Kinesiology

a b s t r a c t Neuromuscular fatigue of the trunk musculature, particularly lumbar paraspinal and abdominal muscles, is important in when evaluating motor control of the trunk. Activation of agonists and antagonists trunk muscles was hypothesized to change during sub-maximal isometric trunk extension efforts. Thirteen women were positioned in 30 of trunk flexion and performed maximal voluntary isometric contraction in trunk extension against an isokinetic dynamometer. One of two sub-maximal efforts (50% and 70%) was performed to induce neuromuscular fatigue on two different days. Surface electromyography of the lumbar paraspinal (LP), rectus abdominis, and external oblique muscles was recorded during each session. Torque output, median frequency of the power density spectrum, and normalized integrated electromyography were analyzed using repeated measures analysis of variance to evaluate trends in the data over time. Paraspinal muscles showed signs of fatigue in both conditions (p < 0.05) Abdominal activity did not increase during the 70% condition, but showed a non-significant trend (p ¼ 0.07), coinciding with the reduced median frequency of LP muscles. The neuromuscular system modulates its motor control strategy to identify the muscle activation levels necessary to maintain force output. This information is necessary in the evaluation of contributing mechanisms to trunk stability in furthering preventative and rehabilitative treatments.

In recent years a great deal of research has been published using peak power (PP) in the jump squat (JS) exercise as a measure of athletic performance. However, no standardized method for the determination of PP exists at this time to accurately evaluate this variable. Our proposed method (PM) for determining PP (PP PM ) in the JS uses the product of vertical ground reaction forces and velocity of the center of mass of both the subject and the external resistance of a loaded Olympic bar. Fifteen male subjects with a mean age of 27 6 3 years, weight of 78 6 17 kg, and height of 175 6 10 cm participated in this study. PP was measured in the JS at five different testing loads (30%, 35%, 40%, 45%, and 50% body weight) based on methods commonly discussed in the literature to compare PP results of previous methods to those obtained using the PM. Paired t-tests at different load levels were used for statistical analysis with an overall a = 0.05. The average PP among five testing loads, measured by the PM, was 3782 6 906 W. PP derived from the product of force and velocity of the bar alone was 72% lower than PP PM at 1057 6 243 W (P , 0.0001). The PP estimated by the product of bar velocity and vertical ground reaction forces of the bar plus the subject was 8% higher than PP PM at 4100 6 844 W (P = 0.0001). Our results indicate that using the methods traditionally reported in the literature may cause an overestimation of PP during athletic performance. Using the PM in future research will facilitate test validity and enable the generalization of results outside the scope of specific research projects.

Human and animal models using electromyography (EMG) based methods have hypothesized that viscoelastic tissue properties becomes compromised by prolonged repetitive cyclic trunk flexion-extension which in turn influences muscular activation including the flexion-relaxation phenomenon. Empirical evidence to support this hypothesis, especially the development of viscoelastic tensionrelaxation and its associated muscular response in passive cyclic activity in humans, is incomplete. The objective of this study was to examine the response of lumbar muscles to tension-relaxation development of the viscoelastic tissue during prolonged passive cyclic trunk flexion-extension. Activity of the lumbar muscles remained low and steady during the passive exercise session. Tension supplied by the posterior viscoelastic tissues decreased over time without corresponding changes in muscular activity. Active flexion, following the passive flexion session, elicited significant increase in paraspinal muscles EMG together with increase in the median frequency. It was concluded that reduction of tension in the lumbar viscoelastic tissues of humans occurs during cyclic flexion-extension and is compensated by increased activity of the musculature in order to maintain stability. It was also concluded that the ligamento-muscular reflex is inhibited during passive activities but becomes hyperactive following active cyclic flexion, indicating that moment requirements are the controlling variable. It is conceived that prolonged routine exposure to cyclic flexion minimizes the function of the viscoelastic tissues and places increasing demands on the neuromuscular system which over time may lead to a disorder and possible exposure to injury.