Association of drop vertical jump displacement with select performance variables.

Drop vertical jumps (DVJs) stimulate enhanced countermovement loading as would occur with a run-up before jumping. A variety of performance variables have been associated with DVJ performance including ground contact time (GCT), reactive strength index (RSI), eccentric utilization ratio (EUR), and elasticity index (EI). This study examined the stability reliability and precision of these variables and their associations with DVJ displacement in trained men and women. The EUR and EI measures were redundant, so only EUR findings were reported. Except for EUR, data for all variables were both reliable and precise (intraclass correlation coefficient ≥ 0.70, coefficient of variation [CV%] ≤ 15.0) although EUR data were precise (CV% ≤ 15.0). Correlations with DVJ displacement were low for GCT, moderate for RSI, and negligible for EUR. Therefore, GCT and EUR likely represent unique performance characteristics not related to DVJ displacement. Furthermore, the variability in DVJ performance accounted for by RSI may primarily reflect the inclusion of displacement as the numerator in the quotient for calculating it.

Gender-based analysis of hamstring and quadriceps muscle activation during jump landings and cutting.

This study evaluated gender differences in the magnitude and timing of hamstring and quadriceps activation during activities that are believed to cause anterior cruciate ligament (ACL) injuries. Twelve men (age = 21.0 +/- 1.2 years; body mass = 81.61 +/- 13.3 kg; and jump height = 57.61 +/- 10.15 cm) and 12 women (age = 19.91 +/- 0.9 years; body mass = 64.36 +/- 6.14 kg; and jump height = 43.28 +/- 7.5) performed 3 repetitions each of the drop jump (jump) normalized to the subject's vertical jump height, and a sprint and cut at a 45-degree angle (cut). Electromyography (EMG) was used to quantify rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), lateral hamstring (LH), and medial hamstrings (MH) activation, timing, activation ratios, and timing ratios before and after foot contact for the jump and cut and normalized to each subject's hamstring and quadriceps maximum voluntary isometric contraction. Data were analyzed using an analysis of variance with results demonstrating that during the postcontact phase of the cut, men demonstrated greater LH and MH activation than women. In the precontact phase of the jump, men showed earlier activation of the VL and VM, than women. Women produced longer RF and VM muscle bursts during the postcontact phase of the cut. Additionally, men showed a trend toward higher hamstring to quadriceps activation ratio than women for the postcontact phase of the cut. This study provides evidence that men are LH dominant during the postcontact phase of the cut compared with women, whereas women sustain RF activation longer than men during this phase. Men activate quadriceps muscles earlier than women in the precontact phase of the jump. Training interventions may offer the potential for increasing the rate and magnitude of hamstring muscle activation. These outcomes should be evaluated using EMG during movements that are similar to those that cause ACL injuries to determine if gender differences in muscle activation can be reduced.

Reliability of surface electromyography during maximal voluntary isometric contractions, jump landings, and cutting.

The reliability of electromyographic (EMG) data has been examined for isometric and slow dynamic tasks, but little is known about the repeatability of this data for ballistic movements. The purpose of this study was to examine the within-session, trial-to-trial reliability of a variety of quadriceps and hamstrings muscles during isometric and ballistic activities. Data were analyzed by way of intraclass correlation coefficients (ICC), intersubject coefficients of variation (CVinter), and intrasubject coefficients of variation (CVintra). Twenty-four subjects performed 3 repetitions each of 2 randomly ordered test exercises, including landing from a depth jump (J) and cutting after a 10-m sprint (C). Data were acquired and processed with root mean square EMG for the muscles assessed, and data were analyzed for each exercise using a repeated measures analysis of variance. Results revealed that all ICC values were greater than 0.80, with most values greater than 0.90, CVinter values ranged from 5.4% to 148.7%, and CVintra values ranged from 11.5% to 49.3%. This study indicates that EMG is a reliable method for assessing the reproducibility of both the quadriceps and hamstrings muscle activation during either isometric or ballistic exercises.

Periodized plyometric training is effective for women, and performance is not influenced by the length of post-training recovery.

This study evaluated the effectiveness of a periodized plyometric training program and the impact of the duration of the post-training recovery period on countermovement jump performance. Fourteen women subjects participated in a 6-week periodized plyometric training program. Ten women subjects served as non-training controls. All subjects' countermovement jump height, peak power, and body mass were assessed before and 2, 4, 6, 8, and 10 days after training. Kinetic data were obtained via a force platform using the average of 3 repetitions of the countermovement jump for each testing session. Jump height was 25.0% greater (p < or = 0.05) after training with no difference (p > 0.05) between recovery periods of 2, 4, 6, 8, or 10 days, for the training group. Peak power was 11.6-14.3% (p < or = 0.001) greater after training for the training group with no difference (p > 0.05) between recovery periods of 2, 4, 6, 8, or 10 days. Analysis revealed no significant difference (p > 0.05) for jump height or peak power from pre- to posttest for the control group. Practitioners should prescribe periodized plyometric programs with decreasing volume and increasing intensity to improve jump performance without a need for a post-training recovery period.

Strength and conditioning practices of United States high school strength and conditioning coaches.

This study describes the results of a survey of the practices of high school strength and conditioning (HS S&C) coaches. Thirty-eight of 128 (29.7%) HS S&C coaches, who were surveyed, responded. This survey examines background information, physical testing, flexibility development, speed development, agility development, plyometrics, strength/power development and program design, nutrition, and injury frequency. High school strength and conditioning coaches average 14.78 years in the profession, and 89% of the survey respondents were certified by the National Strength and Conditioning Association. Coaches assess an average of 5.83 different parameters of fitness, with strength as well as power being the most common parameters assessed. Coaches tested an average of 3.86 times a year with an average of 8.51 specific tests per testing session. All HS S&C coaches used a variety of flexibility development strategies using dynamic stretching more frequently than static. Thirty-six of 38 (95%) coaches follow a periodization model, 37 of 38 (97.4%) coaches indicated that their athletes used Olympic-style lifts, and 37 of 37 (100%) coaches responding to this question used plyometric training with their athletes. The squat and its variations, as well as the Olympic-style lifts and its variations, were most frequently identified as the most important exercises prescribed. All coaches use speed development and agility training strategies with their athletes. This survey examined a variety of other practices and provides detailed information about these practices at the high school level. Coaches are encouraged to review the strength and conditioning practices described in this survey and identify and implement the ideas that may improve their practices.

Using squat testing to predict training loads for the deadlift, lunge, step-up, and leg extension exercises.

The purpose of this study was to determine whether there is a linear relationship between the squat and a variety of quadriceps resistance training exercises for the purpose of creating prediction equations for the determination of quadriceps exercise loads based on the squat load. Six-repetition maximums (RMs) of the squat, as well as four common resistance training exercises that activate the quadriceps including the deadlift, lunge, step-up, and leg extension, were determined for each subject. Subjects included 21 college students. Data were evaluated using linear regression analysis to predict quadriceps exercise loads from 6RM squat data and were cross-validated with the prediction of sum of squares statistic. Analysis of the data revealed that the squat is a significant predictor of loads for the dead lift (R2 = 0.81, standard error of the estimate [SEE] = 12.50 kg), lunge (R2 = 0.62, SEE = 12.57 kg), step-up (R2 = 0.71, SEE = 9.58 kg), and leg extension (R2=0.67, SEE = 10.26 kg) exercises. Based on the analysis of the data, the following 6RM prediction equations were devised for each exercise: (a) deadlift load = squat load (0.83) + 14.92 kg, (b) lunge load = squat load (0.52) + 14.82 kg, (c) step-up load = squat load (0.50) + 3.32 kg, and (d) leg extension load = squat load (0.48) + 9.58 kg. Results from testing core exercises such as the squat can provide useful data for the assignment of loads for other exercises.