NORMAS GENERALES DE TESORERIA

The electrodes which were used to record the EMG were manufactured by Motion Control Inc. and supplied by Oxford Metrics. Each pair of electrodes were encased in plastic cases which were rectangular in shape. Thus the interelectrode distance

was always the same (3.5cm). Between the active electrodes was a reference electrode. Each electrode unit also contained a pre-amplifier with a gain of about 380. The active electrodes were made o f stainless steel and were a.c. coupled. These stainless steel discs, which were 1.2 cm in diameter, and protruded below the casing so that they indented the skin to provide a good contact electrical contact. They were designed to be used without gel, but firm external pressure was necessary for a noise free signal to be obtained, by cutting down the interface resistance

between the skin and electrode as much as possible. They had a high input

impedance (the amplifier also had a high input impedance of IMQ, as recommended by Basmajian (1974)).

Each patient and normal subject was first asked to sit on a couch with their legs extended. The skin over the front of the thigh where the electrodes were applied was prepared by rubbing fairly vigorously with alcohol pre-injection swabs

(Sterets^), so that a slight erythema was produced and the alcohol had removed the

surface grease on the skin. They were then asked to contract statically their

quadriceps muscles, so that the individual portions could be delineated. Each pair of electrodes was placed over the individual portions, in the positions found in the study of motor points, and in the direction o f the muscle fibres in each case. The electrode pair over V.M.O. was placed at an angle 55° to the line of the femur, so that the distal edge was 1.5 cm from the medial edge of the base of the patella. The location for V.L.O. placement was found by taking a line from the iliotibial band to the

lateral edge of the base of the patella, as far down the muscle belly as possible. At an angle of 40° to the shaft of the femur this electrode pair was not as acutely angled as that over V.M.O., because that is the angle at which the fibres approach their patellar insertion (Javadpour et al. 1991). The fibres of V.M.O. are more horizontal (the lowest ones are angled at 80° to the vertical axis of the femur). This was

demonstrated in the dissections made by Bose et al. (1980). The position for R.F. was slightly medial and about midway along the length of the thigh. Since R.F. is a bipennate muscle, the electrodes were angled at 15°. This is, however, an

approximation because the muscle architecture is more complicated in this muscle, with the deep fibres running ventrally. The electrode pairs over V.M.L., and V.L.L. were placed near the motor points (as previously identified), angled to lie along the length of the muscle fibres. The electrodes over these parts of the muscle were not angled as acutely as over either V.M.O. or V.L.O. The electrodes were fixed in position with strips Micropore*, and then the appropriate width oï^Tubigrip*

was rolled onto the leg to apply the required pressure, and keep the electrodes in position during any movement of the thigh which might occur.

When the electrodes were in position the leads, which were well screened against electrical interference, were connected to an amplifier, which had a gain trim to ensure that there was no interelectrode variability. There were different gain settings which could be used (the amplification was from 1000-10000). The raw signal could

^ *Manufactured by 3M

be collected, or it could be rectified, and smoothed by means of a “leaky” integrator, and the time setting constant of this could be changed (50,100or 200ms) to vary the degree of smoothing. The amplifier ran at 6 V supplied by a separate a.c. adapter, thereby isolating the subject from the mains supply. The three channel EMG then passed via a junction box to an analogue to digital converter (A/D) connected to a computer via a serial port. The digital signal could then be displayed on screen, and stored on disc, using the programme Microscope* ”. With this software the duration of sample and the number of sampling points could be adjusted, as well as many other functions. For the isometric experiments the duration of sampling was 4 s and the number of sampling points was 1000. However, for the isokinetic

experiment the duration was increased to 8 s, because a greater time was needed to enable the exercise to be completed. Since the trace had been rectified and

smoothed, in most cases 1000 sampling points were sufficient. However, for any experiment using the raw data, including recording action potentials, the number of sampling points was increased to 4000 so that the maximum amount of data could be saved (see Fig.5.3). The data was further analysed (via ASCII files) using various programmes.