Effects of low frequency electrical stimulation training on muscle contractile properties of paralysed muscle: preliminary results
Zhou, S, Harridge, SDR, Hartkopp, A & Biering-Sorensen, F 1996, 'Effects of low frequency electrical stimulation training on muscle contractile properties of paralysed muscle: preliminary results', Australian Conference of Science and Medicine in Sport: abstract book, Canberra, ACT, 28-31 October, Sports Medicine Australia, Bruce, ACT, pp. 450-451.
It is known from animal studies that muscle composition can be altered in response to different patterns of usage or stimulus (2). However, in man the effects of different types of training on muscle fibre type composition have not achieved a common agreement. This is partly due to the difficulty in controlling the stimuli the muscle receives because the training stimuli can only be superimposed on the daily activities usually undertaken. The muscles of spinal cord injured individuals provide a unique model for studying the process and mechanism of specific training effect, since these muscles are in a highly detrained state containing a low proportion of type I fibres/myosin heavy chain (MHC)-I isoforms (1,3,4,5), and because the stimuli received by the muscles can be more closely controlled by the investigators.
The purpose of present study was to investigate the effects of chronic low frequency electrical stimulation training on the contractile properties and muscle composition of the tibialis anterior muscle in a group of spinal cord injured individuals. This paper reports the preliminary results of muscle contractile property changes after four weeks of training.
Five male paraplegic subjects, aged 29 to 48 years, with 5 to 12 years post spinal cord injury, gave their consent to participate in the study which was approved by the local ethical committee. The subjects were trained in the laboratory under supervision, five days a week for four weeks. The training was carried out on a custom build device that held the leg in a position of knee joint angle of 90deg and ankle joint angle of 10deg of plantar flexion. A rigid foot piece was applied on the dorsum of the foot to restrict any movement of the foot and toes, and to provide a resistance to dorsiflexion when the anterior tibialis muscle was stimulated. Percutaneous electrical stimulation was provided by a Uni-Tens 2031 Stimulator (ASAH medico, Denmark), with the stimulation electrodes placed over the motor point area of the muscle. Surface EMG electrodes were placed on the muscle belly to record M-waves. Force responses were monitored using a sensitive strain gauge force transducer attached to a analogue-to-digital converter.
The training stimulation frequency was 10 Hz, with a duty cycle of 5 s on and 5 s off, and a pulse width of 350 us and current of 60 to 75 mA. This intensity was equivalent to approximately 60-70% of the muscle’s maximal tension at this stimulation frequency. The daily training time started from 1.5-2 hours in the first week and increased gradually till 5-6 hours were performed in the last training week. In each training day, a recovery period of 15-25 min was inserted between stimulation sessions of 30-90 min. Muscle contractile properties of both right (training) and left (control) legs were measured before and after the four weeks of training. The functional tests included single twitches, tetanic contractions evoked at 10, 20, 50 and 100 Hz, and a fatigue test where the muscle was stimulated at 20 Hz for 2 s with 3 s recovery for 5 min. The major parameters analysed included peak torque (Pt), time to peak torque (TPT), half-relaxation time (1/2RT), fatigue index (FI), and electromechanical delay (EMD). Muscle biopsy samples were obtained before and after the training to analyse MHC composition. Student t-test (paired) was used to detect any significant changes between the mean values pre and post training and between the legs.
All five subjects successfully completed the training program. The average training time during the four weeks was 68.2 (SD 9.1) hours. No significant differences in the functional tests were found between the control and training legs before training. After the training, the trained leg showed significantly increased TPT and decreased FI (both p<0.05) compared with that of pre-training values. The trained leg also showed an elongated EMD and decreased FI compared with the control leg (both p<0.05). The relative changes of the parameters (post compared with pre-training) are shown in the Figure 1. DISCUSSION
The training stimulation in the present study was given at a lower frequency (1,5) and for a longer time period (3,4) than that in some other similar studies. The trend of muscle contractile property changes due to the low frequency training was as one might expect from similar studies on animal muscles, notably the increased TPT and resistance to fatigue. Further evidence of a ‘slowing’ in muscle properties is provided by the increase in EMD. These contractile changes were, however, not coupled to any significant increase in torque production evoked tetanically, indicating no change in muscle size, despite the loading applied to the muscle during training. A tendency towards an increase in torque at 10 Hz was observed which may also be indicative of a slower muscle requiring a lower fusion frequency.
The changes in MHC composition, muscle fibre cross sectional area, and metabolic characteristics due to the training are under analysis.
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