Dimensions of the functional degrees of freedom of the first serve in tennis

We examined the influence of court side and target location of the ball on the coordination and control of the tennis serve. Five male and five female experienced players performed 10 'first-serves' to centre and wide targets. 3D kinematics of the service were analysed from two frames of reference: joint position (v) and joint angle (ω) with emphasis on the qualitative aspects of movement coordination. Principal component analysis (PCA) showed in all service location conditions the first two components accounted for ~80% of the total variance with the external frame of reference (elbow and shoulder v), and internal frame of reference (ω: left and right hip) contributing most to component 1 (~60%). Arm (ω: shoulder, elbow, wrist) contributed (~20%) to component 2. Serving toward the court T centre led to more variables involved in the organization of the motion. Peak mean racket head resultant velocity was similar with each serving condition, but higher in males than females. The PCA showed that the number of components (Functional Dimensions) was less than the number of joint space physical DFs. There was quantitative variation in individual kinematic variables within- and between- players but a common qualitative kinematic structure to the coordination solution.

Increasing Movement Amplitude in Speeded Hitting Enhances Contact Velocity Without Affecting Directional Accuracy or Movement Variability

Performance of sport-related ballistic motor skills, like ball hitting in golf and baseball, requires wide movements to produce highly fast and spatially accurate movements. In this study, we assessed the effect of movement amplitude on directional accuracy in a ballistic hitting task. Participants performed the task of moving a manual handle over a flat surface to hit with high speed a moveable disc, aiming to propel it towards a frontal target. Five movement amplitudes were compared, ranging from 11.5 cm to 27.5 cm in steps of 4 cm. Kinematic analysis evaluated motions of the handle, disc, and arm joints. Results showed that greater movement amplitudes led to longer acceleration phases, with delayed peak velocities at the handle, shoulder and elbow, leading to higher contact and peak linear velocities of the handle, and higher angular velocities at the shoulder and elbow. Manipulation of movement amplitude led to no evidence for effects on either disc directional accuracy or variability. Results also revealed no evidence for differences in variability of contact velocity, peak velocity and time of peak velocity across movement amplitudes in the shoulder, elbow, and wrist. Our results indicated that greater movement amplitudes in hitting a spatial target lead to increased contact velocity while not affecting directional accuracy or movement variability.

Task Specific and General Patterns of Joint Motion Variability in Upright- and Hand-Standing Postures

The preservation of static balance in both upright- and hand-stance is maintained by the projection of center of mass (CM) motion within the region of stability at the respective base of support. This study investigated, from a degrees of freedom (DF) perspective, whether the stability of the CM in both upright- and hand-stances was predicted by the respective dispersion and time-dependent regularity of joint (upright stance—ankle, knee, hip, shoulder, neck; hand stance—wrist, elbow, shoulder, neck) angle and position. Full body three-dimensional (3D) kinematic data were collected on 10 advanced level junior female gymnasts during 30 s floor upright- and hand-stands. For both stances the amount of the dispersion of joint angle and sway motion was higher than that of the CM and center of pressure (CP) with an inverse relation to time-dependent irregularity (SampEn). In upright-standing the variability of neck motion in the anterior–posterior direction was significantly greater than that of most joints consistent with the role of vision in the control of quiet upright posture. The findings support the proposition that there are both task specific and general properties to the global CM control strategy in the balance of upright- and hand-standing induced by the different active skeletal-muscular organization and the degeneracy revealed in the multiple distributional variability patterns of the joint angle and position in 3D.