Yoichi Ohta

Kinesthetic aftereffects induced by a weighted tool on movement correction in baseball batting

We investigated the kinesthetic aftereffects of a weighted tool on interceptive performance. Eight college baseball players performed three warm-ups before the interceptive task: a normal warm-up, a recalibrated warm-up with a standard 850-g bat and a 1200-g weighted bat, and a weighted warm-up with a 1200-g bat. For the interceptive task, subjects were asked to swing the standard bat coincident with the arrival and position of a moving target. After the warm-ups with the weighted bat, participants felt that the bat was lighter and swung faster. When participants needed to correct their swings to the targets velocity change, larger timing errors were produced in the weighted than in the normal practice condition. These results indicate that warm-ups with a weighted tool create adverse effects for the movement (re)programming processes in interceptive action. This suggests that warm-ups with a weighted tool for an interceptive task affect the central nervous system and not the peripheral system.

In vivo behaviour of human muscle architecture and mechanomyographic response using the interpolated twitch technique

This study investigated the origin of curvilinear change in the superimposed mechanomyogram (MMG) amplitude of the human medial gastrocnemius muscle (MG) with increasing contraction intensity. The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening were measured using ultrasonic images of electrical stimulation during isometric plantar flexions at levels 20%, 40%, 60%, 80%, and 100% of the maximal voluntary contraction (MVC). The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening decreased with increasing contraction intensity. The superimposed MMG amplitude and the extent of fascicle shortening showed a curvilinear decrease, while the superimposed twitch amplitude showed a linear decrease at levels up to 80% of the MVC. There was a linear relationship between the superimposed MMG amplitude and the extent of fascicle shortening at different contraction intensities. These results indicate that the superimposed MMG amplitude reflects changes in the extent of fascicle shortening at different contraction intensities better than the superimposed twitch amplitude. Our study suggests that the origin of the curvilinear decrease of superimposed MMG amplitude is associated with a curvilinear decrease of the extent of fascicle shortening with increasing contraction intensity in the human MG.

Changes in force and tendinous tissue elongation during the early phase of tetanic summation in in vivo human tibialis anterior muscle

The purpose of this study was to determine the changes that occur in tendinous tissue properties during the early phase of tetanic summation in the in vivo human tibialis anterior muscle (TA). The torque response and tendinous tissue elongation following single stimuli, two-pulse trains, and three-pulse trains were recorded in the TA during isometric contractions. The elongation, compliance, and lengthening velocity of tendinous tissue were determined by real-time ultrasonography. The contribution of the response to the second stimulation (C2) was obtained by subtracting the response to the single stimulation (C1) from the response of doublet. The third contribution (C3) was obtained by subtracting the response to the doublet from that of the triplet. C2 (7.8+/-0.5 Nm) and C3 (7.3+/-0.6 Nm) had torque responses significantly higher than C1 (3.6+/-0.7 Nm). In contrast, the elongations of tendinous tissue for C2 (2.8+/-0.4mm) and C3 (1.7+/-0.2mm) were significantly lower than for C1 (4.9+/-0.3mm), indicating that the summation pattern of tendinous tissue elongation is different from the summation pattern of torque response. In addition, this showed considerable difference both between C1 (0.12+/-0.01 mm/N; 83+/-4.6mm/s) and C2 (0.03+/-0.005 mm/N; 50+/-6.3mm/s) and between C1 and C3 (0.02+/-0.002 mm/N; 39+/-6.4mm/s) in the compliance and lengthening velocity of tendinous tissue. These results suggest that changes in tendinous tissue properties between first and second contraction are related to different summation patterns of force and tendinous tissue elongation during early phase of tetanic summation.

Muscle Activation Characteristics of the Front Leg During Baseball Swings with Timing Correction for Sudden Velocity Decrease

This study aimed to clarify the activation characteristics of the vastus lateralis muscle in the front leg during timing correction for a sudden decrease in the velocity of a target during baseball swings. Eleven male collegiate baseball players performed coincident timing tasks that comprised constant velocity of 8 m/s (unchanged) and a sudden decrease in velocity from 8 to 4 m/s (decreased velocity). Electromyography (EMG) revealed that the muscle activation was typically monophasic when responding unchanged conditions. The type of muscle activation during swings in response to decreased velocity condition was both monophasic and biphasic. When biphasic activation appeared in response to decreased velocity, the impact time and the time to peak EMG amplitude were significantly prolonged and the timing error was significantly smaller than that of monophasic activation. However, the EMG onset from the target start was consistent both monophasic and biphasic activation in response to conditions of decreased velocity. In addition, batters with small timing errors in response to decreased velocity were more likely to generate biphasic EMG activation. These findings indicated that timing correction for a sudden decrease in the velocity of an oncoming target is achieved by modifying the muscle activation characteristics of the vastus lateralis muscle of front leg from monophasic to biphasic to delay reaching peak muscle activation and thus prolong impact time. Therefore, the present findings suggests that the extent of timing errors in response to decreased velocity is influenced by the ability to correct muscle activation after its initiation rather than by delaying the initiation timing of muscle activation during baseball swings.

Warm-up with Weighted Bat and Adjustment of Upper Limb Muscle Activity in Bat Swinging under Movement Correction Conditions

The effects of weighted bat warm-up on adjustment of upper limb muscle activity were investigated during baseball bat swinging under dynamic conditions that require a spatial and temporal adjustment of the swinging to hit a moving target. Seven male college baseball players participated in this study. Using a batting simulator, the task was to swing the standard bat coincident with the arrival timing and position of a moving target after three warm-up swings using a standard or weighted bat. There was no significant effect of weighted bat warm-up on muscle activity before impact associated with temporal or spatial movement corrections. However, lower inhibition of the extensor carpi ulnaris muscle activity was observed in a velocity-changed condition in the weighted bat warm-up, as compared to a standard bat warm-up. It is suggested that weighted bat warm-up decreases the adjustment ability associated with inhibition of muscle activation under movement correction conditions.

The effect of summation of contraction on acceleration signals in human skeletal muscle.

The purpose of this study was to determine the effects of summation of contraction on acceleration signals in human skeletal muscle. The torque parameters of dorsiflexion and acceleration signals in the tibialis anterior muscle were measured during evoked isometric contractions. In an examination of two-pulse trains with different inter-pulse intervals, the torque and accelerometer responses to inter-pulse intervals of 10-100 ms were recorded. In an investigation of the effects of different numbers of stimuli, the torque and accelerometer responses to 1-8 pulses with a constant inter-pulse interval of 10 ms were recorded. The present study found that there was a difference in acceleration amplitude between the single-pulse and two-pulse trains with an inter-pulse interval of 10 ms but not two-pulse trains with an inter-pulse interval of 20 ms or more. In the investigation of different numbers of stimuli, we found a similar MMG amplitude across 2-8 pulses. Moreover, we observed that the maximal time to the peak acceleration signal was approximately 27 ms. In a comparison of torque parameters with acceleration signals, the present study clearly shows that acceleration amplitude is poorly correlated to changes in force parameters when the inter-pulse interval or the number of stimuli are increased. These results suggest that the absence of associated changes in acceleration peak is due to the long interval for the subsequent pulses relative to the time at which acceleration peak is achieved ( approximately 27 ms). These findings will provide useful information concerning the method for assessing summation of contraction with an accelerometer.

Possibility of intrinsic muscle contractile properties in force summation and postactivation potentiation as indices of maximal muscle strength and muscle fatigue

We investigated whether intrinsic muscle contractile properties in force summation and postactivation potentiation (PAP) can be used as indices of maximal muscle strength and muscle fatigue.

The effects of muscle architectural change with a pre-motion silent period on the subsequent muscular output during rapid voluntary movement

This study aimed to determine the characteristics of the in vivo behaviour of human muscle architecture during a pre-motion silent period (PMSP) using ultrasonography. Subjects were requested to perform rapid knee extension with vertical jumping. Electromyographic signals were recorded from the vastus lateralis (VL), vastus medialis, and biceps femoris muscles. Ultrasonic images were recorded from the VL. We found that the cross point between the fascicle and deep aponeurosis in the VL moved to the distal side before the rapid vertical jumps with PMSP. This cross point movement with PMSP was of low amplitude (mean: 1.0+/-0.3mm) and velocity (22.2+/-6.1mm/s). The amplitude and velocity of the cross point movement were significantly positively related to the angular peak velocity of knee extensor during rapid vertical jumping in trials with PMSP. These results suggest that although low levels of pre-movement muscle architectural change with PMSP may be the result of muscle relaxation behaviour rather than the result of muscle stretching behaviour, this pre-movement effect can influence subsequent muscular performance during a rapid voluntary movement. PMSP may allow pre-movement muscle architectural change to generate a better muscular condition to increase neural activation during the subsequent rapid voluntary contraction.