How EMS-trainings influence on nerve conduction and muscle architecture

This article describes the study on the efficiency of EMS-trainings regarding neuromuscular conduction and muscular tissue structure. The results of the experiment, conducted in France, were presented in Medicine & Science in Sports & Exercise in 2005.

The content of the article

1. Introduction
2. Methods
2.1 Subjects
2.2 EMS-trainings
3. Testing
3.1 Torque measurement
3.2 Electromyography and data analysis
3.3 Measurement of cross-sectional area
4. Results
5. Conclusions

Electromyostimulation is widely used in strength training for healthy people. Many specialists noted the increase of maximum voluntary contraction force after several EMS-sessions, however, basic mechanisms that are responsible for these changes, remain under-explored.

A number of studies showed that neuron factors influence on the increase of velocity-strength indicators. Even one single stimulation can cause the response on molecular level. However, muscular hypertrophy, as a response to EMS, appears only after certain number of sessions.

The purpose of the study was to explore the influence of 4- and 8-weeks EMS-training course on nerve conduction and knee extensor strength with the detailed analysis of adaptation dynamics.

To estimate neuron adaptations, electromyographic activity and muscle activation were examined at maximum voluntary contraction. In the analysis of muscular changes, torque indicators were taken into account as well as anatomical cross-sectional area (ACSA) and pennation angle (vastus lateralis or VL).

20 male students participated in the study – 12 in the study group and 8 in the control group.
The average age of the participants was 23 ± 5,0 years, height 178,4 ± 8,9 cm, body weight 73,7 ± 9,4 kg. The second group indicators were the following: the average age – 24,3 ± 1,6 years, height 176,4 ± 4,7 cm, bogy weight 69,3 ± 7,4 kg. None of the subjects has tried EMS-trainings before. The experiment was conducted with the approval of Human Research Committee of the University of Burgundy and according to Declaration of Helsinki.

Study group training program included 32 EMS-sessions, 18-minutes each, for 8 weeks (4 sessions every week). 40 isometric contractions were done during the session. The subjects were placed in sitting position with the knee fixation at the angle of 60°. Impulses with 75 Hz frequency and the duration 400 µs were transmitted with the rise time of 1,5 seconds, the time of steady tetanic impulsation is 4 s and the decay time is 0.75 s. The pause of 20 seconds followed every stimulation. There was a standard warm-up before each workout, including 5 minutes of submaximal EMS (5 Hz frequency, duration 200 µs).

The evaluation of indicators examined was done 4 times – 2 weeks before the study, just before it, in 4th and 8th weeks.

Instantaneous isometric torque was registered by dynamometer. The participants were placed in sitting position (same as during EMS-session). Dynamometer axe was aligned with the knee joint axe.
The individual stimulation intensity indicator was set by its gradual increase to the value 20% higher than the maximum effective one (super-maximum intensity). During the test, the participants were doing two maximum voluntary contractions of the knee extensor. In case of more than 5% difference in results between 1st and 2nd attempts the 3rd attempt was done.

The record of EMG was done in such a way to avoid innervation area and to get an optimal amplitude. Values registered during EMG were normalized in accordance with the maximum amplitude of M-wave. Muscles activation was estimated in percentage: 1 – superimposed doublet / potentized doublet * 100%. The co-activation level was calculated by normalization of average square values, obtained in the moment of knee flexion (also in percentage). The following contraction characteristics were analyzed: peak torque (Pt, highest torque production value), time to peak torque (tPt), half relaxation time (hRt, time required to decrease the peak torque up to 50%).

The estimation of ASCA was done using ultrasound. At the beginning of the study, the subjects were placed in laying position for 20 min. Scanning was done at the level of the middle of distance between upper the border of patella and the trochanter major.

Pennation angle was measured using ultrasound, too, at the center of the hip. Every picture with the values of all three angles was estimated, and then the calculation of the average value was done.

The results of the study showed the significant growth of every neuron and muscle indicator analyzed. This experiment was the 1st that implied these indicators after involuntary training sessions.

The measurements have demonstrated vividly, that the neuron indicators dynamics at earlier stages of the study exceeded muscle dynamics, the significant growth of which was observed by the end of the 8th week.

The experiment clearly showed the effectiveness of EMS-training for both muscular and neuron adaptation. It provides the opportunity to use EMS not only in sports but also in different locomotor apparatus rehabilitation programs.