Examining impulse-variability in overarm throwing.

The purpose of this study was to examine variability in overarm throwing velocity and spatial output error at various percentages of maximum to test the prediction of an inverted-U function as predicted by impulse-variability theory and a speed-accuracy trade-off as predicted by Fitts' Law Thirty subjects (16 skilled, 14 unskilled) were instructed to throw a tennis ball at seven percentages of their maximum velocity (40-100%) in random order (9 trials per condition) at a target 30 feet away. Throwing velocity was measured with a radar gun and interpreted as an index of overall systemic power output. Within-subject throwing velocity variability was examined using within-subjects repeated-measures ANOVAs (7 repeated conditions) with built-in polynomial contrasts. Spatial error was analyzed using mixed model regression. Results indicated a quadratic fit with variability in throwing velocity increasing from 40% up to 60%, where it peaked, and then decreasing at each subsequent interval to maximum (p < .001, η2 = .555). There was no linear relationship between speed and accuracy. Overall, these data support the notion of an inverted-U function in overarm throwing velocity variability as both skilled and unskilled subjects approach maximum effort. However, these data do not support the notion of a speed-accuracy trade-off. The consistent demonstration of an inverted-U function associated with systemic power output variability indicates an enhanced capability to regulate aspects of force production and relative timing between segments as individuals approach maximum effort, even in a complex ballistic skill.

An examination of possible quadriceps force at the time of anterior cruciate ligament injury during landing: A simulation study.

Anterior cruciate ligament (ACL) rupture is a common and traumatic injury. Although, identifying the mechanism of ACL injury has received considerable research attention, there are still many unanswered questions. One proposed mechanism asserts that the ACL is injured due to an aggressive quadriceps muscle contraction. However, recently it has been questioned if the magnitude of quadriceps force needed to tear the ACL is physiologically realistic under the conditions where injury occurs during landing (e.g. near full knee extension and within 50ms after impact). To answer this question, a simple simulation model was developed to examine the upper bounds of quadriceps force that can be developed under these conditions. The model included force-length, and force-velocity properties as well as activation dynamics. Model parameters were chosen to provide a high estimate for possible quadriceps force in a young healthy man. The effects of varying quadriceps pre-activation levels were also examined. When using realistic pre-activation levels, the simulated quadriceps force was less than half of what has been shown to cause ACL injury. Even when using maximum pre-activation, the quadriceps force still did not reach close to the level shown to cause injury. Therefore, we conclude that quadriceps force alone seems to be an unlikely mechanism for ACL injury.

Influences of variation in force application on tibial displacement and strain in the anterior cruciate ligament during the Lachman test.

OBJECTIVE: The purpose of this study was to examine the influence of Lachman test performance technique on tibial displacement and strain in the anterior cruciate ligament. DESIGN: Model simulation of experimental Lachman test performance by trained clinicians. BACKGROUND: Differences in clinician hand placement during Lachman test performance have been observed. METHODS: A two-dimensional computer sagittal plane model of the knee was designed to simulate experimentally observed Lachman test performance, and determine anterior cruciate ligament strain and tibial translation that occurred during variation in clinician hand placement and force magnitude. RESULTS: Anterior cruciate ligament strain and tibial translation were greater under conditions mimicking clinician hand placement utilizing a more proximal force application on the tibia. CONCLUSIONS: Tibial translation and strain behavior of the anterior cruciate ligament during the Lachman test appear to be influenced by clinician hand position used in the application of force to the tibia.

Effect of warm-up on the standing broad jump in trained and untrained men and women.

The effect of 3 warm-up routines on standing broad jump (SBJ) performance was investigated. Thirty-two men and women participated as subjects. Following the determination of 1-repetition maximum (1RM) squat, subjects completed warm-up routines and broad jumps on 4 occasions in a randomized order. Subjects performed SBJ immediately (POST) and 15 min following (POST15) the given warm-up routine. The routines were high force, consisting of high % 1RM, low repetition squats; high power, consisting of low % 1RM, low repetition speed squats; stretching, consisting of static stretches; and no activity, a control condition. Repeated measures analysis of covariance (ANCOVA) revealed no differences among broad jump performance following any of the warm-up routines (p = 0.157). A strong correlation (R = 0.805) was found between 1RM squat and SBJ. These data indicate that warm-up of any type has little effect on jump performance and that maximum strength is strongly related to jumping ability.

Further evidence on the dynamics of self-injurious behaviors: impact forces and limb motions.

The dynamics of self-injurious behaviors (SIBs) were examined in 8 adults with mental retardation. The trajectories of the arm movements and the impact forces of the head blows were determined from a dynamic analysis of videotapes of discrete bouts of self-injury. The results revealed a high degree of cycle-to-cycle consistency in the qualitative dynamics of the limb motions, indicating that the motions involved in SIB are often stereotyped in nature. The resultant individual peak impact forces ranged from 50 to 1560 N. The impact forces of SIB as a percentage of body mass are either near or at the low end of forces generated in boxing blows and karate hits.