ABSTRACT
SUMMARY: As it is currently played, volleyball is a game in which success depends in large measure on the athleticism of the participants. The aim of this research was to point out the importance of the cycle of stretching and shortening in different jumps for elite volleyball players. Thus, it is common for volleyball athletes to place considerable emphasis on jump training. Not surprisingly, overload injuries of the knee and ankle joints, both acute and chronic, occur frequently among volleyball players and are related to the volume of jump training and skill repetition. Understanding the biomechanics of jumping is therefore a prerequisite for designing effective training programs which minimize the risk of overuse injuries that may result from excessive jumping, and the repetitive mechanical loading of muscles and joints that are involved in jump training Muscles acting about a joint function naturally through a combination of eccentric (lengthening) and concentric (shortening) activations. In the lower limb, the stretch-shortening cycle is a reflex arc in which the tendomuscular system acting about the knee or ankle is eccentrically preloaded (stretched) in the loading or impact phase of the jump before concentrically shortening in the push-off or take-off phase.
RESUMEN: Actualmente, el voleibol es un juego en el que el éxito depende en gran medida del atletismo de los participantes. El objetivo de esta investigación fue señalar la importancia del ciclo de estiramiento y acortamiento en diferentes saltos para jugadores de voleibol de élite. Es común que los atletas de voleibol pongan un énfasis considerable en el entrenamiento de salto. No es sorprendente que las lesiones por sobrecarga de las articulaciones de la rodilla y el tobillo, tanto agudas como crónicas, ocurran con frecuencia entre los jugadores de voleibol y estén relacionadas con el volumen de entrenamiento de saltos y la repetición de destrezas. Comprender la biomecánica del salto es, por lo tanto, un requisito previo para diseñar programas de entrenamiento efectivos que minimicen el riesgo de lesiones por uso excesivo que pueden resultar de un salto excesivo y la carga mecánica repetitiva de los músculos y las articulaciones que están involucrados en el entrenamiento del salto. Los músculos que actúan sobre una articulación funcionan de forma natural a través de una combinación de activaciones excéntricas (alargamiento) y concéntricas (acortamiento). En el miembro inferior, el ciclo de estiramiento-acortamiento es un arco reflejo en el que el sistema tendomuscular que actúa sobre la rodilla o el tobillo se precarga (estira) excéntricamente en la fase de carga o impacto del salto antes de acortarse concéntricamente en el despegue o toma. -fase de apagado.
Subject(s)
Humans , Muscle, Skeletal/physiology , Volleyball , Reflex , Biomechanical Phenomena , Muscle Strength , Muscle ContractionABSTRACT
Objective To analyze muscle pre-activation and surface electromyography (sEMG) characteristics of knee and ankle joints of long-term Tai Chi practitioners during brush-knee twist-step and normal walking, and explore the neuromuscular control strategies of Tai Chi to prevent falls. Methods Vicon motion capture system, Kistler force plate, and Noraxon sEMG system were synchronously used to collect the EMG signals of the rectus femoris, biceps femoris, tibialis anterior muscle, lateral head of gastrocnemius and body posture information during brush-knee twist-step and normal walking. The pre-activation and co-contraction of knee and ankle joints were calculated by integrated EMG of the rectus femoris/biceps femoris, tibial anterior/lateral gastrocnemius muscles. Results Compared with normal walking, the average time of brush-knee twist-step in four phases was significantly increased. There was a significant difference in the percentage of time in four phases. The knee joint co-contraction level and pre-activation level decreased, and the ankle joint co-contraction level and pre-activation level increased. Conclusions Long-term Tai Chi exercises may increase the activation level of the muscles around knee joints and enhance the synergy in muscle groups to help stabilize the joint. The results provide references for rehabilitation assessment and training of neuromuscular control disorders.
ABSTRACT
OBJECTIVE: To quantify, for each activation, the effect of preactivations of differing distribution and intensity on the neutral position of T-loops (7-mm height), specifically the horizontal force, moment to force (M/F) ratio, and load to deflection ratio. METHODS: A total 100 loops measuring 0.017 x 0.025 inches in cross-section were divided into two groups (n = 50 each) according to composition, either stainless steel or beta-titanium. The two groups were further divided into five subgroups, 10 loops each, corresponding to the five preactivations tested: preactivations with occlusal distribution (0degrees, 20degrees, and 40degrees), gingival distribution (20degrees), and occlusal-gingival distribution (40degrees). The loops were subjected to a total activation of 6-mm with 0.5-mm iterations. Statistical analysis was performed using comprised ANOVA and Bonferoni multiple comparison tests, with a significance level of 5%. RESULTS: The location and intensity of preactivation influenced the force intensity. For the M/F ratio, the highest value achieved without preactivation was lower than the height of the loop. Without preactivation, the M/F ratio increased with activation, while the opposite effect was observed with preactivation. The increase in the M/F ratio was greater when the preactivation distribution was partially or fully gingival. CONCLUSIONS: Depending on the preactivation distribution, displacement of uprights is higher or lower than the activation, which is a factor to consider in clinical practice.
Subject(s)
Analysis of Variance , Stainless SteelABSTRACT
The purpose of the present study was to examine the importance of the agonist muscle activity of the post-impact 30 ms phase during drop jump (DJ) for effective rebound performance by comparing those of sprint runners and swimmers. The eight sprint runners (SPRINT) and twelve swimmers (SWIM) were participated in this study. They performed DJ from a 0.3-m height box with maximal rebound efforts. Electromyograms (EMG) of the lower leg muscles (medial gastrocnemius [MG], soleus [SOL] and tibialis anterior [TA]), and vertical ground reaction force together with kinematic data were measured simultaneously during DJ. In addition, the onsets of fascicle stretching of the MG and SOL muscles were measured by using high-speed ultrasonography (521Hz) during DJ. The onsets of the fascicle stretching of SOL during DJ were not significantly different between SPRINT and SWIM (15 ± 7 ms and 16 ± 6 ms, respectively). During DJ, SPRINT showed onset of the SOL EMG before the ground contact (-26 ± 19 ms). Meanwhile, SWIM showed the onset of the SOL EMG after the ground contact of DJ (16 ± 19 ms). These results suggest that the SOL muscles for SWIM cannot be fully-activated during the braking phase. Consequently, the rate of force development during the braking phase of DJ and subsequently rebound height could be reduced in SWIM.
ABSTRACT
The purpose of this study was to examine the effect of the different visual and vestibular inputs on EMG response of ankle muscles during movement to absorb impact in sliding down a slope. Thereby, our research focused on the relationship between preactivation (PA) in central program and stretch reflex induced by dorsiflexion immediately after impact. The subjects were nine healthy males. Movements were conducted using a special sliding apparatus. Conditions included sliding down a lower 15°slope with eyes open (Low) and with eyes closed (Low-Closed), and a higher 20°slope with eyes open (High) . PA prior to impact indicated the co-contraction of the medial gastrocnemius (MG) and tibialis anterior (TA) . PA levels in those muscles were significantly higher during High than during Low-Closed (p<0.05) . In contrast, PA of the coleus (Sol) was low in all test conditions. After impact, however, the stretch reflex of Sol during Low-Closed was greater than other test conditions. Because muscle stretch velocities and PA levels of Sol among all test conditions remained unchanged, these results suggest that different visual inputs could change the response of stretch reflex by modulations in reflex gain.