Ízületi szöghelyzet-specifikus erődeficit és EMG aktivitás a quadriceps femoris izomban mikrosérülést okozó edzést követően

Due to the microdamage, caused by high tension in the muscle, contractile strength decreases along withthe modification of the muscle’s myoelectric (EMG) activity.In the present study we hypothesized that strength attenuation of knee extensors as well as the change inknee extensor and biceps femoris EMG activity is joint-angle-specific.Using a dynamometer, maximal voluntary isometric knee extension torque was measured at 30, 50 and70° knee joint angles in seventeen healthy, physically active male participants. Quadriceps and biceps femoris EMG activity was measured during the knee extensions. All tests were repeated 24 hours after a singlebout of maximal intensity eccentric-concentric knee extension exercise training performed on the samedynamometer.As a result of the acute exercise, a substantial amount of muscle pain developed, indicating the presenceof muscle damage. The isometric knee extensor torque decreased in all 3 joint angles; the smaller thejoint angle was, the bigger the torque deficit was. Quadriceps femoris EMG activity measured at knee extensions remained unchanged after exercise. However, biceps femoris co-activation increased significantlyin all joint angles. The biceps femoris/quadriceps femoris EMG activity ratio increased only at 70° and 50°.The quadriceps femoris EMG activity/knee extension torque ratio increased at 30° and 50° but remainedunchanged at 70°.Our results indicate that, in case of exercise induced muscle damage, the torque deficit is greater at smaller muscle length. At the same time, the antagonist co-activation increases towards when the isometricknee extension test is performed at greater muscle length.We conclude that an acute bout of knee extensor resistance exercise induces joint-specific alterations inknee stability in terms of changes in torque and agonist-antagonist activation ratio.A korábbi kutatások igazolták, hogy a vázizom mikrosérülései erődeficitet és az izom myoelektromos (EMG) aktivitásának módosulását okozhatják. Vizsgálatunkban feltételeztük, hogy a térdextenzorok erődeficitje, valamint a térdextenzorok és a biceps femoris izom EMG aktivitásának módosulása ízületi szöghelyzet-specifikus.Tizenhét egészséges, fizikailag aktív fiatal férfi esetében vizsgáltuk elektromiogáf és Multicont II. dinamométer segítségével, hogy miként változik a 30, 50 és 70°-os ízületi szöghelyzetben kifejtett maximális izometrikus forgatónyomaték mértéke, valamint az agonista és antagonista izmok aktivációja egy maximális intenzitású excentrikus-koncentrikus térdextenzió edzést követően 24 órával. A térdextenzorok izometriás forgatónyomatéka az edzés után mindhárom ízületi szöghelyzetben csökkent. Az nyomatékdeficit annál nagyobb volt, minél kisebb ízületi szöghelyzetben mértük azt. Az edzés egyetlen ízületi szöghelyzetben sem okozott változást a térdextenzorok EMG aktivitásában izometriás kontrakció alatt. A biceps femoris izom ko-aktivitása azonban nőtt izometriás térdextenzió alatt valamennyi ízületi szöghelyzetben. A biceps femoris/quadriceps femoris EMG arány nőtt 70 és 50°-os szöghelyzetű izometriás kontrakció alatt. A quadriceps EMG/extenzios nyomaték arány az edzés hatására a 30 és 50°-os izometriás extenzioban nőtt, 70°-os ízületi szöghelyzetnél változatlan maradt. Kutatásunk alapján elmondhatjuk, hogy minél kisebb izomhossznál történik az izometrikus térdextenzió, annál nagyobb forgatónyomatékdeficit következik be mikrosérülést okozó edzést követően. Ugyanakkor az antagonista biceps femoris izom ko-aktiváció nagyobb izomhossznál nagyobb mértékben fokozódik.Az eredményekből arra következtetünk, hogy intenzív terhelést követően a tédízület stabilitása mind a térdextenzorok forgatónyomatékának, mind pedig az extenzor/flexor aktivitási arány tekintetében ízületi szöghelyzet-specifikusan módosul

The effects of eccentric hamstring exercise training in young female handball players

Purpose The multidimensional role of hamstring muscle group strength in athletic performance and injury prevention is well documented, and nordic hamstring exercise (NHE) is a popular method for the development of hamstring strength. Our aim was to examine the EMG characteristics of the eccentric NHE as well as the effects of long-term eccentric NHE training on muscle strength and vertical jump performance in 10- to 11-year-old female handball players. Methods Players from the same handball team were randomly assigned to an eccentric NHE training (13 players) or a control group (10 players). Both groups continued their regular handball training routine, but the NHE group performed additional eccentric NHE exercises once or twice a week, with progressively increasing volume, over 20 weeks. To test training effects, countermovement jump (CMJ) height, eccentric hamstring impulse, peak torque, and angle of peak torque were evaluated before, during and after the training period. In the pre-exercise test, EMG activity of the medial and lateral hamstring muscle was also assessed during NHE. Results Hamstring activities ranged between 98 and 129%. Lateral hamstring activity was greater than medial only in the right leg during NHE. Eccentric hamstring impulse improved in both legs at 10 weeks in both groups. Then, at 20 weeks, it remained unchanged in the NHE but decreased in controls. A similar adaptation was seen in eccentric hamstring torque, without change in the optimum knee angle. CMJ height improved only in the NHE. Conclusion It is concluded that NHE activates the hamstring musculature effectively, and a favourable mechanical adaptation to long-term NHE exercise in girls can be triggered as early as 11 years of age

No Difference in the Acute Effects of Randomization vs. Blocking of Units of Lower-Extremity Proprioceptive Training on Balance and Postural Control in Young Healthy Male Adults

Random practice is a form of differential learning and its favorable acute effects on motor performance are well described when visual tasks are practiced. However, no study to date has investigated the acute effects of differential learning using variable proprioceptive stimuli instead of the visual cues. The aim of the present study was to compare the acute effects of randomized versus blocked lower-extremity proprioceptive training stimuli on balance and postural adjustments. In two conditions, healthy young males (n = 15, age = 23 years) performed 16 one-legged landings on a board tilted in four directions: 1) tilt direction unknown and randomized and 2) tilt direction known with order of presentation blocked. Multi-segmental angular sway while balancing on an unstable surface and postural responses to perturbation stimulus by surface tilts were measured before and 4 min after training. Overall frontal-plane postural sway on the unstable surface decreased (p < 0.05, η2 = 0.022) in both conditions, while sagittal-plane postural sway remained unchanged. When the surface was toes-up tilted in the perturbation test, the sagittal-plane shank-thigh-pelvis alignment improved in both conditions (p < 0.05, η2 = 0.017), but the direction of the segmental positioning was non-uniform across participants. We conclude that randomization vs. blocking of units of lower-extremity proprioceptive training did not affect balance and postural control in our cohort of healthy young adults but the improvements were test-specific

Frontal Plane Neurokinematic Mechanisms Stabilizing the Knee and the Pelvis during Unilateral Countermovement Jump in Young Trained Males

(1) The unilateral countermovement jump is commonly used to examine frontal plane kinetics during unilateral loading and to identify athletes with an increased risk of lower limb injuries. In the present study, we examined the biomechanical mechanisms of knee and pelvis stabilization during unilateral vertical jumps. (2) Healthy males performed jumps on a force plate with the dominant leg. Activity of the dominant-side gluteus medius and the contralateral-side quadratus lumborum and erector spinae muscles was recorded with surface EMG. The EMG data were normalized to the EMG activity recorded during maximal voluntary isometric hip abduction and lateral trunk flexion contractions. During jumps, the propulsive impulse was measured, and the pelvis and thigh segmental orientation angles in the frontal plane were recorded and synchronized with the EMG data. (3) The magnitude of knee valgus during the jump did not correlate with hip abduction force, but negatively correlated with gluteus medius activity. This correlation became stronger when gluteus medius activity was normalized to hip abduction force. Propulsive impulse did not correlate with any neuromechanical measurement. (4) We conclude that hip abduction force itself does not regulate the magnitude of knee valgus during unilateral jumps; rather, the gluteus medius should be highly activated to increase frontal-plane knee joint stability

Frontal Plane Neurokinematic Mechanisms Stabilizing the Knee and the Pelvis during Unilateral Countermovement Jump in Young Trained Males

(1) The unilateral countermovement jump is commonly used to examine frontal plane kinetics during unilateral loading and to identify athletes with an increased risk of lower limb injuries. In the present study, we examined the biomechanical mechanisms of knee and pelvis stabilization during unilateral vertical jumps. (2) Healthy males performed jumps on a force plate with the dominant leg. Activity of the dominant-side gluteus medius and the contralateral-side quadratus lumborum and erector spinae muscles was recorded with surface EMG. The EMG data were normalized to the EMG activity recorded during maximal voluntary isometric hip abduction and lateral trunk flexion contractions. During jumps, the propulsive impulse was measured, and the pelvis and thigh segmental orientation angles in the frontal plane were recorded and synchronized with the EMG data. (3) The magnitude of knee valgus during the jump did not correlate with hip abduction force, but negatively correlated with gluteus medius activity. This correlation became stronger when gluteus medius activity was normalized to hip abduction force. Propulsive impulse did not correlate with any neuromechanical measurement. (4) We conclude that hip abduction force itself does not regulate the magnitude of knee valgus during unilateral jumps; rather, the gluteus medius should be highly activated to increase frontal-plane knee joint stability