The effect of strength and flexibility training on skeletal muscle electromyographic activity, stiffness, and viscoelastic stress relaxation response.

The present study examined whether isometric strength training alone or isometric strength training combined with flexibility training of the hamstring muscles altered the viscoelastic response during stretch. Twelve male subjects performed isometric training (strength) on one side and isometric and flexibility training (strength and flexibility) on the other side for 13 weeks; 10 other subjects served as controls. Passive torque offered by the hamstring muscle group was measure during passive knee extension using a dynamometer. The knee was passively extended to a predetermined final position at 0.0875 rad/sec (dynamic phase), where it remained stationary for 90 seconds (static phase). The slope of the line (stiffness) and the area under the curve (energy) in the dynamic phase, and the decline in passive torque (viscoelastic stress relaxation) in the static phase were analyzed. Isometric strength was determined with a dynamometer. A strength test and a stretch maneuver were administered before and after the training period. All variables were unchanged in the control group. Isometric strength increased similarly on both training sides by 43%. The stretch maneuver showed that energy, stiffness, and passive torque increased on both training sides while low-level electromyographic recordings remained constant. Furthermore, the viscoelastic stress relaxation response (31% to 33%) was unaffected by the training. The addition of flexibility exercise had no significant effect on these strength training responses. These data suggest that an increase in isometric strength is accompanied by changes in the material properties of the muscle that are unaffected by flexibility exercises.