J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.8.774 on 1 August 1985. Downloaded from Journal of Neurology, Neurosurgery, and Psychiatry 1985;48:774-781 Effects of chronic low frequency electrical stimulation on normal human tibialis anterior muscle OM SCOTT, G VRBOVA,* SA HYDE, V DUBOWITZt From the Departments of Physiotherapy and Paediatrics, t Hammersmith Hospital and Department of Anatomy and Embryology, * Centre for Neuroscience, University College, London, UK SUMMARY The loss of force that occurred during intermittent electrically evoked tetanic contrac- tions was determined for the tibialis anterior muscle of normal subjects. Adult muscles showed a characteristic reduction of tension over the first two to three minutes until a steady plateau was reached. Muscles of young children showed no comparable decrease of the initial tension in response to this method of fatigue testing. After fatigue the muscles of both groups of subjects produced a higher proportion of tension at lower rates of stimulation. Following prolonged chronic low frequency stimulation at 8-10 Hz, adult muscles showed a significant increase (p < 0.01) in fatigue resistance compared to unstimulated control: the muscles of the normal child showed no measured change. It is concluded that it is possible to alter the properties of adult human muscle by superimposed low frequency electrical stimulation. Protected by copyright. Investigations on animals have shown that it is poss- longed electrical stimulation are easier to interpret. ible to modify the properties of mammalian skeletal In this case, fatigue is probably caused by the muscles by chronic electrical stimulation. Fast twitch decrease in tension of high threshold motor units, muscles of cats and rabbits have been induced by which normally are not used during sustained volun- chronic low frequency stimulation to exhibit proper- tary contraction (for review see ref 6). ties of slow contracting twitch muscles. An early Most data on human muscles are obtained from effect of such stimulation is an increase in resistance adult subjects, and little is known of how human to fatigue with an increase in capillary density and skeletal muscles change during development in oxidative enzymes. These changes occur before an childhood. In this study, an attempt was made to observable change in contractile properties.' The compare some physiological characteristics of adult purpose of this study was to establish whether simi- muscle with that of young children. On the basis of lar increases in resistance to fatigue could be work with animals, it can be expected that chronic induced in human skeletal muscle. electrical stimulation will increase the resistance to In previous experiments, measurements of resis- fatigue of mature muscle. We were concerned to in resistance to fatigue fol- tance to fatigue of human muscles have been based investigate any changes http://jnnp.bmj.com/ upon the decrease in tension, or changes in electrical lowing a prolonged period of chronic low frequency activity using electromyography, either during vol- stimulation as shown by changes of response to an untary contraction, or prolonged electrical stimula- electrically stimulated test of muscle fatigue.7 tion.>5 In assessing resistance to fatigue during vol- untary contraction, account must be taken of the Method motivation of the subject and other influences that control. In contrast, tests using pro- Subjects are difficult to Sixteen normal adults (six males and 10 females), varying in age from 17 to 53 years, and 18 normal children (15 on September 23, 2021 by guest. Addlress for reprint requests: Dr 0\4 Scott, Physiotherapy boys and three girls) of age range 3 to 13 years took part in Department, Hamimersmith Hospital, Du Catie Road, Lolndon the study. All the subjects were normal, healthy volunteers Wl2 OHS, UK. and the studies were approved by the Committee on Ethics of Clinical Investigations of Hammersmith Hospital. were made of maximum voluntary and 1984 and in final revised form 5 December Measurements Received 30 March electrically evoked contractions of the tibialis anterior 1984. of Accepted 15 December 1984 muscles in all subjects. In order to evaluate the effects 774 J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.8.774 on 1 August 1985. Downloaded from Effects of chronic low frequency electrical stimulation on normal human tibialis anterior muscle 775 chronic low frequency stimulation, five female adults and subject pulled was adjusted so that it was comfortable. one 11-year-old girl stimulated their tibialis anterior mus- Particular care was taken to ensure that the foot was cles at a frequency of 10 Hz or less for an hour, three times positioned so that the bar was aligned with the head of the daily for 6 weeks. first metatarsal bone. The subject was asked to make max- imal effort to dorsiflex against the bar and the output from Assessment procedure the strain gauges was fed through amplifiers on to a pen To ensure standardisation of position and fixation of the recorder. The best of three attempts was taken as the max- limb during assessment, a special chair was designed imum voluntary contraction, unless the third was greatest (fig 1). The position of the seat and leg supports were in which case a fourth attempt was made and if greater, adjusted to the individual and then firmly secured. Two repeated until a maximum was established. A standard sets of footplates were provided, one was made of stainless constant voltage stimulator (Digitimer Type 3072) con- steel and of dimensions suitable for adults, the other set nected to two 4 cm square carbon rubber electrodes placed made of aluminium, was built on the same design principle over the motor point and on the belly of the muscle was and was used for measuring in children. Strain gauges used in stimulation. Good contact was ensured by using mounted on the upper surface of a mild steel bar measured electrode jelly and the electrodes were secured by adhesive the tension exerted by the tibialis anterior muscles. The bar tapes. Isometric contractions of the tibialis anterior muscle was mounted at 20° to horizontal on two supporting pillars were thus elicited by stimulating the intramuscular and its height could be readily adjusted by two self-locating branches of the lateral popliteal nerve using a square wave screws. Moulded padding on the under surface of the bar pulse of 50 ,us duration, applied at selected frequencies for ensured optimum force transmission from the contracting 250 ms each second. A routine sequence of testing was muscle group. Two bars were provided, one of 3 mm established to record the increments of tension in response thickness which gave a measuring range suitable for adult to brief trains of stimulation at 1, 10, 20 and 40 Hz. To subjects and one of 1 mm thickness which was more sensi- determine resistance to fatigue, the muscle was stimulated tive and was suitable for measuring the forces exerted by at 40 Hz for 250 ms, every second for 5 minutes and the the children. The subjects were secured to the seat by an percentage decrease of tension after three minutes was adjustable seat belt, sat upright with their thighs sup- measured. ported, their knees flexed to 90° and their feet resting on The effect of this fatigue test on the tension elicited by two rigid footplates as shown in fig 1. Two correctly sized brief trains of stimulation was established (fig 2), by Protected by copyright. heel cups were fitted and the padded bar against which the 5kg] Ii 6 1i 23 4 Time (min) http://jnnp.bmj.com/ 5kg] I I I I I I 0 1 2 3 4 5 Time (min) on September 23, 2021 by guest. Fig 2 Records ofroutine sequence of testing ofthe tibialis anterior muscles oftwo normal adult subjects, (a) shows the response to testing ofsubject (AT), her maximum voluntary contraction and the increments oftension in response to brieftrains ofstimulation at 1, 10, 20 and 40 Hz before and Fig 1 Apparatus for measuring strength oftibialis anterior after fatigue. (b) shows the typical response to fatigue muscle. testing, subject (MD) FI = 23 %. J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.48.8.774 on 1 August 1985. Downloaded from Scon, Vrbova, l)ubowitz 776 HY'de, 10, 20 and 40 Hz normal school children varied from one week to a recording the tension developed at 1, these results are before and after fatigue. The following ratios were deter- maximumn of three months and mined: shown in table 2. Ihese indicate more variability this be accounted for and Tension (20 Hz) x 100 than in the adult studvy; can Tension ( 10Hz) x 100 interval between the two tests, (40 Hz) Tension (40 Hz) partly by the longer Tension the inherent variability of young childrel. was then calculated as and A Fatigue Index (FI) at 40 Hz It has been suggested that percutaneous stimula- follows: a muscle may be unsatisfactory and after tion of part of Initial Tetanic Tension-Tetanic Tension that the relationship between high and low 3 minute stimulation frequency force generation is voltage dependent.' Initial Tetanic Tension Further investigations have shown that the relation- A prelimary study of six normal adults and six normal ship is robust except at the lowest voltages when school children aged 5 to 12 years indicated that this 10% of maximum voluntary con- 20 forces of less than method of assessment was well tolerated and between traction are generated." and 50% of the maximum voluntary force could be elicited on stimulation at 40 Hz. Fatigue testing in adults was measured in six- stimulation The response to fatigue testing Chronic low frequency A typical record is shown in The subjects stimulated their tibialis anterior muscles with teen normal adults.