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\u27s 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\u27s 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

Antagonist conditioning contractions impair agonist functioning

This study assessed the effect of antagonist conditioning contractions (ACC) on the subsequent force and electromyography of an agonist. Twelve subjects performed isokinetic elbow flexion on a dynamometer in 4 test conditions including a baseline condition without, and 1, 3 and 6 seconds after, isometric triceps extension. Average peak torque (T), peak torque/body weight (T:BW), average power (P), and rate of torque development (RTD) were assessed. Electromyographic data were obtained from elbow extensors and flexors. A repeated measures ANOVA with post hoc analysis demonstrated that T, T:BW, P, and RTD were higher in the baseline, compared to the post ACC conditions (P ≤ 0.05), and appears to be due to higher brachioradialis activation in the baseline condition in compared to some post ACC conditions (P ≤ 0.05)