The Influence of Dynamic Resistance Training on Isometric Muscle Strength, Architecture and Biomechanical Effectiveness of the Lower Limb Extensor Muscles when Performing Vertical Jumps from Different Heights

The purpose of this study was to evaluate changes in early adaptation of muscle architecture, isometric and dynamic leg extensor muscle strength in response to light load training. A group of young novice participants (n = 6, age 21.8 ± 2.3 years, body weight 74.8 ± 9.2 kg, height 1.75 ± 0.08 m) performed low-speed slow concentric and eccentric training for 6 weeks 3 times a week. The workout consisted of concentric calf raises and eccentric lowering for 10 repetitions in 5 sets. In the concentric mode, the subject performed ankle extension to full plantar extension within 2 s, and then in the eccentric mode performed ankle flexion and returned to the original neutral position within 2 s. Before and after the training period, the following were recorded: maximum voluntary contraction (MVC) using a Biodex isokinetic dynamometer (USA), force of voluntary “explosive” contraction, force with an interval of 50 ms from the beginning of the effort (F50, F100, F150, F200, F250 and F300) during voluntary explosive isometric contraction, jump height, relative and absolute power, take-off speed when jumping from a squat jumps (SJ), countermovement jumps (CMJ) and during jumping (DJ) from a height of 20, 40 and 60 cm (DJ20 - DJ60). Kinetic data were collected using a contact platform. The muscle structure of the medial gastrocnemius muscle (MG) was visualized using an Edge ultrasound scanner (USA) at 30% of the distance between the popliteal crease and the center of the lateral malleolus at rest with the ankle joint in neutral position. In this position, longitudinal ultrasound images of the MG were obtained in a relaxed state with determination of the length (Lf) and angle of inclination of the fibers (Θf) relative to the aponeurosis and muscle thickness (Tm). After training, an increase in Tm (+2.7%, p < 0.05) and Θf (+10.4%, p < 0.05), MVC (+17.0%, p < 0.05), average force created in the first 50 ms (+25.0%, p < 0.05) with a slight decrease in Lf (-2.1%). The repulsion speed during DJ20 was 2 times higher than the speed during DJ60 and was higher than at DJ60. The time without support phase during DJ60 was not significantly shorter than during DJ20. The power value varied depending on the jump height, but was significantly higher during DJ20 than during DJ60. The absolute power value during DJ20 was also significantly higher than during DJ60. Our results show that light load dynamic resistance training leads to increases in MVC, explosive voluntary force in the early phase of contraction, Tm and Θf. The latter may indicate increased stiffness of the musculo-tendinous complex, allowing more efficient transmission of force from fibers to the tendons, increasing rapid force production, providing new evidence of peripheral adaptation in just 6 weeks of training.