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.

THE RELATIONSHIP BETWEEN THE RUNNING ECONOMY AND THE HOPPING ECONOMY IN THE LONG DISTANCE RUNNER / 体力科学

Running economy is an important factor in determining a performance of a long distance running. The purpose of this study was to examine the relationship between the running economy and the submaximal hopping economy. Twelve long-distance runners performed a submaximal repeated hopping exercise on a force platform at a frequency of 2.2Hz and the maximum five-repeated rebound jumping (5RJ). Jumping height, contact time, maximum ground reaction force and oxygen intake were recorded during submaximal repeated hopping exercise. In addition, they performed the submaximal running for a distance of 3200m on an outdoor 400m track. Oxygen intake was recorded during the submaximal running. Then running economy (RE = V/VO₂) was calculated by using oxygen intake (VO₂) and running speeds (V). Hopping economy (HE = h/VO₂) was calculated by using oxygen intake (VO₂) and average jumping height (h). As a result of this study, we confirmed that a submaximal repeated hopping exercise performed for a ten-minute period was an aerobic exercise and a steady-state exercise. There was a significant positive correlation between RE and HE (r = 0.805, p＜0.01). These results suggest that hopping economy is an important factor in running economy. On the other hand, we did not find a significant correlation between HE and 5RJ. Furthermore, between RE and RJ index of the submaximal hopping exercise, a significant positive correlation was found (r = 0.735, p＜0.01). Therefore, RE seems to be connected with the Stretch-shortening cycle (SSC) function of the legs. We conclude that the SSC ability of a long distance runner can be evaluated appropriately by using the submaximal repeated hopping exercise of this study.

PHYSICAL ACTIVITY PATTERNS AND PHYSICAL FITNESS LEVELS OF HOMEBOUND ELDERLY PEOPLE LIVING IN THE COMMUNITY / 体力科学

There have so far been no studies examining the physical fitness and physical activity (PA), measured using objective measures, in homebound elderly people. The purpose of this study was to examine physical fitness levels and PA patterns and evaluate their relationships in homebound elderly people. In 2004, a total of 3964 community-dwelling elderly aged 65 years and over participated in a base line survey. The subject data were directly collected by in-home interviewer. Subjects were defined as being homebound if they went outdoors less than once a week. However, the subjects who could not go out without assistance due to sickness and/or disability were excluded from the analysis. In 2005, 38 homebound (22 men, 16 women) and 70 non-homebound (33 men, 37 women) older adults who participated in the base line survey were selected. Measurements of physical fitness levels and PA patterns measured using an accelerometer, were taken in 2005. The total steps per day did not significantly differ between homebound and non-homebound men and women. The time spent in 1.8 METs activity (corresponding to activity level 1 of the accelerometer) was significantly higher in women than in men for non-homebound and homebound, respectively. The time spent in lower-, moderate-, and high-intensity PA did not significantly differ between homebound and non-homebound men and women, respectively. Handgrip strength, knee extensor strength, leg extensor power, stepping and maximum walking speed were significantly higher in non-homebound than in homebound men and women. These results suggest that the physical fitness levels of homebound were lower than those of non-homebound, but no difference was observed in the PA levels between homebound and non-homebound.

EFFECTS OF PLYOMETRICS ON THE ABILITIES OF THE JUMP, FOOTWORK AND THE CHEST PASS IN COMPETITIVE BASKETBALL PLAYERS / 体力科学

The purpose of this study was to determine the effects of plyometrics on jump, footwork and chest pass ability in competitive basketball players. Ten male basketball players competing at the college level in Japan practiced plyometrics by using the drop jump and the medicine ball throw three sessions per week for seven weeks. All players had continued regular training and competition in basketball over ten years. Height and contact time of the rebound drop jump and the jump shot were tested for each player along with sprint running velocity in a straight direction and with four footwork phases for changing direction, ball velocity and contact time of hands for chest pass pre and post plyometrics. Drop jump training significantly shortened contact time but did not enhance jumping height of the jump shot with a basketball. Moreover, drop jump training significantly shortened average velocity of sprint running with four footwork phases for changing direction, but did not enhance sprint running in a straight direction. Medicine ball training significantly enhanced ball velocity and contact time of hands for the chest pass. These results suggest that plyometrics, by using the drop jump and medicine ball throw, are effective training methods for improving jump, footwork and chest pass ability in competitive basketball players.

Characteristics of torque production at the lower limb joints with special reference to type of muscle contraction / 体力科学

A study was conducted to investigate the relationship between the characteristics of torque production with special reference to eccentric and concentric muscle contraction and the structural and functional properties at the lower limb joints in the human kinetic chain. Ten male college athletes were tested with a isokinetic dynamometer (biodex) for eccentric and concentric torques during extension and flexion at the hip and knee, and plantar flexion and dorsiflexion at the ankle (angular velocity ; 30, 60, 120 deg/s) . The peak eccentric and concentric torques were higher in the order hip, knee and ankle joints, as the size of each muscle acting on its joint increased. However, the rate of peak concentric to eccentric torque (CON/ECC, %) was higher at the ankle joint than at the hip and knee joints. They tended to increase in the order of angular velocity ; 30, 60, 120 deg/s. These results suggest that the ankle joint is charactered by higher eccentric torque production. This characteristic is probably due to the fact that (1) the ankle joint is located at the end of the human kinetic chain and plays a role in transmitting the power to the ground effectively, (2) the muscle tendon complex acting at the ankle joint must have increased stiffness and tolerate great stretch loads to store and reuse the amount of elastic energy, (3) the hip and knee joints located at the center are the main sources of power supply. On the other hand, no significant correlations were observed among the torques at the hip, knee and ankle. These results suggest that torque production is independent at each of the lower limb joints, and is based on the structural and functional properties of each joint. These findings seem to be useful to clarifying the methods of strength and power training that can be adapted to the characteristics of torque production and the structural and functional properties of the hip, knee and ankle joints.

Effects of landing motion on power during takeoff in rebound drop jump. With special reference to angle at the knee joint / 体力科学

This study clarified the most appropriate landing motion for enhancement of the rebound drop jump index (RDJ<SUB>index</SUB>), which can evaluate the ability to perform ballistic and stretch-shortening cycle (SSC) movements. The RDJindex was calculated using the formula RDJ<SUB>index</SUB>= (1/8⋅g⋅RDJt<SUB>a</SUB><SUP>2</SUP>) /RDJt<SUB>c</SUB> where RDJt<SUB>c</SUB> and RDJt<SUB>a</SUB> are the contact and air times during a rebound drop jump from a height of 0.3 m (RDJ), a typical SSC movement. The relationships between the RDJ<SUB>index</SUB> and the characteristics of the knee and ankle joint motions during RDJ were examined in nine male jumpers. The results were as follows:<BR>(1) The characteristics of leg motion during the descending phase were that the ratio (%K-ANG) of displacement in flexion of the knee during the descending phase to that during both descending and takeoff phases was 48.6% and this preliminary motion started 53.6 ms before touchdown. Furthermore, as %K-ANG increaced, the contact time decreased (r=-0.784, p<0.05), the air time increased (r=0.874, p<0.01) and consequently, the RDJ<SUB>index</SUB> increased (r=0.891, p<0.01) . These results suggested that quick knee flexion just before touchdown is an important factor in increasing the RDJ<SUB>index</SUB>.<BR>(2) A characteristic of the leg motion during the takeoff phase was that the end point of knee flexion appeared 13.3 ms earlier than that of ankle dorsiflexion. That time increased as %K-ANG increaced (r=0.830, p<0.01), but conversely, as %K-ANG decreased markedly, end point of ankle dorsiflexion appeared earlier than that of knee flexion. These results suggested that preliminary motion of the knee would result in appropriate timing of the knee and ankle motions for shock absorption during the takeoff phase of RDJ.<BR>(3) An other characteristic of the leg motion during the takeoff phase was that the ratio of displacement of the knee in extension to that in flexion was 310.2%. This ratio increased as %K-ANG increaced (r=0.903, p<0.001) . These results suggested that preliminary motion of the knee would cause to increase displacement in extension for kick motion and yet to decrease displacement in flexion for shock absorption.<BR>These findings led to the conclusion that quick and short range flexion at the knee just before touchdown was an effective landing motion for enhancing the ability to perform ballistic and SSC movements.

Determinants of ability to achieve ballistic stretch-shortening cycle movement. With special reference to strength and power / 体力科学

Long jump and high jump, and several types of jumping and footwork with rapid changes in speed and direction of movement in ball games are characterized not only by a combination of eccentric and concentric movement (stretch-shortening cycle movement, SSC) but also a ballistic movement of very short duration (100-200 ms) . This study was conducted to examine the effects of strength and power on ballistic stretch-shortening cycle movement (BSSC) ability to clarify the methods of strength and power training necessary for jumpers and ball game players. The subjects were 99 male college athletes. The index for BSSC ability was RDJ<SUB>index</SUB> [RDJ<SUB>index</SUB> = (1/8⋅g⋅RDJt<SUB>a</SUB><SUP>2</SUP>) /RDJt<SUB>c</SUB>] which was calculated using contact time (RDJt<SUB>c</SUB>) and air time (RDJt<SUB>a</SUB>) in a rebound drop jump from a height of 0.3 m with a small angular range of motion for the knee. The index for strength was maximum leg extension strength exerted in a squat position at a 90° knee angle (Smax/BW) . The index for power was the height of a counter movement jump (CMJh) . Results were summarized as follows;<BR>(1) There were significant correlations between RDJ<SUB>index</SUB>and Smax/BW (r=0.219, p <0.05), RDJ<SUB>index</SUB>and CMJh (r=0.584, p<0.001), and Smax/BW and CMJh (r=0.487, p<0.001), but they were fairly low, suggesting that these indices do not have similar significance for estimating training effects and sports talent.<BR>(2) There was no significant correlation between RDJt<SUB>c</SUB>and RDJt<SUB>a</SUB>, of which the RDJ<SUB>index</SUB>was composed (r=-0.145, ns) . This result suggests that BSSC ability consists of two independent abilities, i. e. the ability to jump higher and the ability to shorten one's movement time.<BR>(3) There were significant correlations between RDJt<SUB>a</SUB>and Smax/BW (r=0.340, p<0.001), RDJt<SUB>a</SUB>and CMJh (r = 0.647, p <0.001), but not RDJt<SUB>c</SUB>and Smax/BW (r=0, 035, ns), or RDJt<SUB>c</SUB>and CMJh (r=-0.187, ns), suggesting that the ability to jump higher is affected by strength and power whereas the ability to shorten one's movement time is not.<BR>(4) When subjects with equal RDJ<SUB>index</SUB>were compared, one subject was dominant in RDJtc and another in RDJt<SUB>a</SUB>. This result suggests that we should consider individual differences based on RDJt<SUB>c</SUB>and RDJt<SUB>a</SUB>in training for increasing the RDJ<SUB>index</SUB>.<BR>These findings seem to be useful to clarifying the methods of strength and power training for jumpers and ball game players who need to improve their ballistic stretch-shortening cycle movement ability.