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This study investigates the effect of initial leg angle on horizontal jump performance. Eleven highly skilled male and female long jumpers (national and Olympic level) performed a series of horizontal jumps for distance. Within-jumper differences in initial leg angle, normalized horizontal and net vertical impulses, contact time, and average reaction force during the impact interval, postimpact interval, and in total were measured using high-speed video (240 or 300 Hz) and a force plate (1200 Hz). Pearson's correlations, Winsorized correlations, and the HC4 method were used to determine significant correlations between variables (α =.05). Within-jumper analysis indicated that when jumpers initiate the takeoff phase with a larger leg angle they are able to generate significantly greater negative horizontal and positive net vertical impulses (n = 7). Increased impulse generation was the result of increased contact time (n = 5 of 7) and/or increased average reaction force (n = 4) during the impact interval (n = 3) and/or postimpact interval (n = 4), depending on the individual. Initial leg configuration at contact and individual specific impulse generation strategies are important to consider when determining how an athlete with initial momentum can increase impulse generation to jump for distance. [ABSTRACT FROM AUTHOR]

Golf shots off uneven terrain often require modifications in address position to complete the swing successfully. This study aimed to determine how golf players coordinate the legs to regulate linear and angular impulse (about an axis passing vertically through the center of mass) while modifying the lower-extremity address position during the swing. Nine highly skilled golf players performed swings with a 6-iron under the Normal, Rear Leg Up, and Target Leg Up conditions. Components of linear and angular impulse generated by the rear and target legs (resultant horizontal reaction force, resultant horizontal reaction force angle, and moment arm) were quantified and compared across the group and within a player (α =.05). Net angular impulse did not change between conditions. Target leg angular impulse was greater in the Target Leg Up condition than Rear Leg Up condition. Regulation of linear and angular impulse generation occurred while increasing stance width and redirecting resultant horizontal reaction forces to be more parallel to the target line under modified address positions. Net linear impulse perpendicular to the target was near 0 or slightly posterior. Net linear impulse parallel to the target was less toward the target in the Target Leg Up condition compared with Normal and Rear Leg Up conditions. These results indicate individuals utilized player-specific mechanisms to coordinate the legs and regulate impulse generation during the golf swing under modified address positions. [ABSTRACT FROM AUTHOR]

This study determined how dancers regulated angular and linear impulse during the initiation of pirouettes of increased rotation. Skilled dancers (n = 11) performed single and double pirouette turns with each foot supported by a force plate. Linear and angular impulses generated by each leg were quantified and compared between turn types using probability-based statistical methods. As rotational demands increased, dancers increased the net angular impulse generated. The contribution of each leg to net angular impulse in both single and double pirouettes was influenced by stance configuration strategies. Dancers who generated more angular impulse with the push leg than with the turn leg initiated the turn with the center of mass positioned closer to the turn leg than did other dancers. As rotational demands increased, dancers tended to increase the horizontal reaction force magnitude at one or both feet; however, they used subject-specific mechanisms. By coordinating the generation of reaction forces between legs, changes in net horizontal impulse remained minimal, despite impulse regulation at each leg used to achieve more rotations. Knowledge gained regarding how an individual coordinates the generation of linear and angular impulse between both legs as rotational demand increased can help design tools to improve that individual's performance. [ABSTRACT FROM AUTHOR]

Angular impulse generation is dependent on the position of the total body center of mass (CoM) relative to the ground reaction force (GRF) vector during contact with the environment. The purpose of this study was to determine how backward angular impulse was regulated during two forward translating tasks. Control of the relative angle between the CoM and the GRF was hypothesized to be mediated by altering trunk--leg coordination. Eight highly skilled athletes performed a series of standing reverse somersaults and reverse timers. Sagittal plane kinematics, GRF, and electromyograms of lower extremity muscles were acquired during the take-off phase of both tasks. The magnitude of the backward angular impulse generated during the push interval of both tasks was mediated by redirecting the GRF relative to the CoM. During the reverse timer, backward angular impulse generated during the early part of the take-off phase was negated by limiting backward trunk rotation and redirecting the GRF during the push interval. Biarticular muscles crossing the knee and hip coordinated the control of GRF direction and CoM trajectory via modulation of trunk--leg coordination. [ABSTRACT FROM AUTHOR]

Abstract: Repetitive loading of the upper extremity musculature during activities like wheelchair propulsion can lead to fatigue of surrounding musculature causing irregular segment kinematics. The goal of this study was to determine the effect of increase in load on the kinematics of the scapula in users with paraplegia and tetraplegia. Data were collected on 18 participants (11 with paraplegia and 7 with tetraplegia) using an electromagnetic motion tracking system (100Hz) and force sensing pushrim (200Hz). The participants propelled under no load and loaded conditions at their customary propulsion velocity. On average a 60N increase in force was elicited with the experimental protocol. Users with tetraplegia showed significant increases (p <.05) in the rate of change of scapular angles in the upward/downward rotation and the retraction/protraction direction under the loaded conditions, whereas users with paraplegia only showed difference in the retraction/protraction rotation direction. Overall both user populations moved towards position of increased downward rotation, anterior tilt and protraction with increase in load hence increasing the risk of impingement. This experiment adds depth to our understanding of dynamic scapular kinematics during wheelchair propulsion under different loading conditions and differences in scapular control between users with paraplegia and tetraplegia. [Copyright &y& Elsevier]

Weight-bearing tasks performed by humans consist of a series of phases with multiple objectives. Analysis of the relationship between control and dynamics during successive phases of the tasks is essential for improving performance without sustaining injury. Experimental evidence regarding foot landings suggests that the distribution of momentum among segments at contact influences stability during interaction with the landing surface. In this study, we hypothesized that modification of control in one subsystem, in our case shoulder torque, during the flight phase of an aerial task would enable the performer to maintain behavior of other subsystems (e.g.lower extremity kinematics) and initiate contact with momentum conditions consistent with successful task performance. To test this hypothesis, an experimentally validated multilink dynamic model that incorporated modifications in shoulder torque was used to simulate the flight phase dynamics of overrotated landings. The simulation results indicate that modification in shoulder torque during the flight phase enables gymnasts to maintain lower extremity kinematics and initiate contact with trunk angular velocities consistent with those observed during successful landings. These results suggest that modifications in the control logic of one subsystem may be sufficient for achieving both global and local task objectives of landing. [ABSTRACT FROM AUTHOR]