Effects of strategy use on acquisition of a motor task during various stages of learning.

To specify the optimal point for introducing a learning strategy, 50 participants were randomly assigned into five groups based on the timing of strategy introduction while learning a badminton serve. Groups were instructed in the use of Singer's Five-step Strategy either prior to starting their acquisition trials (100% group) or following acquisition Trial Blocks 1 (83% group), 3 (50% group), 5 (17% group) or were assigned to a control (0% group) group). Participants were asked to complete six acquisition trial blocks of 10 serves each, followed by a break and then two retention trial blocks. Scores were obtained by hitting shuttles into a scoring grid, which served as the dependent measure. Data were analyzed using a mixed-model analysis of variance with a group x trial blocks design, which yielded significant main effects for both factors during acquisition. Introduction of a learning strategy may be more efficient once participants have become familiar with the task. No significant differences were observed between groups who received the strategy early and the control group. Thus, it appears that learning strategies should be introduced later in the learning process and may distract if provided too early.

Spinal cord modulation associated with isometric contractions.

OBJECTIVE: The dual-strategy hypothesis explains single-joint voluntary movement by dividing movements into two different strategies and suggesting that different excitation pulses modulate these movements. The existence of this excitation pulse was evaluated by quantifying magnitude and timing changes in the H-reflex (changes in spinal excitability) prior to a voluntary contraction. These changes in spinal excitability were assessed during a ballistic plantar flexion isometric contraction, where both the target size and force level was manipulated. METHODS AND MATERIALS: Subjects were seated in a modified chair with a force transducer placed under the metatarsal heads to measure ankle force output. Following a visual stimulus subjects were trained to produce a plantar flexion force of 25% and 50% of a maximum voluntary contraction, within target sizes of 5% and 15% of the selected force level. Soleus motor neuron reflex excitability was analyzed by measuring changes in the H/M ratio. The H-reflex was randomly elicited by tibial nerve stimulations at 15, 30, 45, 60, 75 and 90 ms prior to the recorded average soleus premotor time for each of the force and target size conditions. RESULTS: A two-way repeated-measures analysis of variance indicated a significant effect among target sizes for the time of change in spinal excitability, slope of facilitation (rate of rise of spinal excitability), and peak facilitation. A significant difference was also established between force levels for the slope and peak facilitation, but there was no difference with time of facilitation. CONCLUSIONS: These results indicate that changes in both target size and force level can influence slope and peak of facilitation. However, only target size appears to affect the time of facilitation. Results clearly support the existence of an excitation pulse that is regulated by the type of movement.

The Effect of Life Events on Incidence of Injury in High School Football Players.

OBJECTIVE: To investigate the potential relationship between life events and injury among high school football players. DESIGN AND SETTING: This was a prospective cohort study. We collected athlete-exposure and injury data through 2 consecutive seasons at 13 high schools. Injury data consisted of injury status, frequency, severity, practice versus game, time of season, and total time lost from football participation. A life event survey was administered at the end of each season to obtain total, negative, and positive life event scores. SUBJECTS: A total of 331 varsity high school football players. MEASUREMENTS: Data collected included athlete exposures, injury reports, and life event surveys. Statistical analysis was completed using the Wilcoxon rank sum W test, the Pearson chi(2) test using an iterative proportional fitting procedure, and contingency chi(2) test. RESULTS: Total and negative life change measures significantly affected injury status and frequency of injury (P <.05). However, they did not reflect differences in severity of injury or time lost due to injury, and the positive life change measure was not associated with significant differences for any injury factor (P >.05). Preseason injury rates were significantly higher than season and playoff injury rates (P <.05). CONCLUSIONS: High school football players who experience high degrees of total and negative change were at greater risk of becoming injured and of sustaining multiple injuries. Playing situation and time of season also affected risk of injury.