ACCURACY OF PREDICTED PEAK FORCES DURING THE POWER DROP EXERCISE

The utility of a new method for estimating upper body plyometric forces was demonstrated and results compared with recent training load equations. Videography and force platform data were collected for subjects performing power drop exercises with drop heights between 49 and 124 cm. Within-subject analyses demonstrated that vertical forces in power drop exercises were not correlated or were weakly (r < 0.71) associated with drop height. Other factors like technique and medicine ball properties determine the majority of the variance in peak forces in the power drop exercise. The data supported the utility of the method but did not support the validity of the Ebben et al. (1999) training load equations

EFFECT OF GRIP MODELS ON REBOUND ACCURACY OF OFF-CENTER TENNIS IMPACTS

INTRODUCTION AND PURPOSE The accuracy of strokes is of prime importance in winning tennis play. Most studies of ball rebound accuracy have used firmly clamped rackets for experimental control. Recent modeling and experimental data suggest that hand-held rackets may behave mechanically at impact as if they were unrestrained rather than clamped (Brody, 1987; Casolo & Ruggieri, 1991; Hatze, 1992, 1993; Knudson, 1997). The purpose of this study was to compare ball rebound accuracy of three models of off-center tennis impacts, free standing (FS), hand held (H), and firmly clamped. METHODS The effect of the three grip models on the rebound accuracy impacts was documented by measuring lateral deviation of ball rebound in the transverse plane. An oversized tennis racket was strung with nylon at 311 N (70 lbs) of tension. The racket stood in a vertical position on an immobilized bench, standardized by markings and a carpenters square. Six new tennis balls were projected by a ball machine at 24.1 m/s at a 14 degree angle to the racket face. Impact locations on the racket face were monitored by 15 (five for each model) measurements of ball imprints made on paper taped to the strings. Mean impact location was 20 f 5 mm off-center. Rebound accuracy was defined as the lateral displacement of the rebounding ball measured from ball imprints made by carbon paper on poster board taped to a wall 1.52 m from the racket. Twelve impacts were measured for each grip model. Displacement data were analyzed with a one-way ANOVA and Tukey-Kramer HSD post hoc tests with statistical significance accepted at the p < 0.05 level. RESULTS AND CONCLUSIONS ANOVA analysis revealed a significant (F2.33 = 11.4, p = 0.0002) effect of grip model on rebound accuracy. Forty-one percent of the variance (h2 = 0.409) of rebound accuracy was related to grip model. Tukey-Kramer post hoc tests demonstrated that the C model was significantly more accurate than the FS and H models. The rebound accuracy of the FS and H models were not significantly different. The mean angles of rebound (measured normal to the racket face) that correspond to the lateral displacements observed in the C, H, and FS models were 14.6, 16.3, and 17.0 degrees. Since most impacts occur off-center on the racket during tennis play, studies of rebound accuracy in tennis should use H or FS models rather than C models. Previous studies of tennis impact mechanics using clamped racket models should be interpreted with caution. REFERENCES Brody, H. (1987b). Models of tennis racket impacts. International Journal of Sport Biomechanics, 3, 293-296. Casolo, F., & Ruggieri, G. (1991). Dynamic analysis of the ball-racket impact in the game of tennis. Meccanica, 26.67-73. Hatze, H. (1992). Objective biomechanical determination of tennis racket properties. International Journal of Sport Biomechanics, 8,275-287. Hatze, H. (1993). The relationship between the coefficient of restitution and energy losses in tennis rackets. Journal of Applied Biomechanics, 9, 124-142

WHAT HAVE WE LEARNED FROM TEACHING CONFERENCES AND RESEARCH ON LEARNING IN BIOMECHANICS?

A narrative review was conducted of biomechanics teaching/learning papers published in teaching conference proceedings and in journals since 1980. The majority of the papers published focused on course concepts and technology, rather than reporting data on student learning. Recent progress has been made in standardized tests of biomechanical concepts and identifying factors that are associated with learning these concepts. Future research should use these tests to focus on learning-related factors and active learning strategies from physics education research to improve student mastery of biomechanical concepts

ABDOMINAL MUSCLE ACTIVATION IN TWO TRUNK-CURL TESTS

INTRODUCTION Trunk-curl exercises have been advocated as the safest and most effective exercises for the abdominal muscles. Trunk-curl tests of abdominal endurance have begun to replace the bent-knee sit-up as the muscular endurance measures in health-related fitness batteries. The purpose of this study was to compare the abdominal muscle activation in two current trunk-curl tests of abdominal endurance. METHODS Twenty-three subjects (12 female and 11 male) between 19 and 24 years of age volunteered and gave informed consent for the study. Electromyographic activity of the right rectus abdominis (RA) and the left external oblique (EO) were recorded with surface electrodes (Noraxon Myosystem 2000) as the subjects performed the bench trunk-curl (Knudson & Johnston, 1995), the modified curl-up (CIAR, 1992), and two maximal voluntary contractions (MVC). Trunk motion was measured with a Penny & Giles M180 electrogoniometer attached to the lateral illiac crest and ribs. EMG and goniometer data were A/D converted at 1000 Hz and stored on disk. Curl-up tests were performed to a cadence of 20 repetitions per minute. Mean rectified EMG of the RA and EO were calculated during the first 500 ms of the concentric phase for six repetitions of each trunk-curl for each subject. Subject mean RA and EO EMG were expressed as a percentage of the maximum rectified EMG in the MVC's. Mean normalized EMG of the RA and EO were compared across trunk-curls with dependent t-tests. Statistical significance was accepted at p < 0.05. RESULTS AND CONCLUSIONS - There were no significant differences in RA and EO activation between the two trunk-curl tests. Mean normalized activation of the RA and EO were between 10.9 and 14.2% of MVC. This activation was slightly smaller than previous EMG research on sit-up/curl-ups (Ekholm, Arborelius, & Fahlcrantz, 1979). Each subject showed a consistent response of the two abdominal muscles to the trunk-curl tests, but there were between subject differences in abdominal muscle activation across tests. Most subjects (70% EO and 61% RA) had greater abdominal muscle activation in the BTC compared to the MTC. The n o d variability of abdominal muscle activation (Ekholm et al., 1979; Gilleard & Brown, 1994) could account for the no significant difference in mean activation of the RA and EO across trunk-curl tests. REFERENCES Cooper Institute for Aerobic Research. (1992). The prudential fitness grarn: test administration manual. Dallas, TX: CIAR. Ekholm, J., Arborelius, E., & Fahlcrantz, A. (1979). Activation of abdominal muscles during some physiotherapeutic exercises. ScandinavianJournal of Rehabilitation Medicine, 11,75-84. Gilleard, W., & Brown, J. (1994). An electromyographic validationof an abdominal muscle test. Archives of Physical Medicine and Rehabilitation, 75,1002- 1007. Knudson, D., & Johnston, D. (1995). Validity and reliability of a bench trunk-curl test of abdominal endurance. Journal of Strength and Conditioning Research, 9, 165-169

Highly Excited States of a Hydrogen Atom in a Strong Magnetic Field

Classical trajectories and semiclassical energy eigenvalues are calculated for an atomic electron in a high Rydberg state in an external magnetic field. With the use of perturbation theory, a classical trajectory is described as a Kepler ellipse with orbital parameters evolving slowly with time. As they evolve, the ellipse rocks, tilts, and flips in space, but the length of its major axis remains approximately constant. Exact numerical calculations verify that perturbation theory is quite accurate for the cases considered (principal quantum number ≃ 30, magnetic field ≲ 6 T). Action variables are calculated from perturbation theory and from exact trajectories, and semiclassical eigenvalues are obtained by quantization of action. Excellent agreement is found with observations

EFFECT OF TYPE 3 BALL ON UPPER EXTREMITY EMG AND ACCELERATION IN THE TENNIS FOREHAND

The effect of the larger type 3 tennis ball on upper extremity EMG and impact acceleration was studied in a diverse sample of adult tennis players. Twelve players from intermediate to advanced levels of ability played no-add sets of six games using the larger and the regular tennis ball. Racket acceleration and upper arm muscle activation were measured during forehands with a Noraxon telemetry system. Within-subject statistical analysis showed that half of the players had significantly (p < 0.05) greater mean normalized-rectified EMG of the biceps or triceps with the larger ball compared to the regular ball. Half of the players with complete acceleration data also had significantly (p < 0.05) greater mean rectified acceleration after impact with the larger ball

INFLUENTIAL LITERATURE IN APPLIED SPORTS BIOMECHANICS

This study documented the perception of prestige of applied sports biomechanics journals, as well as influential articles and books. Recent ISBS members were surveyed to rate the quality/prestige of 35 journals. Descriptive statistics of ratings were calculated for respondents and correlated with the 2008 impact factor (IF) reported in the Journal Citation Reports. Mean ratings showed that international perception of influential journals were weakly (r = 0.48) correlated with the IF. These results confirm previous studies that the IF is a poor index for evaluating the influence of journals publishing applied sports biomechanics research, and there was considerable diversity among the respondent’s nominations of the most influential books and articles in the field