Jubeau M.

INTRODUCTION Isometric contractions of the quadriceps femoris evoked by electromyostimulation (EMS) induce muscle damage indicated by decreases in maximal voluntary contraction strength (MVC), delayed onset muscle soreness, and increases in creatine kinase (CK) activity in the blood (1). It is known that muscle damage is attenuated in a subsequent bout eccentric exercise (2). A recent study (3) reported that such adaptation effect was induced in eclectically stimulated eccentric exercise. However, no previous study has examined if this is also the case for muscle damage induced by EMS isometric contractions. Therefore, this study compared between first and second bouts of EMS isometric exercise of the knee extensors for changes in MVC, muscle soreness and tenderness, and plasma CK activity. METHODS Nine healthy men (31 ± 4 yrs) with no resistance training experience for the last six months had two EMS bouts separated by 2-3 weeks. One of the knee extensors (randomly chosen) was stimulated by an Intelect Stim (Chattanooga, USA) while seating on a Biodex isokinetic dynamometer with the knee joint angle of 100° (0°: full extension). Four self-adhesive electrodes were placed on the anterior surface as follows: 2 positive electrodes over the motor point of the vastus lateralis and vastus medialis, and 2 negative electrodes on the proximal portion of the quadriceps femoris. Forty isometric contractions were induced by EMS with biphasic rectangular pulses (75 Hz, 400 µs, on-off ratio 5-15 s), while the current amplitude was consistently increased throughout contractions until maximal tolerable level. Criterion measures consisted of maximal isometric strength at 100°, muscle soreness (a 100-mm visual analogue scale), pressure pain threshold of the muscles, and plasma CK activity. These measures were taken before and 1, 24, 48, 72 and 96 h after EMS bout. Changes in the measures over time were compared between bouts by a two-way repeated measures ANOVA with a Tukey’s post-hoc test. RESULTS The evoked torque produced during both bouts was approximately 30% of MVC, and no significant difference between bouts was evident for the changes in torque over 40 contractions. MVC decreased significantly (P<0.05) by 26% at 1 h after both bouts, but the recovery was significantly (P<0.05) faster after the second bout (100% at 96h) than the first bout (81% at 96h). Development of muscle soreness and increases in CK were significantly (P<0.05) smaller after the second bout than the first bout. DISCUSSION These results showed that the second bout resulted in less muscle damage than the first bout. This adaptation is similar to that shown in eccentric exercise-induced muscle damage (2,3). It appears that the protective adaptation is induced locally rather than centrally. REFERENCES 1) Jubeau M et al. (2008) Appl Physiol, 104:75-81 2) Clarkson P et al. (1992) Med. Sci Sports Exerc, 24:512–520 3) Black C & McCully K (2008) Med Sci Sports Exerc, 40:1605-15