout the entire electrical circuit. Results 100 0 1nV degrees "I jneurc iJOms r Irotatj gran on aiigle Fig 2. Detection of the contraction of the dog ankle in the passive mode of the brace. Tine upper trace stiows a neurogram recorded from the sciatic nerve after rotation of the brace and the dog leg with a DC motor system at 50 degrees. In the passive mode (Fig. 2) the brace is able to rotate to a maximum extension of 45 degrees and of maximum flexion of 55 degrees, according to the neutral position of the ankle. The DC motor/gear system is able to perform movement of the artificial ankle with a speed of up to 78 degrees/second. However, practically we could not exceed 7 passive movements with maximum amplitude of flexion and extension of a dog ankle per minute during the experiments because of the combination of endurance and fric- tion of tlie complete system and resistance of the dog an-l