Effects of Loaded Squat Exercise with and without Application of Superimposed EMS on Physical Performance

The aim of the present study was to investigate the effects of a multiple set squat exercise training intervention with superimposed electromyostimulation (EMS) on strength and power, sprint and jump performance. Twenty athletes from different disciplines participated and were divided into two groups: strength training (S) or strength training with superimposed EMS (S+E). Both groups completed the same training program twice a week over a six week period consisting of four sets of the 10 repetition maximum of back squats. Additionally, the S+E group had EMS superimposed to the squat exercise with simultaneous stimulation of leg and trunk muscles. EMS intensity was adjusted to 70% of individual pain threshold to ensure dynamic movement. Strength and power of different muscle groups, sprint, and vertical jump performance were assessed one week before (pre), one week after (post) and three weeks (re) following the training period. Both groups showed improvements in leg press strength and power, countermovement and squat jump performance and pendulum sprint (p < 0.05), with no changes for linear sprint. Differences between groups were only evident at the leg curl machine with greater improvements for the S+E group (p < 0.05). Common squat exercise training and squat exercise with superimposed EMS improves maximum strength and power, as well as jumping abilities in athletes from different disciplines. The greater improvements in strength performance of leg curl muscles caused by superimposed EMS with improvements in strength of antagonistic hamstrings in the S+E group are suggesting the potential of EMS to unloaded (antagonistic) muscle groups. Key points Similar strength adaptations occurred after a 6 week 10 RM back squat exercise program with superimposed EMS (S+E) and 10 RM back squat exercise (S) alone. Specific adaptations for S+E at the leg curl muscles were evident. S and S+E improved SJ, CMJ and pendulum sprint performance. No improvement occurred in linear sprint performance. Key words: Electrical stimulation, strength training, MVC, peak power output, sprint, change of direction speed, jump height Introduction Resistance training is the predominant method to enhance strength and performance in power related abilities like sprinting and jumping (Comfort et al., 2012). Squat exercise with additional load is one common exercise shown to improve lower limb strength and muscular power, as well as jump and sprint performance (Chelly et al., 2009; Cormie et al., 2010). Furthermore, Electromyostimulation (EMS) is known to be an effective method for improving the aforementioned factors of athletic performance too (Filipovic et al., 2012). The reasons for the improvements and improved adaptations with EMS are the higher number of motor units recruited during exercise with EMS compared to dynamic voluntary contractions only (Kots and Chiwlon, 1971) and, additionally, activation of fast-twitch fibers at relatively low force levels (Gregory and Bickel, 2005). Furthermore, EMS superimposed to dynamic movements can also increase activation levels at different muscle length and during different contraction modes, e.g. during eccentric work phases (Westing et al., 1990). Willoughby and Simpson (1998) hypothesized that type II muscle fibers remain active during EMS in contrast to the normal continuing de-recruitment of motor units during the eccentric phase. Therefore, intensification of loaded squat exercise by superimposed EMS can potentially induce an increase in recruitment of high-threshold motor units (Dudley, 1992). EMS potentially supports the athlete to achieve power and sport-specific movement velocities within resistance training (Young, 2006) by increased firing rate and a synchronization of motor units (Gregory and Bickel, 2005). Further advantages on muscular strength and power could be achieved by whole body EMS devices that are able to stimulate several muscle groups simultaneously, e.g. muscle chains or agonist/antagonist during multi joint movement like squat exercise. Stimulation of muscle chains could support squats by compensating usual weak points like hip extensor (Lynn and Noffal, 2012) or the lower back muscles (Hamlyn et al., 2007). Furthermore, it is possible that counterproductive firing of agonist and antagonist evoke demands on voluntary contraction, especially on a reduced co-activation of antagonistic muscles, to continue the required dynamic exercise. To improve muscle strength and power by the use of EMS the settings such as impulse intensity; stimulation frequency; impulse width; pulse type; and stimulation ratio have to be taken into account (Filipovic et al., 2011). In most studies a biphasic impulse type rather than a monophasic is applied (Babault et al., 2007; Maffiuletti et al., 2009). This offers advantages for applying high stimulation intensities and, therefore, has a higher influence on the enhancement of strength abilities. Furthermore, muscle contraction force can be regulated by varying the level of impulse intensity (Lake, 1992). However, due to the resistance of different tissue structures it is not possible to precisely determine the impulse intensity (mA) that ultimately reaches the muscles. Most studies used the maximum pain threshold (maximum tolerated amperage) to regulate the maximum impulse intensity (Brocherie et al., 2005; Maffiuletti et al., 2009). However, a high level of muscle tension due to EMS limits the range of dynamic movements. Therefore, in dynamic exercise modes with superimposed EMS, the impulse intensity has to be adjusted to ensure sufficient movement. There are lacks of studies dealing with dynamic exercise and superimposed EMS. It has been shown that 70% of maximum pain threshold is practicable and might be auspicious, because of subjective feeling of increased intensity (Doermann, 2011). With regards to the stimulation frequency, a wide range between 2-200 Hz is recommended (Bossert et al., 2006). Comprehensive recommendations for high stimulation intensities range between 50-100 Hz (Filipovic et al., 2011). In addressing the level of impulse width, a compromise needs to be found in order to activate deeper motor units without being unpleasant for the athlete. Longer impulse durations result in deeper and more intensive muscle stimulation, which results in more motor units being recruited (Baker et al., 1993; Bossert et al., 2006). Bossert et al. (2006) recommend a level between 300-400 microseconds due to the unpleasant or even painful sensation above that level. Regarding the stimulation ratio, Filipovic et al. (2011) revealed a predominant use of short impulse on-times of 6.0 ± 2.4 seconds in all EMS methods for enhancing strength abilities. During dynamic exercise, on-times should be synchronized to movement and repetitions. A number of different studies have documented the positive effects of applying EMS on physical performance parameters such as muscle strength and power (Babault et al., 2007), sprint and jumping performance (Herrero et al., 2006) or anaerobic performance (Herrero et al., 2010b). A review by Filipovic et al. (2012) found that EMS methods are also effective in enhancing maximal strength and power in elite athletes and consequently increasing jumping and sprinting ability. Enhancing performance parameters with the application of EMS training periods from 4-6 weeks, consisting of three sessions per week has been shown to be sufficient (for review see Filipovic et al. (2011)). Most of the previous research has focused on EMS at maximal intensities during isometric contractions in one muscle (group), e.g. the m. quadriceps femoris. Only one study has addressed dynamic movements during EMS or EMS superimposed to leg strength training (Willoughby and Simpson, 1998) with positive effects. Currently, no study has investigated the effects of EMS applied to several muscle groups superimposed to loaded back squat exercise. Superimposed EMS could improve the quality of squat exercise during specific block training phases and increase training adaptations on a high level. Therefore, the aim of this study was to investigate the effects of a 10 repetition maximum (RM) loaded back squat exercise program with EMS superimposed to several leg and trunk muscles on athletic performance. Athletic performance parameters were differentiated in maximum isometric strength and isoinertial power of several leg and core muscle groups, jump height and sprint time. It is hypothesized that squat training with superimposed EMS will increase strength and power and these improvements will enhance jump and sprint performance more than squat training alone.