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Purpose: The aim of this study was to evaluate concomitantly the changes in leg-spring behavior and the associated modifications in the lower limb muscular activity during a constant pace run to exhaustion at severe intensity. Methods: Twelve trained runners performed a running test at the velocity associated with VO2max (5.1±0.3ms-1; mean time to exhaustion: 353±69s). Running step spatiotemporal parameters and spring-mass stiffness were calculated from vertical and horizontal components of ground reaction force measured by a 6.60m long force platform system. The myoelectrical activity was measured by wireless surface electrodes on eight lower limb muscles Results: The leg stiffness decreased significantly (-8.9%; P<0.05) while the vertical stiffness did not change along the exhaustive exercise. Peak vertical force (-3.5%; P<0.001) and aerial time (-9.7%; P<0.001) decreased and contact time significantly increased (+4.6%; P<0.05). The myoelectrical activity decreased significantly for triceps surae but neither vastus medialis nor vastus lateralis presented significant change. Both rectus and biceps femoris increased in the early phase of swing (+14.7%; P<0.05) and during the pre-activation phase (+16.2%; P<0.05). Conclusion: The decrease in leg spring-stiffness associated with the decrease in peak vertical ground reaction force was consistent with the decline in plantarflexor activity. The biarticular rectus femoris and biceps femoris seem to play a major role in the mechanical and spatiotemporal adjustments of stride pattern with the occurrence of fatigue during such exhaustive run. [ABSTRACT FROM AUTHOR]

Abstract: Background: Although numerous studies revealed that isokinetic dynamometers were valuable tools for assessing spastic hypertonia, no standard methodology using such devices is currently widespread in clinical setting. The aim of this study was to standardize a protocol to assess spastic hypertonia in the triceps surae. Methods: The passive resistance during dorsiflexions imposed from 10 to 300°/s with an isokinetic dynamometer was measured at the neutral position in 15 patients with spastic hypertonia and 12 healthy subjects. The normalized passive resistance was obtained by expressing raw passive resistance as a percent of the values measured at the lowest velocity (10°/s). EMG signals from plantar and dorsiflexors were also recorded. Findings: While no significant difference between spastic patients and control subjects was observed in raw passive resistance values, the difference was significant for each tested velocity when considering the normalized values. Furthermore, the Ashworth score was significantly correlated with the normalized passive resistance for each velocity whereas no correlation was observed with the raw passive resistance. For the patients, except at the highest velocity, the normalized passive resistance was not affected by the fact that reflex responses in the triceps surae were elicited or not. Interpretation: The normalized passive resistance, expressed with respect to the initial one, i.e., measured at very low velocity, seems a very effective parameter to quantify the velocity-dependent increase in resistance to passive stretch in spastic plantarflexors. However, while the simplicity of the isokinetic tests and the reduced time of data treatment seems to support the clinical use of this methodology, further investigations are required to definitely standardize the protocol. [Copyright &y& Elsevier]

Abstract: Objective: The purpose of this study was first to compare the kinematic parameters of imposed ankle mobilizations measured during Ashworth or isokinetic tests and, second, to better understand why the stretch reflex was more or less easily elicited by one method or the other. Methods: Passive dorsiflexions were applied on eight adult patients with plantarflexor spasticity in two conditions: (i) manually, using the Ashworth test where passive dorsiflexions were performed freely by seven rehabilitation clinicians, and (ii) instrumentally, using an isokinetic device (Cybex® Norm™) and a dorsiflexion velocity at 300°/s. Mean values of initial ankle position, maximal angular velocity (θ′max), maximal angular acceleration (θ″max) and plantarflexor reflex responses obtained with each method were compared. Results: During the Ashworth test, all the patients presented reflex activities in the triceps surae while, during the isokinetic mobilization, only three out of the eight patients tested shown reflex responses. θ′max values were significantly higher (P&#60;0.05) in the manual test (308±80°/s vs 216±5.5°/s for the isokinetic test). The most marked difference concerned the θ″max values (5046±2181°/s2 for the Ashworth test vs 819±18°/s2 for the isokinetic test, P&#60;0.001). This parameter was significantly correlated with the mean rms-EMG values of the gastrocnemius lateralis (GL) and the soleus (SOL). Conclusions: This study indicates that passive dorsiflexions imposed during Ashworth and isokinetic tests largely differ in velocity and acceleration, and the higher dynamic parameters evaluated during the Ashworth test could mainly explain that the stretch reflex was more easily elicited during this manual testing. Significance: If isokinetic devices offer numerous advantages in the assessment of passive resistance to spastic muscle stretch, they cannot be used to simulate the manual test. [Copyright &y& Elsevier]

Overground sprint studies have shown the importance of net horizontal ground reaction force impulse ( IMP H ) for acceleration performance, but only investigated one or two steps over the acceleration phase, and not in elite sprinters. The main aim of this study was to distinguish between propulsive ( IMP H +) and braking ( IMP H − ) components of the IMP H and seek whether, for an expected higher IMP H , faster elite sprinters produce greater IMP H +, smaller IMP H − , or both. Nine high-level sprinters (100-m best times range: 9.95–10.60 s) performed 7 sprints (2×10 m, 2×15 m, 20 m, 30 m and 40 m) during which ground reaction force was measured by a 6.60 m force platform system. By placing the starting-blocks further from the force plates at each trial, and pooling the data, we could assess the mechanics of an entire “virtual” 40-m acceleration. IMP H and IMP H + were significantly correlated with 40-m mean speed ( r =0.868 and 0.802, respectively; P <0.01), whereas vertical impulse and IMP H − were not. Multiple regression analyses confirmed the significantly higher importance of IMP H + for sprint acceleration performance. Similar results were obtained when considering these mechanical data averaged over the first half of the sprint, but not over the second half. In conclusion, faster sprinters were those who produced the highest amounts of horizontal net impulse per unit body mass, and those who “pushed more” (higher IMP H +), but not necessarily those who also “braked less” (lower IMP H − ) in the horizontal direction. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the effect of loading on lower limb muscle coordination involved during ballistic squat jumps. Twenty athletes performed ballistic squat jumps on a force platform. Vertical force, velocity, power and electromyographic (EMG) activity of lower limb muscles were recorded during the push-off phase and compared between seven loading conditions (0–60% of the concentric-only maximal repetition). The increase in external load increased vertical force (from 1962 N to 2559 N; P = 0.0001), while movement velocity decreased (from 2.5 to 1.6 m s −1 ; P = 0.0001). EMG activity of tibialis anterior first peaked at 5% of the push-off phase, followed by gluteus maximus (35%), vastus lateralis and soleus (45%), rectus femoris (55%), gastrocnemius lateralis (65%) and semitendinosus (75%). This sequence of activation ( P = 0.67) and the amplitude of muscle activity ( P = 0.41) of each muscle were not affected by loading condition. However, a main effect of muscle was observed on these parameters (peak value: P < 0.001; peak occurrence: P = 0.02) illustrating the specific role of each muscle during the push-off phase. Our findings suggest that muscle coordination is not influenced by external load during a ballistic squat jump. [ABSTRACT FROM AUTHOR]

Abstract: Purpose: This study investigated neuromuscular fatigue following high versus low-intensity eccentric exercise corresponding to the same amount of work. Methods: Ten volunteers performed two eccentric exercises of the elbow flexors: a high-intensity versus a low-intensity exercise. Maximal voluntary contraction torque and surface electromyography of the biceps brachii muscle were recorded before, immediately and 48h after exercises. Maximal voluntary activation level, neural (M-wave) and contractile (muscular twitch) properties of the biceps brachii muscle were analysed using electrical stimulation techniques. Results: Maximal voluntary contraction torque was significantly (P <0.01) reduced immediately and 48h after exercise but the reduction was not different between the two conditions. Electromyography associated with maximal voluntary contraction significantly decreased (P <0.05) immediately and 48h after exercise for both conditions while maximal voluntary activation level was only significantly reduced immediately after the high-intensity exercise. Peak twitch alterations were observed immediately and 48h after exercise for both conditions while M-wave did not change. Conclusion: High and low-intensity eccentric exercises with the same amount of work induced the same reduction in maximal strength capacities of the biceps brachii muscles. The magnitude of peripheral and central fatigue was very similar in both conditions. [Copyright &y& Elsevier]