Effect of Dynamic Resistance Training on Isometric Strength, Architecture, and Biomechanical Efficiency of Muscles when Performing Vertical Jumps from Various Heights

The objective of the study was to evaluate the changes in early adaptation of the architecture and isometric and dynamic strengths of leg extensor muscles 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 exercises in three training sessions per week for 6 weeks. The workout consisted of concentric calf raises and eccentric lowerings; 10 repetitions were performed in five sets with 1-min breaks between the sets. Each subject performed ankle extension to full plantar extension within 2 s in the concentric mode and then performed ankle flexion in the eccentric mode to return to the original neutral position within 2 s. The following parameters were recorded before and after the training period: maximum voluntary contraction (MVC) as measured with a Biodex isokinetic dynamometer (United States); force of voluntary “explosive” contraction; force at 50-ms intervals from beginning the effort (F50, F100, F150, F200, F250, and F300) during voluntary explosive isometric contraction; jump height; relative and absolute powers; and take-off speed during squat jumps (SJs), countermovement jumps (CMJs), and drop jumps (DJs) from drop heights 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 (United States) at 30% of the distance between the popliteal fossa and the center of the lateral malleolus at rest with the ankle joint in neutral position. Longitudinal ultrasound images of the MG were obtained in a relaxed state in this position, and the fiber length (Lf), the fiber inclination angle (Θf) relative to the aponeurosis, and the muscle thickness (Tm) were determined. Training was observed to increase Tm (+2.7%, p < 0.05), Θf (+10.4%, p < 0.05), MVC (+17.0%, p < 0.05), and the average force created in the first 50 ms (+25.0%, p < 0.05), while Lf decreased slightly (–2.1%). The take-off speed during DJ20 and CMJ was substantially higher than during SJ. The flying time in DJ60 was nonsignificantly shorter than in DJ20. The power varied depending on the drop height and was significantly higher in DJ20 compared with DJ60. The absolute power output during DJ20 was also significantly higher than during DJ60. The results showed that light-load dynamic resistance training increased MVC, explosive voluntary force in the early phase of a contraction, Tm, and Θf. An increase in Θf suggests an increased stiffness of the musculotendinous complex and, therefore, a more efficient transmission of force from fibers to tendons and higher rapid force production. The finding provides new evidence for peripheral adaptation developing as early as 6 weeks of training.