In the last decades, electromyostimulation (EMS) has been widely used in rehabilitation medicine (Deley et al. 2005; Snyder-Mackler et al., 1995) and in sports (Babault et al., 2007; Lattier et al., 2004; Maffiuletti et al., 2002) to counteract the effects of pathology/hypoactivity and to increase strength. High and low frequencies of stimulation (>40 Hz vs.
The aim of this preliminary report was therefore to investigate the effects of a six-week low-frequency EMS training program at the different levels of exercise performance: respiratory, neuromuscular and cardiovascular. Since low-frequencies have been suggested to mimic endurance-type training (Atherton et al., 2005), we can expect improvements of the subject's aerobic capacity, resistance to fatigue and cardiovascular control (muscle sympathetic nervous activity, MSNA; resting blood pressure and bradycardia). Also, results obtained in patients (Deley et al., 2005; 2008) suggested that this low-frequency EMS training might have significant effects on muscle strength attributable to neural and/or muscular adaptations.
A 33 year-old man (height 1.73 m, body mass 73 kg), free from previous knee injury, participated in this study. The subject was recreationally active and had never engaged in systematic strength training or had experience with EMS. The investigation conformed to the principles outlined in the Declaration of Helsinki and the subject gave his written informed consent after being clearly advised about the protocol, which was approved by the Institutional Ethics Committee.
Design of the study
The subject underwent a 6-week EMS training on knee extensor muscles of both legs. Exercise capacity, neuromuscular properties, muscle architecture and sympathetic activity were evaluated at least four days before the first training session and four days after the last training session. The subject was familiarized with all measurements before entering the study.
The bilateral low-frequency training program consisted of 45-min sessions, five days per week for six weeks. Knee extensors of both legs were stimulated using a portable battery-powered stimulator (Compex-2, Medicompex SA, Ecublens, Switzerland). Two self-adhesive positive electrodes (each measuring 25 [cm.sup.2], 5 x 5 cm), which have the property of depolarizing the membrane, were placed on the thigh as close as possible of motor points of vastus medialis (VM) and vastus lateralis (VL) muscles, near the proximal insertion of each muscle. Rectangular negative electrodes, each measuring 50 [cm.sup.2] (10 x 5 cm, Medicompex SA, Ecublens, Switzerland), were placed over the femoral triangle of each thigh (1-3 cm below the inguinal ligament). During stimulation, the subject was in the supine position and the lower limbs were positioned without hip or knee flexions. Rectangular wave pulsed currents of 10 Hz lasting 250 ps were used and the stimulus was alternately on for 9 s and off for 2 s. The duration of each training session was 40 min, following a 5-min warm-up (submaximal contractions at 3 Hz). After warm-up, stimulation intensity for each muscle was increased according to the patient's tolerance, to always be at the maximal tolerated level. The validity of these stimulation characteristics for the improvement of knee extensor muscles strength and exercise capacity have been confirmed by previous low-frequency EMS training studies (Deley et al. 2005, Dobsak et al. 2006). During the study period, the subject was asked not to perform any physical activity, except the EMS program, in order to exclude confounding parameters.
Measurements and data analysis
Before and after training, the subject came four times to the laboratory for testing sessions: (i) cardiopulmonary exercise test, (ii) neuromuscular tests on knee extensor muscles, (iii) quadriceps muscle architecture, and (iv) cardiovascular measurements (Figure 1).
Cardiopulmonary exercise test
The cardiopulmonary exercise test was performed on an electromagnetically braked cycle ergometer (Lode, Groningen, Nederlands). The exercise protocol began with 3 min resting followed by 3 min of warm-up pedalling at 50 watts (W), and then power was increased by 25 watts every minute. The exercise test was terminated when the subject was unable to maintain the imposed pedalling rhythm of 80 revolutions per min, limited generally by dyspnea and/or muscular fatigue. Six minutes of passive recovery followed the incremental exercise test. Heart rate was monitored throughout the test and recovery (Polar Electro Oy, Kempele, Finland). Gas exchanges were measured breath-by-breath using a portable system (K4b2, Cosmed, Rome, Italy). Before each test, the system was calibrated with a 2-liter Rudolph syringe and a standard gas of known concentration.
Maximal oxygen uptake (V[O.sub.2]), power and heart rate (HR) were defined as the mean values during the last 30 s of exercise. The ventilatory threshold (VT) was determined visually using the Beaver et al.'s method (Beaver et al., 1986).
Neuromuscular tests on knee extensor muscles
During this session, the subject was seated in comfortable upright position on an isokinetic dynamometer (BIODEX system 2, Biodex Corporation, Shirley, NY) with a 95[degrees] hip angle. Velcro straps were applied tightly across the thorax. The leg was fixed to the dynamometer lever-arm and the axis of rotation of the dynamometer was aligned to the lateral femoral condyle, indicating the anatomical joint axis of the knee. Mechanical traces were digitized online and stored for analyses (Biopac sytems, Inc., MP System hardware and Acknowledge software).
The session began with a standardized warm-up composed of 10 to 15 progressive dynamic leg extensions performed at 120[degrees]-[s.sup.-1]. Then, the subject performed 30 maximal knee extensions (60[degrees]-[s.sup.-1] angular velocity, 100[degrees] range of motion) preceded and immediately followed by (i) maximal voluntary contractions (MVC) of the right quadriceps with activation level assessment (see below) and (ii) electrically evoked contractions at rest.
MVC: Two 5-s MVC (separated by 15 s) were performed and the best was retained for further analysis (75[degrees] knee flexion, 0[degrees] corresponding to full extension).
Evoked contractions: Quadriceps contractile properties were studied using femoral nerve stimulations. Electrical impulses were delivered through a pair of surface electrodes. The anode (self-adhesive stimulation electrode, 10 cm x 5 cm) of the electrical stimulator (Digitimer DS7, Hertfordshire, England) was pasted halfway between the superior aspect of the greater trochanter and the inferior border of the iliac crest. The cathode (10 mm diameter ball probe) was pressed in the femoral triangle and moved to the position allowing the biggest contraction. Optimal stimulation intensity was determined in isometric condition using series of three square-wave stimuli (1-ms duration, 400 V maximum voltage and...