THE EFFECTS OF ELECTROSTIMULATION TRAINING ON THE PLANTARFLEXOR CONTRACTILE PROPERTIES MAFFIULETTI Nicola A. – MARTIN Alain Groupe Analyse du Mouvement UFR STAPS Dijon, Faculté des Sciences du Sport Introduction High-frequency electrostimulation (ES) training increases the maximal force-generating capacity of human lower limb muscles (Delitto et al. 1989; Nobbs and Rhodes 1986; Martin et al. 1993; Maffiuletti et al. 2000). Although no electromyographic (EMG) activity has been recorded in these studies, some of the investigators have searched for their basis in neural adaptations affecting the agonist muscles. However, strength increases could also be attributed to adaptations occurring at peripheral sites (e.g. hypertrophy and/or contractile muscular adaptations), since ES is applied at the muscle level with no apparent involvement of the central nervous system. For instance, modifications in twitch and/or tetanus contractile properties are the result of changes in muscle properties and not the result of alterations in a central recruitment pattern. Therefore we aimed to study the effects of isometric ES training on the maximal torque obtained under voluntary conditions and following electrical stimulation of the motor nerve, in order to investigate the plantar-flexor (PF) contractile properties. Also, the EMG activity from the triceps surae muscular group was recorded to quantify the potential neural adaptations. Materials and Methods Height healthy males (mean age ± SD 20.4 ± 2.1 yr; height 186.4 ± 8.0 cm; mass 83.5 ± 9.6 kg) completed sixteen 18-min sessions of isometric ES over a 4-week period, with four sessions per week. During the stimulation, subjects were seated in a calf machine with the ankle, knee and hip joints flexed at about 90°. A portable battery-powered stimulator (Compex Sport, Medicompex SA, Ecublens, Switzerland) discharged square-wave pulsed currents (75 Hz) lasting 400 micros. Three 2mm-thick, self-adhesive electrodes were placed over each leg. Each 4 s contraction was followed by a pause lasting 20 s. In these conditions, 45 isometric contractions where carried out during each training session. The individual level of isometric force developed during ES varied between 50-70% of the pre-training maximal voluntary contraction. Before and after ES training, the posterior tibial nerve was stimulated at rest using a cathode ball electrode (0.5-cm diameter) to investigate PF contractile properties. The current of a rectangular pulse (1 ms) was progressively increased until maximal twitch torque. This intensity was then maintained for (i) single twitch, (ii) paired stimuli (10-ms interval) and (iii) tetanic contraction (100 Hz; 250 ms). The following parameters were then measured on the respective torque traces: peak torque (Pt), contraction time (CT), maximal rate of tension development (RD) and relaxation (RR). Two maximal voluntary plantar-flexions lasting 5 s were also carried out at the training angle, and the associated EMG activity from the soleus and gastrocnemius medialis was recorded by means of silver-chloride surface electrodes. The differences between pre- and post-training results were tested by a Student’s t-test for paired observations. Results Table 1. Plantar-flexor contractile properties recorded before and after ES training for single, paired stimuli and tetanic contraction. Values SINGLE TWITCH Pt, Nm CT, ms RD, Nm⋅ms-1 RR, Nm⋅ms-1 PAIRED STIMULI Pt, Nm CT, ms RD, Nm⋅ms-1 RR, Nm⋅ms-1 TETANIC CONTRACTION Pt, Nm RD, Nm⋅ms-1 RR, Nm⋅ms-1

Before

After

20.95 ± 4.28 128.49 ± 11.76 0.302 ± 0.048 0.186 ± 0.037

20.37 ± 3.81 127.55 ± 8.46 0.300 ± .058 0.166 ± 0.04

41.20 ± 7.76 145.93 ± 11.88 0.540 ± 0.092 0.339 ± 0.055

38.93 ± 9.04 144.94 ± 11.58 0.510 ± 0.123 0.296 ± 0.068

88.34 ± 30.45 0.676 ± 0.218 0.667 ± 0.207

101.33 ± 21.76 0.817 ± 0.235* 0.848 ± 0.140*

n. stimuli 1

2

25

15 Nm 150 ms

are means ± SD. The respective torque traces are also presented for one representative subject (right). *Post-training values significantly higher than baseline at p