Influence of exercise intensity and joint angle on endurance time prediction of sustained submaximal isometric knee extensions

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Neuromuscular adaptations to 8-week strength training: isotonic versus isokinetic mode

International audiencePrevious studies attempted to compare the effectiveness of isokinetic and isotonic training. However, they have provided conflicting results. The purpose of this study was to compare the effects of isotonic versus isokinetic standardized concentric strength training programs of the knee extensors on the neuromuscular system. The standardization of these two training programs was ensured by the equalization of the total external amount of work performed and the mean angular movement velocity. Thirty healthy male students were randomly assigned to the isotonic (IT; n = 11), the isokinetic (IK; n = 11) or the control (C; n = 8) group. Both IT and IK groups trained their dominant lower leg 3 sessions/week for 8 weeks on a dynamometer. The IT group exercised using a preset torque of 40% of the maximal voluntary isometric torque at 70 degrees (0 degrees = leg in horizontal position). The IK group exercised at a velocity ranging between 150 degrees and 180 degrees s(-1). Isotonic, isokinetic and isometric tests were performed on a dynamometer before and after strength training. Surface electromyographic activity of vastus lateralis, vastus medialis, rectus femoris, semitendinosus and biceps femoris muscles was recorded during the tests. Significant strength increases in both dynamic and static conditions were noticed for IT and IK groups without any significant difference between the two trained groups. Agonist muscle activity also increased with training but no change in antagonist muscle co-activity was observed. The two training methods could be proposed by clinicians and athletic coaches to improve concentric muscle strength in dynamic and static conditions

Adaptations neuromusculaires et musculo-tendineuses à l'exercice excentrique isotonique et isocinétique

International audienceTo present the properties of an eccentric contraction and compare neuromuscular and muscle-tendon system adaptations induced by isotonic and isokinetic eccentric trainings. Synthesis. – An eccentric muscle contraction is characterized by the production of muscle force associated to a lengthening of the muscle-tendon system. This muscle solicitation can cause micro lesions followed by a regeneration process of the muscle-tendon system. Eccentric exercise is commonly used in functional rehabilitation for its positive effect on collagen synthesis but also for resistance training to increase muscle strength and muscle mass in athletes. Indeed, eccentric training stimulates muscle hypertrophy, increases the fascicle pennation angle, fascicles length and neural activation, thus inducing greater strength gains than concentric or isometric training programs. Eccentric exercise is commonly performed either against a constant external load (isotonic) or at constant velocity (isokinetic), inducing different mechanical constraints. These different mechanical constraints could induce structural and neural adaptive strategies specific to each type of exercise. Conclusion. – The literature tends to show that isotonic mode leads to a greater strength gain than isokinetic mode. This observation could be explained by a greater neuromuscular activation after IT training. However, the specific muscle adaptations induced by each mode remain difficult to determine due to the lack of standardized, comparative studies. # Résumé Objectif. – Présenter les propriétés de la contraction excentrique et comparer les adaptations neuromusculaires et musculo-tendineuses induites par les modes isotonique et isocinétique. Synthèse. – Une contraction musculaire excentrique se caractérise par la production d'une force musculaire associée a ` un allongement du complexe muscle-tendon. Cette sollicitation musculaire peut provoquer des microlésions auxquelles succèdent des processus de régénération des structures musculaires et tendineuses. L'exercice excentrique est donc utilisé en rééducation fonctionnelle pour son effet positif sur la synthèse du collagène et dans le domaine de l'entraıˆnemententraıˆnement pour développer la force et la masse musculaire. L'entraıˆnemententraıˆnement excentrique stimule en effet l'hypertrophie musculaire, augmente l'angle de pennation, la longueur des fascicules musculaires et l'activation nerveuse, engendrant ainsi des gains de force supérieurs aux entraıˆnementsentraıˆnements concentriques ou isométriques. L'exercice excentrique peut e ˆtre réalisé contre une charge externe constante (isotonique) ou a ` vitesse constante (isocinétique). Ces contraintes mécaniques différentes pourraient induire des adaptations structurales et nerveuses spécifiques a ` chaque mode d'exercice.Pre´senter les proprie´te´s de la contraction excentrique et comparer les adaptations neuromusculaires et musculo-tendineuses induitespar les modes isotonique et isocine´tique.Synthe`se. – Une contraction musculaire excentrique se caracte´rise par la production d’une force musculaire associe´e a` un allongement ducomplexe muscle-tendon. Cette sollicitation musculaire peut provoquer des microle´sions auxquelles succe`dent des processus de re´ge´ne´ration desstructures musculaires et tendineuses. L’exercice excentrique est donc utilise´ en re´e´ducation fonctionnelle pour son effet positif sur la synthe`se ducollage`ne et dans le domaine de l’entraıˆnement pour de´velopper la force et la masse musculaire. L’entraıˆnement excentrique stimule en effetl’hypertrophie musculaire, augmente l’angle de pennation, la longueur des fascicules musculaires et l’activation nerveuse, engendrant ainsi desgains de force supe´rieurs aux entraıˆnements concentriques ou isome´triques. L’exercice excentrique peut eˆtre re´alise´ contre une charge externeconstante (isotonique) ou a` vitesse constante (isocine´tique). Ces contraintes me´caniques diffe´rentes pourraient induire des adaptations structuraleset nerveuses spe´cifiques a` chaque mode d’exercice

Keskustelunanalyyttinen tutkimus opettajan kannustavista palautevuoroista kuvataideopetuksen kontekstissa

International audienceThe present study aimed to standardize isotonic (IT) and isokinetic (IK) eccentric exercises by equalizing the amount of work and the angular velocity at two intensity levels, to be able to compare specific effects of these exercise modes on the neuromuscular system. Fourteen subjects participated in three test sessions consisting of two IT and two IK sets on a customized isokinetic dynamometer. IT sets were comprised of 8 eccentric contractions of the knee extensors at 120% of the maximal repetition (1RM) in the first two sessions, and 100% in the third session. IK sets were performed at the same mean angular IT velocity and stopped when the amount of work performed corresponded to the IT set. External work, angular velocity and Root Mean Square (RMS) of electromyographic activity of three superficial muscles of the quadriceps femoris were calculated. Results showed concordance of work and angular velocity for each test session. Both modes involved the same number of repetitions at 120% and fewer repetitions in IK mode at 100% of 1RM. Work and RMS values remained steady in all sets. This study allowed the standardization of isotonic and isokinetic eccentric exercises, a first step before determining their specific effects on neuromuscular function

Impact of ankle muscle fatigue and recovery on the anticipatory postural adjustments to externally initiated perturbations in dynamic postural control

International audienceThe aim of this study was to determine whether and how young participants modulate their postural response to compensate for postural muscle fatigue during predictable but externally initiated continuous and oscillatory perturbations. Twelve participants performed ten postural trials before and after an ankle muscle fatigue protocol. Each postural trial was 1 min long and consisted of continuous backward and forward oscillations of the platform. Fatigue was induced by intermittent, bilateral isometric contractions of the ankle plantar- and dorsiflexors until the force production was reduced to 50 % of the pre-fatigue maximal voluntary contraction. Changes in the center of mass (COM) displacement, center of pressure (COP) displacement, and anterior-posterior location of the COP within the base of support were quantified as well as the activity of the tibialis anterior (TA), medial gastrocnemius (MG), quadriceps, and hamstring. All participants demonstrated postural stability post-fatigue by maintaining the displacement of their COM. Everyone also demonstrated a general forward shift in the anterior-posterior location of the COP within the base of support; however, two distinct postural modifications, corresponding to either an immediate fatigue-induced increase or decrease in the COP displacement during the backward platform translation, were recorded immediately post-fatigue. The changes in muscle onset latencies lasted beyond the recovery of the force production of the fatigued postural muscles. By 10 min post-fatigue, the participants showed a decrease in the COP displacement as well as an earlier activation of the postural muscles and an increased TA/MG co-activation relative to pre-fatigue. Although different strategies were used, the participants were able to adjust to and overcome postural muscle fatigue and remain balanced during the postural perturbations regardless of the direction of the platform movement. These adjustments lasted beyond the recovery of the ankle muscle force production indicating that they may be part of a centrally mediated protective response as opposed to a peripherally induced limitation to performance

A Methodologic Approach for Normalizing Angular Work and Velocity During Isotonic and Isokinetic Eccentric Training

International audienceContext: Resistance exercise training commonly is performed against a constant external load (isotonic) or at a constant velocity (isokinetic). Researchers comparing the effectiveness of isotonic and isokinetic resistance-training protocols need to equalize the mechanical stimulus (work and velocity) applied. Objective: To examine whether the standardization protocol could be adjusted and applied to an eccentric training program. Design: Controlled laboratory study. Setting: Controlled research laboratory. Patients or Other Participants: Twenty-one sport science male students (age = 20.6 ± 1.5 years, height = 178.0 ± 4.0 cm, mass = 74.5 ± 9.1 kg). Intervention(s): Participants performed 9 weeks of isotonic (n = 11) or isokinetic (n = 10) eccentric training of knee extensors that was designed so they would perform the same amount of angular work at the same mean angular velocity. Main Outcome Measure(s): Angular work and angular velocity. Results: The isotonic and isokinetic groups performed the same total amount of work (–185.2 ± 6.5 kJ and –184.4 ± 8.6 kJ, respectively) at the same angular velocity (21 ± 1°/s and 22°/s, respectively) with the same number of repetitions (8.0 and 8.0, respectively). Bland-Altman analysis showed that work (bias = 2.4%) and angular velocity (bias = 0.2%) were equalized over 9 weeks between the modes of training. Conclusions: The procedure developed allows angular work and velocity to be standardized over 9 weeks of isotonic and isokinetic eccentric training of the knee extensors. This method could be useful in future studies in which researchers compare neuromuscular adaptations induced by each type of training mode with respect to rehabilitating patients after mus-culoskeletal injury. Key Points • Equalizing mechanical stimuli through a method previously developed on concentric muscle loading can be applied successfully to the study of long-term isotonic and isokinetic eccentric resistance training. • This standardization procedure is valid and adaptable to progressive resistance exercise commonly used in strength training and rehabilitation

Muscle architecture and EMG activity changes during isotonic and isokinetic eccentric exercises

International audienceThe present study aimed to compare musclearchitecture and electromyographic activity during isotonic(IT) and isokinetic (IK) knee extensors eccentric contractions.Seventeen subjects were assigned in test and reproducibilitygroups. During test session, subjects performedtwo IT and two IK sets of eccentric contractions of kneeextensor muscles. Torque, angular velocity, VL architectureand EMG activity of agonist (vastus lateralis, VL;vastus medialis; rectus femoris) and antagonist (semitendinosus;biceps femoris, BF) muscles were simultaneouslyrecorded and averaged on a 5 window. Torque–angle andangular velocity–angle relationships exhibited differencesin mechanical load between the IT and IK modes. Changesin muscle architecture were similar in both modes, sinceVL fascicles length increased and fascicle angle decreased,resulting in a decrease in muscle thickness during eccentriccontraction. Agonist activity and BF co-activity levels werehigher in IT mode than in IK mode at short muscle lengths,whereas agonist activity was higher in IK mode than in ITmode at long muscle lengths. Differences in mechanicalload between both modes induced specific neuromuscularresponses in terms of agonist activity and antagonist coactivity.These results suggest that specific neural adaptationsmay occur after IT or IK eccentric training. Thishypothesis needs to be tested in order to gain new insightsconcerning the most effective eccentric protocols based onwhether the objective is sportive or clinical

Impact of forearm fatigue on the postural response to an externally initiated, predictable perturbation

International audiencePurpose: The objective was to examine the impact of non-postural muscle fatigue on anticipatory postural control, during postural perturbations induced by platform translations. The experimental setup investigated the central changes caused by fatigue without the potential confounding influence of peripheral fatigue within the postural muscles.Methods: Fatigue induced in forearm muscles by a maximal handgrip contraction has been previously shown to influence forearm force production for 10 min, reduce ankle plantarflexion force for 1 min and create measureable central fatigue for 30 s. The peak-to-peak anterior/posterior displacement of the center of mass and center of pressure (COP) and muscle activity were measured during the postural perturbation tasks performed before the fatigue protocol and for 10 min post-fatigue.Results: The fatigue protocol decreased the peak-to-peak COP displacement from 128.0 ± 12.3 mm pre-fatigue to 81.9 ± 7.8 mm post-fatigue during the forwards platform translation (p < 0.05) and from 133.8 ± 12.0 to 89.2 ± 7.9 mm during the backwards translation (p < 0.05). The fatigue protocol also caused the tibialis anterior (TA pre-fatigue = -0.25 ± 0.04 s, TA post-fatigue = -0.41 ± 0.02 s, p = 0.001) and medial gastrocnemius muscles (MG pre-fatigue = -0.39 ± 0.03 s, MG post-fatigue = -0.48 ± 0.02 s, p = 0.028) to be recruited significantly earlier relative to the pre-fatigue condition.Conclusion: This experimental setup ensured that peripheral fatigue did not develop in the postural muscles; therefore, a general fatigued-induced modification of the postural strategy is proposed as the origin of the postural changes and delayed recovery

Specific neuromuscular fatigue induced by repetitive isoload concentric knee extension

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No evidence of expertise-related changes in muscle synergies during rowing

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