Grip responses to object load perturbations are stimulus and phase sensitive.

Responses to load changes of a held object that challenge grasp stability are known to be adept and fast, but the responses to changes in load where grasp stability is not challenged are not well understood. In order to compare responses to these functionally opposite perturbations, the grasp response to increases and decreases in the load of a held object was examined. A pulling force used to create object load was abruptly altered so that it felt lighter (decreased load) or felt heavier (increased load). The perturbation occurred either during movement of the object (lift) or when the object was held steady (hold). Grip force modulation was earlier, larger, had a faster maximum rate and a smaller change in relative safety margin when load increased. Also, the grip force modulation was earlier, larger, had a faster maximum grip force rate and a smaller change in relative safety margin when the perturbation occurred during active lift. In the decreased loading condition, participants were not required to make a grip force adjustment to maintain grip. Interestingly, participants chose to make the adjustment (decreasing grasp force), albeit more slowly. During the lift phase, the nature of the task is more dynamic and the resulting additional mechanical stimulation may have lead to a facilitated response. The results point to the greater functional significance of increasing load for grip force modulation and the potential for greater sensory or motor facilitation during dynamic lifting.

Effect of an abdominal binder during wheelchair exercise.

The purpose of this study was to determine whether use of an abdominal binder would affect oxygen uptake, trunk range of motion, and duration of the stroke phase during wheelchair propulsion. The subjects were six paraplegic wheelchair athletes with T1-T6 injuries and no abdominal muscle function. Each subject performed two trials, one while wearing the binder and one without the binder. Each trial consisted of submaximal and maximal exercise tests conducted on wheelchair rollers. Oxygen uptake was determined by open circuit spirometry while heart rate was determined by telemetry. Max VO2 values averaged 2.51 l.min-1 while average maximum heart rate values were 190 b.min-1. A 3-D video-based motion analysis system was used to obtain kinematic parameters of wheelchair propulsion. In general, 30% of the cycle time was comprised of the stroke phase, while 70% was comprised of the recovery phase across speeds. There were no statistically significant effects of the abdominal binder on any of the cardiovascular or kinematic variables at submaximal or maximal levels of exercise. Under the conditions of this laboratory investigation, it appears that an abdominal binder does not alter physiological or selected biomechanical measures in highly trained athletes.

Bilateral performance symmetry during drop landing: a kinetic analysis.

The assumption that lower extremity function is bilaterally symmetrical is prevalent throughout the biomechanics literature. The unilateral development of many overuse and acute injuries may suggest that this assumption is inaccurate. Analyses of bilateral function report conflicting results and may be partially attributable to the types of movement activities utilized and to research methodology. The purpose of this study was to evaluate the concurrent bilateral performance of the lower extremities during an apparently symmetrical movement task. Concurrent left/right side vertical ground reaction forces (VGRF) and lower extremity joint moments (JM) were obtained for 10 subjects performing 25 voluntary hanging drop landings (60 cm) on each of 3 consecutive days. Bilateral variability (BV) and systematic bilateral asymmetry (BA) were calculated for each trial. Bilateral variability was less for VGRF variables (12.8%) than JM variables (25.3%). Bilateral asymmetry was identified more often among VGRF variables (52.5%) than JM variables (16.7%). The magnitude of identified BA was greater than either the group BV (25%) or BA (110%) and indicates that bilateral differences are biomechanically meaningful.

A biomechanical analysis of four sprint start positions.

The sprint starts of 12 skilled collegiate sprinter/hurdlers were filmed for four different sprint start conditions. Ground reaction forces were collected for the first step out of the blocks and velocities through a 2-m speed trap immediately following the first support phase were recorded. The subjects employed their preferred right-left leg placement in the blocks, while the anterior-posterior spacing of the front block with respect to the starting line and the amount of forward lean in the set position were varied. Four positions were constructed that accounted for anthropometric differences. The four positions consisted of combinations of two arm orientations (forward lean and perpendicular to ground) and two front block to starting line distances (bunched and elongated). Kinematic data were reduced to provide center of mass position and velocity measures and analyzed for critical periods throughout the starting action. Selected critical events from the first step kinetic records were also analyzed. The results indicated that the elongated starting positions resulted in greater horizontal displacement, greater propelling impulse, increased first step toe-off velocity, and greater average velocity through a 2-m speed trap. Arm orientation effects were less well defined. Forward lean tended to result in greater vertical velocity at block clearance and horizontal velocity at first step toe-off, whereas perpendicular arm positioning resulted in greater 2-m speed trap velocity.