Longitudinal changes in the rate of force development during single-leg press and sports performance for jumpers / 体力科学

The aim of this study was to investigate longitudinal changes in rate for development (RFD) during the early, late, and whole phases, and to determine whether early RFD is exhibited in response to the improvement of explosive force production in training throughout the competitive season. Fifteen male jumpers participated in personal training according to the training cycle (Semi-annual two-cycle periodization model) of their team. The early, late and average RFD along with peak force during isometric single-leg press with a knee angle of 115° were measured. The measurement was performed during the pre-season (January or March) and post-season (September or November). Based on comparison between the two measurements, it was found that 10 participants displayed improved performance in the major jump event. The average RFD increased significantly for all participants (p = 0.02, d = 0.67). Participants who achieved their personal best had seen a significant increase in early RFD (p = 0.03, d = 0.84). In the contrast, late RFD (all participants; p = 0.06, d = 0.54, participants who achieved personal best; p = 0.07, d = 0.65) and peak force (all participants; p = 0.31, d = 0.27, participants who achieved personal best; p = 0.15, d = 0.50) did not show significant changes. These results suggested that changes in early, late, and average RFD varied respectively. Moreover, early RFD was found to be the best indicator that reflects improved jumping performance through explosive force production compared to late RFD, average RFD, and peak force.

The mechanical characteristics of single-leg squat exercise with emphasis on pelvic elevation / 体力科学

Movement control and muscle function for pelvic movement in the frontal plane (pelvic elevation) are important for various single-leg sports activities. We aimed to clarify mechanical characteristics of pelvic squat (P-Sq: single-leg squat exercise with emphasis on pelvic elevation, developed by our research group) compared with the double-leg squat (D-Sq) and single-leg squat (S-Sq). Twelve male track and field athletes performed D-Sq, S-Sq, and P-Sq exercises at various loads (90%, 75%, and 60% of 1-repetition maximum [1RM]), using maximum effort. Kinematic and kinetic data were calculated using data recorded with a motion capture system and force platforms. We observed the highest values with P-Sq, followed by S-Sq and D-Sq under all load conditions as follows: peak vertical ground reaction force and rate of force development (RFD), range of pelvic elevation, peak pelvic elevation velocity, peak powers associated with hip abduction torque and trunk lateral flexion torque. In P-Sq, RFD at 90% 1RM was smaller than under the other load conditions, whereas peak vertical ground reaction force at 90% 1RM was larger than under the other load conditions. There were no differences among load conditions with regard to hip abduction and trunk lateral flexion torques and powers. Therefore, characteristics of P-Sq compared to those of D-Sq and S-Sq are 1) larger and faster pelvic elevation, using related muscles (hip abductors and trunk lateral flexors) under all load conditions, 2) larger peak ground reaction force with pelvic elevation under large load conditions, and larger RFD in pelvic elevation under low load conditions.

Evaluation of specific strength and power at the lower extremity of jumpers using the rebound jump test / 体力科学

<p>We aimed to investigate the characteristics of lower-limb strength and power used for lower-limb mechanical variables in rebound jump (RJ) test by using a new system (Quick Motion Analysis System), which calculates mechanical variables in real time. Thirty-three male jumpers performed the RJ test. The performance (RJ index, contact time, and jump height) and joint kinetics (joint work and joint contribution) in RJ were calculated. IAAF Scoring Tables of Athletics were used to calculate jump event performance (IAAF score). IAAF score was positive correlated with RJ index, jump height, and joint work at the ankle and hip joints. Elite jumpers achieved higher RJ performance by larger ankle and hip joint work. As performance variables, jumping height and contact time were converted to T scores, and evaluation method was proposed to use the relative merits of these values to classify athletes into four types. The IAAF score showed no differences among the four types. These results indicate that there is no relation among jump events performance and characteristics of the four types. Moreover, focusing on stiffness, based on the contact time and jump height, jumpers with a longer contact time and higher jump height type showed lower stiffness (compliant spring characteristics), whereas those with the opposite features showed higher stiffness (stiffer spring characteristics). Therefore, for evaluating lower-limb strength and power characteristics, the use of performance and joint kinetics are effective, in addition to focusing on type characteristics based on the contact time and jump height in RJ.</p>

Time-series relationship to achieve performance on rebound drop jump / 体力科学

<p>The aim of this study was to demonstrate a time-series relationship in drop jump (DJ) from a pre-set state with improved performance. Twelve male college athletes performed a DJ from a height of 0.60 m. DJ performance was assessed with a DJ-index (jump height/contact time). Short-interval intracortical inhibition (SICI) was assessed as intracortical inhibitory circuit excitability in a pre-set state, calculated by using paired-pulse transcranial magnetic stimulation for the medial gastrocnemius muscle (MG). The H-reflex of the left MG and the ankle joint torque were calculated in the early phase of take-off. A significant correlation was shown between ⊿SICI during the pre-set state and the DJ index. Thus, we examined the relationships between phases, focusing on time-series relationships throughout the jump period. The results showed a significant correlation between ⊿SICI during the pre-set state and %H-reflex during the early phase of take-off, and peak ankle joint torque during take-off was also significantly correlated with %H-reflex during the early phase of take-off. A significant correlation was also demonstrated between ankle joint torque during take-off and the DJ-index. In conclusion, we observed a time-series relationship between DJ from a pre-set state and improved performance. A decrease of intracortical inhibitory circuit excitability in the pre-set state affects stretch-reflex facilitation during the early phase of take-off; stretch-reflex facilitation results in the development of a large force in the ankle joint during take-off, and this force develops ankle joint torque. These findings may be used to improve jump performance.</p>

Effect of short-interval intracortical inhibition in motor cortex during pre-set on rebound drop jumping performance / 体力科学

This study aimed to assess the effect of short-interval intracortical inhibition (SICI) before a jump off a platform (pre-set) on drop jump (DJ); the purpose was to investigate the relationship between this activity and performance, and the different effects of SICI on agonist and antagonist muscles during pre-set for jump athletes. Jump athletes (Jumper group, n=13) and Other athletes (Other group, n=9) performed DJ from drop heights of 0.30, 0.45, and 0.60 m). DJ performance was evaluated with DJ-index which was calculated from contact time and jump height. SICI was calculated from motor evoked potentials (MEP) recorded using paired-pulse transcranial magnetic stimulation for the medial gastrocnemius (MG) and tibialis anterior (TA) muscles in 7 jump athletes. Significantly higher DJ performance was observed for the Jumper group at all drop heights, and the Jumper group exhibited greater performance for the highest drop height than the Other groups. Significant decreases in SICI for MG were observed for the Jumper groups, and this decrease in inhibition was more prominent for the highest drop height during pre-set. Furthermore, the correlation between SICI for MG and DJ-index was stronger for increased drop heights, and a significantly positive correlation between these variables was observed at a height of 0.60 m. However, the SICI during the pre-set for TA exhibited no significant change under any of the conditions. The results of the present study suggest the importance of selective disinhibition of brain areas associated with the agonistic muscles during pre-set for higher DJ performance.

Comparison of loads and characteristics of lower extremity joint kinetics during clean exercises / 体力科学

The purpose of this study was to identify the ground reaction force and lower extremity joint kinetics during clean exercises performed with light and heavy loads. Thirteen male track and field athletes performed the clean exercises from 20 kg to one repetition maximum (1RM). Kinetic data were collected from recorded data using a Vicon motion system (250 Hz) and force platforms (1,000 Hz). The results of the analyses were as follows: (1) Ground reaction force and joint kinetics of hip and ankle were increased by increasing loads in clean exercises. (2) From light to heavy loads, we found relationships between ground reaction force and joint torque of hip in clean exercises. (3) In progressing from light to heavy loads, some subjects continued to involve the muscle group controlling the hip joint as the main power source, while others did not. The results indicate that clean exercises yield different individual characteristics for training. These results suggest that the clean exercise is effective for selectively the hip extensor in all loads.

EFFECT OF THE DIFFERENCE OF AEROBIC CAPACITY ON ENERGY CONTRIBUTION RATE IN WINGATE TEST / 体力科学

The purpose of this study was to investigate the influence of physiological factors which effect oxygen kinetics and energy system contribution on the power of Wingate test (WT), with focusing on the difference of aerobic capacity. Twenty three male track and field athletes (sprinters, long distance runners and decathletes) performed the WT on electromagnetic-braked cycle ergometer. The applied resistance was 7.5% of body weight, and the duration was 60 seconds. Moreover, aerobic capacity (maximal oxygen uptake [VO₂max]) was determined by an incremental test, and anaerobic capacity (maximal accumulated oxygen deficit [MAOD]) was determined by a supramaximal constant load test. The oxygen uptake during each test was recorded by a breath-by-breath method. The participants were divided into two group which was high VO₂max group (High group; n = 11) and low VO₂max group (Low group; n = 12). In the results, although the VO₂max was significantly higher in the High group, the MAOD was not significantly different between two groups. The oxygen uptake during WT was significantly higher in the High group, and the accumulated oxygen deficit during WT was significantly higher in the Low group. The aerobic contribution was significantly higher in the High group than in the Low group. In contrast, the anaerobic contribution was significantly higher in the Low group than in the High group. These results suggest that by the difference of aerobic capacity, aerobic and anaerobic energy supply contribution was different in WT.