Kazuyoshi Miyaguchi

Specific factors that influence deciding the takeoff leg during jumping movements.

Miyaguchi, K and Demura, S. Specific factors that influence deciding the takeoff leg during jumping movements. J Strength Cond Res 24(9): 2516-2522, 2010-The definition of the dominant leg (lateral dominance) is not clear, and there has been little reporting related to lateral dominance in the legs. To uncover the practical factors influencing which leg to use as the takeoff leg in 1-legged jumping movements, this study aimed to investigate the subjective dominance side of fundamental movements and to examine the lateral dominance of motor functions between the takeoff leg and non-takeoff leg. The subjects consisted of 27 young men who exercised regularly. They had not trained particularly on unilateral jumping. Fifteen men are the athlete group (left-legged jumpers group [LG]) using a left leg and 12 men are the athlete group (right-legged jumpers group [RG]) using a right leg as determined by a preliminary survey related to takeoff leg during high jump. The fundamental motions of the subjective dominant leg were investigated and the differences between the motor functions of takeoff and lead legs, such as sole shapes, single-leg vertical jump, 20-m hopping, ladder hopping, single-leg balance, and isokinetic strength were examined. It was found that many RG subjects (83%) tended to select the right leg for hopping, and many LG subjects (87%) tended to select the left leg for 1-legged balance. It was suggested that skilled movements show right-leg dominance in both takeoff leg groups. In the LG subjects, the left leg showed a higher value than the right leg in sole shape. The RG subjects showed a higher value in the right leg than in the left leg in a single-leg vertical jump. However, marked dominance was not found in the takeoff leg. The lower limbs may not show marked lateral dominance such as in the upper limbs.

Relationships between stretch-shortening cycle performance and maximum muscle strength

This study aimed to examine the relationships between muscle power output using the stretch-shortening cycle (SSC) and maximum strength, as measured by the 1 RM (1 repetition maximum) test and the isokinetic dynamometer under elbow flexion. Sixteen trained, young adult males pulled a constant load of 40% MVC (maximum voluntary elbow flexion contraction) by ballistic elbow flexion under the following two preliminary conditions: 1) the static relaxed muscle state (SR condition) and 2) using the SSC (SSC condition). Muscle power was determined from the product of the pulling velocity and load mass by a power measurement instrument with a rotary encoder. The 1 RM bench press (1RM BP) and isokinetic maximum strength under elbow flexion with the Cybex-325 were measured as indicators of dynamic maximum strength. 1) The early power output exerted under the SSC condition showed a significant and high correlation with the 1 RM BP (r = 0.83), but only moderate correlation with the isokinetic muscle strength (r = 0.50-0.67). 2) The contribution of the 1 RM BP to the early muscle contraction velocity exerted under the SSC condition was large. These results suggested that muscle power exerted using the SSC shows a stronger relationship with maximum muscle strength measured by a 1 RM test rather than isokinetic maximum strength.

Effectiveness of the 1rm Estimation Method Based on Isometric Squat Using A Back-dynamometer

Demura, S, Miyaguchi, K, Shin, S, and Uchida, Y. Effectiveness of the 1RM estimation method based on isometric squat using a back-dynamometer. J Strength Cond Res 24(10): 2742-2748, 2010-This study aimed to clarify the relationships between isometric squat (IS) using a back dynamometer and 1 repetition maximum (1RM) squat for maximum force and muscle activities and to examine the effectiveness of a 1RM estimation method based on IS. The subjects were 15 young men with weight training experience (mean age 20.7 ± 0.8 years, mean height 171.3 ± 4.4 cm, mean weight 64.4 ± 8.4 kg). They performed the IS with various stance widths and squat depths. The measured data of exerted maximum force and the action potential of the agonist muscles were compared with the 1RM squat data. The exerted maximum force during IS was significantly larger in wide stance (140% shoulder width) than in narrow stance (5-cm width). The maximum force was significantly larger with decreased knee flexion. As for muscle activity, the % root mean square value of muscle electric potential of the rectus femoris and the vastus lateralis tended to be higher in wide stance. As for exerted maximum force, wide stance and parallel depth in IS showed a significant and high correlation (r = 0.73) with 1RM squat. Simple linear regression analysis revealed a significant estimated regression equation [Y = 0.992X + 30.3 (Y:1RM, X:IS)]. However, the standard error of an estimate value obtained by the regression equation was very large (11.19 kg). In conclusion, IS with wide stance and parallel depth may be useful for the estimation of 1RM squat. However, estimating a 1RM by IS using a back dynamometer may be difficult.

The difference in output properties between dominant and nondominant limbs as measured by various muscle function tests.

Demura, S, Miyaguchi, K, and Aoki, H. The difference in output properties between dominant and nondominant limbs as measured by various muscle function tests. J Strength Cond Res 24(10): 2816-2820, 2010-Recently, right and left output properties exerted from specific muscle groups have been evaluated using special measurement devices such as an isokinetic dynamometer. However, it remains unclear whether the coach can properly evaluate muscle function corresponding to lateral specificity in athletes. This study aimed to examine the different output properties between the dominant (D) and nondominant (ND) upper limbs as measured by muscle function tests with various muscle contraction patterns. Fifteen right-handed young men participated in this study. Each subject carried out isometric, isokinetic, and isotonic muscle power tests by elbow flexion with right and left arms. When calculating the laterality index, the laterality of the isotonic test (1.17) was the highest. In all tests, significant correlations were found between the measurements of the D and ND limbs. The isometric test was the highest (r = 0.93), followed by the isokinetic test (r = 0.66-0.83) and the isotonic test (r = 0.55). To examine the ratio of the laterality of measurements provided by each muscle function test, each measurement was converted to a standard score (Z-score). There were significant differences between D and ND limbs in the isometric (D:ND = 55.0:45.0) and the isotonic (54.1:45.9) tests but not in the isokinetic test (60°·s−1, 51.4:48.6; 180°·s−1, 50.7:49.2; 300°·s−1, 51.8:48.2). Particularly, the D (right) limb exerted greater muscle power in the isometric and the isotonic tests than in the isokinetic test. Occupational therapists or strength and conditioning professionals should understand that the D-ND differences shown by these muscle function tests may differ because of measurement conditions.

Possibility of Stretch-Shortening Cycle Movement Training Using a Jump Rope

Abstract Miyaguchi, A, Sugiura, H, and Demura, S. Possibility of stretch-shortening cycle movement training using a jump rope. J Strength Cond Res 28(3): 700–705, 2014—Although jumping rope has been said to be a typical stretch-shortening cycle movement (SSC) from the dynamic analysis of muscle contraction, there are few research reports that focus on this point. Recently, the function of SSC of the legs with respect to the jumping movement has been evaluated using the rebound jump index (RJ-index). This study aimed to examine the possibility of using rope jumping in SSC training by comparing the RJ-index of the rebound jump (standard value) and the 2 different methods of rope jumping. The subjects included 76 healthy young men. Most subjects were involved in routine sports training 2–3 times per week. They performed the rebound jump (5 consecutive vertical jumps) and both a basic and a double-under jump with the jump rope, according to each participants individual style (rhythm or timing). The RJ-index was calculated using the ground contact time and the jump height. The reliabilities of the RJ-index in the basic (intraclass correlation coefficient: 0.85) and double-under jump (0.92) were high, and the RJ-index of the latter (1.34 ± 0.24) was significantly higher than that of the former (0.60 ± 0.21). In the case of a group with inferior SSC ability, the RJ-index of the rebound jump only showed a significant correlation with the double-under but not with the basic jump. When using the RJ-index (1.97 ± 0.38) of the rebound jump as a criterion, the double-under—using about 70% of the SSC ability—may be effective for reinforcement of SSC ability.

Relationships between muscle power output using the stretch-shortening cycle and eccentric maximum strength

Miyaguchi, K and Demura, S. Relationships between muscle power output using the stretch-shortening cycle and eccentric maximum strength. J Strength Cond Res 22(6): 1735-1741, 2008-This study aimed to examine the relationships between muscle power output using the stretch-shortening cycle (SSC) and eccentric maximum strength under elbow flexion. Eighteen young adult males pulled up a constant light load (2 kg) by ballistic elbow flexion under the following two preliminary conditions: 1) the static relaxed muscle state (SR condition), and 2) using the SSC with countermovement (SSC condition). Muscle power was determined from the product of the pulling velocity and the load mass by a power measurement instrument that adopted the weight-loading method. We assumed the pulling velocity to be the subjects muscle power parameters as a matter of convenience, because we used a constant load. The following two parameters were selected in reference to a previous study: 1) peak velocity (m·s−1) (peak power) and 2) 0.1-second velocity during concentric contraction (m·s−1) (initial power). Eccentric maximum strength by elbow flexion was measured by a handheld dynamometer. Initial power produced in the SSC condition was significantly larger than that in the SR condition. Eccentric maximum strength showed a significant and high correlation (r = 0.70) with peak power in the SSC condition but not in the SR condition. Eccentric maximum strength showed insignificant correlations with initial power in both conditions. In conclusion, it was suggested that eccentric maximum strength is associated with peak power in the SSC condition, but the contribution of the eccentric maximum strength to the SSC potentiation (initial power) may be low.

Relationship between upper-body strength and bat swing speed in high-school baseball players

Abstract Miyaguchi, K and Demura, S. Relationship between upper-body strength and bat swing speed in high-school baseball players. J Strength Cond Res 26(7): 1786–1791, 2012—This study aimed to clarify the relationship between upper-body strength and bat swing speed in high-school baseball players and to examine the physical characteristics of home run hitters (sluggers). The subjects were 30 male high-school baseball players with national tournament experience at the Koshien Stadium. Bat swing speed exerted by full effort was measured with a microwave-type speed-measuring instrument. One-repetition maximum (1RM) of a bench press (BP), BP power (bench power) using a light load (30 kg), and isokinetic chest press (0.4, 0.8, 1.2 m·s−1) were measured as upper-body strength. The relationships between bat swing speed and upper-body strength values were examined. Additionally, the t-test was used to reveal the mean differences between 14 home run hitters (group A) and 16 mediocre hitters (group B) for each measurement value. The bat swing speed showed significant and middle correlations with the 1RM BP (r = 0.59), bench power (0.41), and isokinetic chest press (0.48–0.55). Group A had significantly higher values in bench power and isokinetic chest press (high-speed) per kilogram of body weight than did group B. The swing speed showed significant correlations (r = 0.62) with the 1RM BP in group B but not in group A. In conclusion, to improve the hitting power of high-school baseball players, it may also be important to develop bench power with light loads in addition to 1RM BP.

Gender Difference in Ability Using the Stretch-Shortening Cycle in the Upper Extremities

Miyaguchi, K and Demura, S. Gender difference in ability using the stretch-shortening cycle in the upper extremities. J Strength Cond Res 23(1): 231-236, 2009-A gender difference in ability using the stretch-shortening cycle (SSC ability) in the upper extremities has not been studied in detail. This study aimed to devise an index to evaluate SSC ability during powerful elbow flexion and to examine its gender differences. Thirty-three men athletes (19.9 ± 1.0 years) and 21 women athletes (20.6 ± 1.2 years) with mastered SSC movements participated in this study. They pulled a 20% load of maximal voluntary contraction (MVC) by elbow flexion as quickly as possible with the dominant upper extremity from the following two preliminary conditions: a static relaxed muscle state (SR condition) and using a countermovement (SSC condition). The muscle power was measured accurately by a power measurement device, which adopted the weight loading method. The peak power under both conditions showed significantly higher values in men than in women. In both genders, the peak power showed significantly lower values in the SSC condition than in the SR condition (p < 0.05). The potentiation of using the SSC was not found in the peak power test. However, the initial power showed significantly higher values under the SSC condition (men: 37.2 ± 6.4 W; women: 17.4 ± 5.1 W) than in the SR condition (men: 18.3 ± 4.3 W; women: 11.2 ± 3.1 W). Hence, assuming a difference between initial muscle power outputs of the SR and SSC conditions as a difference in SSC ability, an SSC index was devised to evaluate the above ability. The SSC index showed significantly higher values in men (50.1 ± 12.4) than in women (32.1 ± 23.2). However, the individual difference of SSC ability was very large in women. The ability of women to use SSC in the upper extremities may be inferior to that of men.

Evaluation of Muscle Power Exerted by Explosive Gripping

Demura, S and Miyaguchi, K. Evaluation of muscle power exerted by explosive gripping. J Strength Cond Res 23(2): 465-471, 2009-To establish the evaluation method for muscle power exerted by explosive gripping, this study aimed to examine the reliability of parameters and the muscle power output properties in both genders. Fifteen young men and 15 women participated in this study. Each subject carried out explosive grip tests twice using a simple muscle power measurement device (weight loading method), each time with 20, 30, 40, and 50% loads of maximum grip strength (MGS) by isometric contraction. Peak power was drawn from the product of the measured peak velocity and relative loads. The reliability of each parameter in all loads was good (interclass correlation coefficient >0.75) for both genders. The MGS showed insignificant correlation with all parameters. Peak power values were larger in men than in women for all loads, and the womens values were 44.5-52.2% of the mens. A large gender difference was found for 20% MGS. In conclusion, the reliability of muscle power parameters measured by the measurement device in this study is high. It was judged that the device used in this study can properly evaluate the gender difference in muscle power output properties by explosive gripping.

Influence of instruction on evaluation of repeated muscular endurance.

This study was done to examine the influence of instructions on the evaluation of rhythmic repeated muscular endurance, that is, the ability to continue muscle-power exertion at a regular interval. 15 healthy young male adults performed maximal repeated rhythmic grips for 6 min. under the following three instruction conditions: (1) “Squeeze freely during the 2-sec. interval,” (2) “Squeeze quickly and then relax,” and (3) “Squeeze quickly and then hold the exertion for 1 sec.” Time to peak force was slower in Condition 1 than in Condition 2. A decline in exertion of force was found for all conditions after 1 min. The decline in rate of the peak values in Conditions 1 and 3 was about 10 to 15% larger than that of Condition 2. In conclusion, proper instructions are necessary, even for a short 2-sec. interval, when evaluating muscular endurance with repeated rhythmic grips. Condition 2 instruction may be most effective.