Rafael E. Bahamonde

Changes in angular momentum during the tennis serve

Three-dimensional cinematography and the direct linear transformation method were used to obtain the coordinates of the landmarks of five right-handed collegiate tennis players. A 15-segment model was used to calculate the total body angular momentum about three orthogonal axes (X, parallel to the baseline; Y, normal to baseline and pointing towards the net; and Z, pointing upwards) passing through the centre of mass and to obtain the segmental contribution of the trunk, arms and legs. Most of the clockwise angular momentum about the X-axis was concentrated in the trunk and the racket-arm. Between the events of maximum external rotation and ball impact, the clockwise angular momentum about the X-axis of rotation of most body segments was reduced and the racket-arm gained clockwise angular momentum. The body angular momentum about the Y-axis of rotation had two distinct patterns and was the result of the lateral rotation of the trunk as the racket shoulder was elevated in preparation for impact. This body angular momentum was clockwise from the event of maximum external rotation to impact for the players with the greatest ball speed, whereas it was counterclockwise for the other players. The angular momentum about the Z-axis of rotation was small and lacked a consistent pattern. The largest source of angular momentum in the tennis serve derives from the remote angular momentum about the X- and Y-axes of rotation, which are then transferred from the trunk to the racket-arm and finally to the racket. Near impact, most of the angular momentum (75.1%) was concentrated in the racket-arm. Of the angular momentum of the racket-arm, the largest percentages were concentrated in the racket (35.9%) and the forearm segment (25.7%).

Effect of endpoint conditions on position and velocity near impact in tennis

Smoothing to obtain accurate position and velocity data near impacts in sport biomechanics studies is complicated by the accelerations of impact, endpoint effects and the smoothing technique used. Wrist goniometric data with two levels of random noise added were used to examine three endpoint modelling conditions for obtaining accurate position and velocity data at impact in the tennis forehand. The common approach of smoothing through impact created distortions of the position signal up to 100 ms before impact and resulted in consistent underestimations (-3.2%) of wrist angle and angular velocity (-67.9%) at impact. New linear and polynomial extrapolation conditions smoothed with all techniques provided lower root mean squared errors than the smoothing-through-impact condition, with discrete wrist positions at impact within 1.1% of the criterion data. Both the new linear and polynomial conditions can be used to make more accurate angular position and velocity estimates for tennis impacts, whereas the smoothing-through-impact condition creates spurious decreases in speed before impact that in the past have been assumed to be aspects of skilful movement.

Stretching has no effect on tennis serve performance.

&NA; Knudson, D.V., G.J. Noffal, R.E. Bahamonde, J.A. Bauer, and J.R. Blackwell. Stretching has no effect on tennis serve performance. J. Strength Cond. Res. 18(3):654–656. 2004.—Stretching prior to vigorous physical activity has been shown to decrease high‐force muscular performance, but little is known about the effect of stretching on speed and accuracy movements. Serving percentage and radar measurements of ball speed were studied to examine the acute effect of stretching on tennis serve performance. Eighty‐three tennis players from beginning level to advanced volunteered to serve following traditional (T) warm‐up and traditional plus stretching (S) conditions. Service speeds and service percentage of each condition were measured. Dependent t‐tests showed nonsignificant effects of stretching on service speed (p = 0.06) or accuracy (p = 0.35), and this lack of an effect was similar for all skill levels, age, and gender. The large sample and good statistical power in this study indicated that these observations are not likely type II errors. There was no short‐term effect of stretching in the warmup on the tennis serve performance of adult players, so adding stretching to the traditional 5‐minute warm‐up in tennis does not affect serve performance.

A Randomized, Prospective Study of 3 Minimally Invasive Surgical Approaches in Total Hip Arthroplasty: Comprehensive Gait Analysis

Purported advantages of total hip arthroplasty performed with minimally invasive surgical (MIS) approaches are less muscle damage and faster recovery. There are little data scientifically evaluating these claims. Twenty-four consecutive hips were randomized to total hip arthroplasty through 1 of 3 MIS approaches (2-incision, mini-posterior, and mini-anterolateral). Each patient underwent preoperative and postoperative gait analysis. Gait parameters included vertical ground reaction force, velocity, single-leg stance time, limb-loading rate, and abductor torque. All 3 groups demonstrated overall improvements in gait parameters at 6 weeks postoperatively. The anterolateral approach patients showed a decrease in the vertical ground reaction force at mid-stance, whereas the 2-incision and posterior approaches demonstrated no significant change. These results fail to demonstrate any significant advantage of the 2-incision approach over the posterior approach in kinetic gait parameters. Furthermore, the anterolateral approach demonstrates a gait pattern consistent with abductor muscle injury in the early recovery period, despite the MIS approach.

Development and evaluation of a biomechanics concept inventory

Abstract To help instructors in evaluating innovations in biomechanics instruction, a standardised test of the key concepts taught in the introductory biomechanics course was developed. The Biomechanics Concept Inventory (BCI) consists of 24 questions that test four prerequisite competencies and eight biomechanics competencies. Three hundred and sixty seven students from ten universities throughout the United States took the test at the beginning and the end of the introductory biomechanics course. Analysis of a sub‐sample of the students showed that the BCI was reliable with typical errors in internal consistency and test‐retest conditions of 1.4 and 2.0 questions, respectively. Mean pre‐test scores (8.5 ± 2.0) significantly (p < 0.0001) improved to 10.5 ± 3.2 in the post‐test (n = 305). Typical biomechanics students could correctly answer half of the prerequisite questions on the pre‐test. Instruction resulted in a mean normalised gain (g) of 13.0% of maximum possible improvement that was consistent with research on traditional instruction in introductory physics courses. It was concluded that the BCI could be an effective tool to evaluate the overall effect of pedagogical strategies on student learning of key biomechanical concepts in the introductory biomechanics course.

Correlates of Learning in Introductory Biomechanics

Characteristics hypothesized to be related to learning in introductory biomechanics classes were examined. Data from a 2003 national study were supplemented by additional instructor-reported data about experience, tests, laboratory hours, and average course-related expenditures. The number of credit hours of the course (r = .15) and money spent by departments on laboratory classes per year (r = -.18) were significantly and uniquely correlated with learning. Increasing credit hours from 3 to 4 with laboratory experience doubled learning. The results supported national standards for a 4-credit course and an emphasis on conceptual understanding rather than quantitative problem-solving. Researchers should seek to confirm these results as well as explore whether contact hours or the nature of laboratory learning experiences contribute to this effect.