1. . Authors: Anu Shrestha, Biomechanics Laboratory, The University of Texas at Arlington, Arlington, TX Sponsor: Dr. Mark Ricard & Dr. Judy Wilson THE EFFECT OF VELOCITY OF STRETCH (SLOW AND FAST) ON JUMPING PERFORMANCE IN MALES AND FEMALES The purpose of this study is to compare the effects of velocity of stretch on countermovement jump performance in males and females. Each subject performed 15 slow and 15 fast stretch velocity countermovement jumps with the order counterbalanced. Vertical force was sampled at 2000 Hz using an AMTI force platform. A Visual Basic.Net 2008 computer program was used to compute: jumping height, velocity of stretch, length of stretch, force at the end of the stretch, positive energy, concentric impulse, and average positive power from the vertical force – time signal, Figure 2. Separate 2 x 2 repeated measures ANVOAs with one within-subjects factor speed (slow, fast) and one between-subjects factor gender (male, female) were used to compare the effects of velocity of stretch and gender on the following dependent variables: jumping height, positive energy, concentric impulse, and average power. The Sidak post hoc test was used for pair-wise comparisons. Stepwise multiple linear regression was used to predict jumping height from the following predictors: average negative velocity, length of stretch, positive energy, concentric impulse, average power, peak power, force at the end of the stretch, eccentric impulse/kg, average power/kg, peak power/kg, concentric impulse/kg, negative energy/kg, force at the end of the stretch/kg, and eccentric impulse/kg. Alpha was set at 0.05. Figure 1: The steps of Countermovement Jump 1. Performance Variable: Jump Height (cm): Slow VelocityFast VelocityGender Means Female 17.35 ± 2.33 24.06 ± 4.39 20.70 ± 2.07 Male 27.11 ± 7.9934.4 ± 9.80 30.75 ± 2.07 Speed Means 22.23 ± 7.60 29.23 ± 9.11 Speed * Gender: F (1, 18) =.128, p =.725, power =.063 Speed: F (1, 18) = 75.27, p =.000 (p <.05), power = 1.00 Gender: F (1, 18) = 11.71, p =.003 (p <.05), power =.899 2. Performance Variable: Normalized Positive Energy (Epos) [J/kg] Slow VelocityFast VelocityGender Means Female 1.70 ±.23 2.31 ±.39 2.00 ±.19 Male 2.65 ±.783.32 ±.88 2.99 ±.19 Speed Means 2.18 ±.74 2.81 ±.84 Speed * Gender: F (1, 18) =.116, p =.737, power =.062 Speed: F (1, 18) = 86.982, p =.000 (p <.05), power = 1.00 Gender: F (1, 18) = 12.65, p =.002 (p <.05), power =.920 3. Performance Variable: Normalized Concentric Impulse [Ns/kg] Slow VelocityFast VelocityGender Means Female 1.83 ±.12 2.12 ±.17 1.98 ±.07 Male 2.28 ±.342.53 ±.31 2.40 ±.07 Speed Means 2.06 ±.33 2.33 ±.32 Speed * Gender: F (1, 18) =.279, p =.604, power =.079 Speed: F (1, 18) = 94.516, p =.000 (p <.05), power = 1.00 Gender: F (1, 18) = 14.784, p =.001 (p <.05), power =.953 Faster stretch velocity in CMJ jumps significantly improved jumping height, concentric impulse, positive energy, concentric impulse and average power. Normalized: average power, concentric impulse and positive energy were significant predictors of vertical jump performance. 4. Performance Variable: Normalized Average Power [W/kg] Slow VelocityFast VelocityGender Means Female 15.34 ± 1.65 22.36 ± 4.50 18.85 ± 4.66 Male 21.04 ± 4.1227.92 ± 3.90 24.48 ± 4.66 Speed Means 18.91 ± 3.14 25.14 ± 4.21 Speed * Gender: F (1, 18) = 0.01, p =.922, power =.051 Speed: F (1, 18) = 81.57, p =.000 (p <.05), power = 1.00 Gender: F (1, 18) = 16.62, p =.071, power =.444 Figure 2. Vertical jump variables. Multiple Linear Regression R 2 = 0.999, SEE = 0.32 JumpHt = 0.061(AvgPower/kg) −8.884(Con Impulse/kg) + 13.91(Epos/kg) + 9.149 Explosive vertical jump ability can improve performance in sport and leisure activities that require jumping motions. Vertical jump height can be increased by utilizing a countermovement in which the jumper accelerates downward by flexing the hip, knee and ankle joints followed by an immediate upward acceleration which is accomplished by extending the hip, knee and ankle joints, see Figure 1. This enhanced jumping performance in a countermovement jump is primarily due to the use of a stretch-shorten cycle (SSC). A stretch-shorten cycle is defined as an eccentric contraction followed by an immediate concentric contraction. The shortening (concentric) phase of a SSC is more powerful, does more work, than the shortening (concentric) phase of a concentric only contraction. The extra work performed during the shortening (concentric) phase of a stretch-shorten cycle is attributed to stored elastic energy, increased muscular activation from a stretch-reflex and increased time for the active muscles to generate force. Increasing the velocity of stretch during the eccentric motion increases the: force at the end of the stretch, the stretch-reflex and the resulting potentiation of the concentric phase. Unfortunately, many jumpers fail to employ a high stretch velocity when performing a vertical jump. Twenty college age students (10 males, 10 females) served as subjects. Each participant filled the data sheet that includes some general information including name, gender, height, weight, ethnicity and the recent lower extremities injury. Percent body fat was computed using a three site (tricep, suprailiac, and thigh) skinfold regression equation.