1. CHANGES IN VERTICAL JUMP HEIGHT ACROSS EIGHT DAYS IN COUNTERMOVEMENT JUMPS PERFORMED BY NOVICE JUMPERS Michael E. Feltner, FACSM, Priscilla G. MacRae, FACSM, Phil G. Westphal Pepperdine University, Malibu, CA, e-mail: michael.feltner@pepperdine.edu  No Day by Trial interactions were present for any variable examined.  A significant increase in jump height (Table 1 and Figure 1) occurred across Days (F [7,77] = 4.24, p<0.05)  Post hoc analysis (pairwise t-tests) indicated that jump height on Days 2 through 8 significantly (p<0.05) differed from Day 1 (average increase 7.2%).  Jump heights on Days 2-8 were not significantly different from each other. RESULTS Duration of the takeoff phase (t TAKEOFF ) was computed as t TO - t ST. Data Analysis Three 8x10 repeated measures ANOVAs (Days by Trials) were used to analyze the dependent variables.  Jump Height  Average VGRF  Duration of the takeoff phase (t TAKEOFF ) Statistical significance was determined at p < 0.05 If a significant main effect for Days or Trials was identified, post hoc pairwise t-tests were used to analyze changes across Days. Where: = the mass of the jumper. METHODS Subjects  Twelve novice females, age 19.3 ± 1.0 years, performed 10 maximal effort jumps daily for 8 consecutive days from the surface of two Kistler force plates. All subjects provided informed consent. Procedures  Novice status was determined by self-report (no high school or intercollegiate experience in a sport involving jumping) and by assessment of mean jump height on Day 1. Individuals with a jump height greater than 30 cm (1 ft.) were excluded from participation. Actual mean Day 1 jump height for the 12 subjects was 19.8 cm (Range: 12.4 – 24.5 cm).  In an attempt to obtain maximal efforts across trials, performance feedback (jump height) and encouragement were given after each trial.  No feedback regarding jumping technique or style was provided. Data Collection  Two Kistler (Amherst, NY) force plates (Model #9281B) sampling at 1000 Hz were used to collect the VGRF data.  Kistler BioWare Software was used to analyze the data collected from the force plates.  The following variables were determined using the procedures outlined by Street et al., (2001).  Subject Weight (W)  Time of start of movement (t ST )  Time of takeoff (t TO )  Vertical Ground Reaction Force (VGRF) impulse between t ST and t TO. Vertical velocity of the center of mass was calculated using the following equation. = = the impulse due to VGRF. = the impulse due to W. Jump height between t TO and the peak of the CM after takeoff (t PK ) was computed as: Jump Height = ∆s TO-PK = (t TO -t ST ) Average VGRF exerted during the takeoff phase was computed as: = acceleration due to gravity PURPOSE To examine the time course of changes in jump height, VGRF application and impulse duration (t TAKEOFF ) during countermovement jumps in novice jumpers. REFERENCES Bernstein, N. (1967). The Co-ordination and Regulation of Movements. New York: Pergammon Press. Magill, R.A. (2004). Motor Learning and Control: Concepts and Applications. New York, New York: McGraw-Hill. Newell, K.M. (1991). Motor Skill Acquisition. Annual Review of Psychology, 42, 213-237. Snoddy, G. S. (1926). Learning and stability, Journal of Applied Psychology, 10, 1-36. Street, G., McMillan, S., Board, W., Rasmussen, M., Heneghan, J.M. (2001). Sources of Error in Determining Countermovement Jump Height With the Impulse Method. Journal of Applied Biomechanics, 17, 43-53. van Ingen Schenau, G. J. (1989). From rotation to translation: constraints on multi-joint movements and the unique action of bi- articular muscles. Human Movement Science, 8(4), 301-337. CONCLUSION In the absence of technique specific feedback, novice vertical jumpers increased vertical jump height by 5.4% within 24 hours of the initiation of practice and 7.2% across eight days of practice. The jumpers did not use a consistent mechanical strategy to increase vertical jump height, but they did exhibit consistent intraday alterations in average VGRF application and impulse duration. DISCUSSION The significant increase in jump height across Days indicates that the net vertical impulse created by the jumper between t ST and t TO increased across Days. Jump heights on Days 2 through 8 were significantly higher than on Day 1. Jump height did not increase from Day 3 through Day 8. The increases in t TAKEOFF and the decreases in average VGRF across Trials indicates that subjects consistently altered their jumping mechanics within each Day. Despite the consistent intraday alterations in t TAKEOFF and average VGRF, the absence of significant changes in either average VGRF or impulse duration (t TAKEOFF ) across Days indicates that the subjects used different strategies to increase net vertical impulse and ultimately jump height across the eight days. Limitations:  Efforts were taken to continually and consistently encourage subjects to perform maximally on every trial, but eliciting maximal efforts on each trial was challenging. This study is the initial phase of a more comprehensive investigation to evaluate the underlying neuromuscular adaptations associated with improved motor skill performance. The currently experimental protocol is being repeated on a different set of novice subjects while lower extremity EMG data also is obtained. The EMG data should provide greater understanding of the muscular coordination patterns associated with skill acquisition and allow greater insight on the causal mechanism(s) used to increase jump height. ABSTRACT Practice is an important variable for improvement in motor skill performance; however, little is known about the time course of changes in motor skills with practice. Purpose: The purpose was to investigate the time course of changes in jump height, vertical force application, and impulse duration during countermovement jumps. Methods: Twelve novice (mean Day 1 jump height = 19.8 cm) college-age participants performed 10 maximal jumps daily for eight consecutive days from the surface of two Kistler force plates. Following each trial, performance feedback (jump height) and encouragement were given to obtain maximum efforts from the subjects. Jump height, average vertical ground reaction force (VGRF), and the duration of the takeoff phase (t TAKEOFF ) were determined for each trial. Separate 8x10 repeated measures ANOVAs (Days x Trials) were used to analyze the data. Results: Jump height increased across Days (F=4.24, p<0.01) and post hoc analysis (pair-wise t- tests) indicated that jump height on Days 2 through 8 significantly (p<0.05) differed from Day 1 (average increase 7.2%). Average VGRF and t TAKEOFF exhibited a main effect for Trials (F=2.34, p=0.02 and F=2.95, p<0.01, respectively). Post hoc analysis indicated that the normalized average VGRF was smaller in Trials 5-8 relative to Trial 1. Post hoc analysis also revealed that average normalized values of t TAKEOFF increased in Trials 4-8 relative to Trial 1. No Day by Trial interactions were present for any variable examined. Conclusions: Significant increases in jump height occurred during the first 24 hours of the study, but no further significant increases in jump height occurred. As average VGRF and t TAKEOFF did not change across Days, a common mechanism for increased jump height was not present. The results suggest that the subjects adapted differently across Days in order to improve their jump performance. The increases t TAKEOFF and the decreases in average VGRF across Trials indicates that subjects consistently altered their jumping mechanics within each Day, but that these mechanical alterations did not improve jump height after Day 2 of the study. INTRODUCTION Prior to takeoff, two forces control the vertical motions of a jumper: weight (W) acting at the center of mass of the body and the vertical component of the reaction force (VGRF). For an individual, W has a constant magnitude, always acts in the downward direction, and is opposed by the upwardly directed VGRF. The relationship between the vertical velocity of the center of mass at takeoff ( v TO ), W and VGRF is given by the following equation: Where t ST is the time of the start of the countermovement, t TO is the instant of takeoff, and m BODY is the mass of the body, and Thus, to maximize jump height, jumpers must coordinate their muscular actions to properly sequence the resulting segmental motions and simultaneously optimize both the duration between t ST and t TO (t TAKEOFF ) and the average magnitude of VGRF during the takeoff phase. Practice is the single most important variable for improving motor skill performance (Magill, 2004). In jumping, it is likely that practice-related improvements in performance result from alterations in the underlying movement coordination pattern (Bernstein, 1967) and are associated with alterations in the associated muscular activation sequences (van Ingen Schenau, 1989). As changes in segmental movement patterns may alter the time history of forces applied to the ground by the jumper, they also may alter the time history of the VGRF applied to the jumper. Thus, examination of changes in the average magnitude of VGRF or in the period of VGRF application (t TAKEOFF ) serve as an indicator of underlying changes in muscular and movement coordination. Additionally, it is known that the rate of improvement in motor skill performance is inversely related to the length of practice (Snoddy, 1926). However, little is known about the time course of changes in a complex motor skill at the onset of a period of practice. Furthermore, little is also known about changes in the mechanical factors that govern skill performance at the onset of practice. Thus, the purpose of the study was to examine the time course of changes in jump height, VGRF application and impulse duration (t TAKEOFF ) during countermovement jumps in novice jumpers. In turn, jump height (measured as the vertical displacement of the center of mass from t TO to the peak of the jump) is given from the following equation. is the net vertical impulse exerted on the jumper. TO Table 1: Absolute jump height (cm) and jump height as a percentage of Day 1 height. Figure 1: Mean jump height expressed as a percentage of Day 1 jump height across 8 days. Error bars indicate ± 1 standard deviation (SD).  As seen in Figure 2, individual subjects’ responses varied, but most subjects had increases in average jump height on Days 2-8 when compared to Day 1.  As seen in Figure 3, VGRF and impulse duration (t TAKEOFF ) did not change across Days. Average VGRF and t TAKEOFF exhibited a main effect for Trials (F[9,99] = 2.34, p<0.05 and F[9,99] = 2.95, p<0.05, respectively).  Post hoc analysis indicated that the normalized average VGRF was smaller in Trials 5-8 relative to Trials 1-4.  Post hoc analysis also revealed that average normalized impulse duration (t TAKEOFF ) increased in Trials 4-8 relative to Trials 1-4. Figure 3: Mean VGRF and impulse duration expressed as a percentage of Day 1 values, respectively, for all subjects across 8 days. Error bars indicate ± 1 SD. Figure 4: Normalized average VGRF across trials for all subjects. VGRF is expressed as percentage of the mean daily VGRF. Error bars indicate ± 1 SD. Figure 5: Normalized average impulse duration (t TAKEOFF ) across trials for all subjects. Impulse duration is expressed as percentage of the mean daily impulse duration. Error bars indicate ± 1 SD. Figure 2: Individual jump height as a percentage of Day 1 jump height.