Quantifying Paddling Kinematics through Muscle Activation and Whole Body Coordination during Maximal Sprints of Different Durations on a Kayak Ergometer: A Pilot Study.

Paddling technique and stroke kinematics are important performance factors in flatwater sprint kayaking and entail significant energetic demands and a high strength from the muscles of the trunk and upper limbs. The various distances completed (from 200 m to 1000 m) require the athletes to optimize their pacing strategy, to maximize power output distribution throughout the race. This study aimed to characterize paddling technique and stroke kinematics during two maximal sprints of different duration. Nine nationally-trained participants (2 females, age: 18 ± 3 years; BMI: 22.2 ± 2.0 Kg m-1) performed 40 s and 4 min sprints at maximal intensity on a kayak ergometer. The main findings demonstrated a significantly greater mean stroke power (237 ± 80 W vs. 170 ± 48 W; p < 0.013) and rate (131 ± 8 spm vs. 109 ± 7 spm; p < 0.001) during the 40 s sprint compared to the 4 min sprint. Athletes used an all-out strategy for the 40 s exercise and a parabolic-shape strategy during the 4 min exercise. Despite the different strategies implemented and the higher muscular activation during the 40 s sprint, no change in paddling technique and body coordination occurred during the sprints. The findings of the present study suggest that the athletes constructed a well-defined profile that was not affected by fatigue, despite a decrease in power output during the all-out strategy. In addition, they regulated their paddling kinematics during the longer exercises, with no change in paddling technique and body coordination.

Motor resonance mechanisms are preserved in Alzheimer's disease patients.

This study aimed to better characterize the sensorimotor mechanisms underlying motor resonance, namely the relationship between motion perception and movement production in patients suffering from Alzheimer's disease (AD). This work first gives a kinematic description of AD patients' upper limb movements, then it presents a simple paradigm in which a dot with different velocities is moved in front of the participant who is instructed to point to its final position when it stopped. AD patients' actions, as well as healthy elderly participants, were similarly influenced by the dot velocity, suggesting that motor resonance mechanisms are not prevented by pathology. In contrast, only patients had anticipatory motor response: i.e. they started moving before the end of the stimulus motion, unlike what was requested by the experimenter. While the automatic imitation of the stimulus suggests an intact ability to match the internal motor representations with that of the visual model, the uncontrolled motion initiation would indicate AD patients' deficiency to voluntarily inhibit response production. These findings might open new clinical perspectives suggesting innovative techniques in training programs for people with dementia. In particular, the preservation of the motor resonance mechanisms, not dependent on conscious awareness, constitutes an intact basis upon which clinicians could model both physical and cognitive interventions for healthy elderly and AD patients. Furthermore, the evaluation of the inhibitory functions, less sensitive to the level of education than other methods, might be useful for screening test combined with the traditional AD techniques. However, further investigations to understand if this feature is specific to AD or is present also in other neurodegenerative diseases are needed.

Pointing to double-step visual stimuli from a standing position: motor corrections when the speed-accuracy trade-off is unexpectedly modified in-flight. A breakdown of the perception-action coupling.

The time required to complete a fast and accurate movement is a function of its amplitude and the target size. This phenomenon refers to the well known speed-accuracy trade-off. Some interpretations have suggested that the speed-accuracy trade-off is already integrated into the movement planning phase. More specifically, pointing movements may be planned to minimize the variance of the final hand position. However, goal-directed movements can be altered at any time, if for instance, the target location is changed during execution. Thus, one possible limitation of these interpretations may be that they underestimate feedback processes. To further investigate this hypothesis we designed an experiment in which the speed-accuracy trade-off was unexpectedly varied at the hand movement onset by modifying separately the target distance or size, or by modifying both of them simultaneously. These pointing movements were executed from an upright standing position. Our main results showed that the movement time increased when there was a change to the size or location of the target. In addition, the terminal variability of finger position did not change. In other words, it showed that the movement velocity is modulated according to the target size and distance during motor programming or during the final approach, independently of the final variability of the hand position. It suggests that when the speed-accuracy trade-off is unexpectedly modified, terminal feedbacks based on intermediate representations of the endpoint velocity are used to monitor and control the hand displacement. There is clearly no obvious perception-action coupling in this case but rather intermediate processing that may be involved.

How the ankle joint angle alters the antagonist and agonist torques during maximal efforts in dorsi- and plantar flexion.

The aim of this study was to assess, via an EMG bio-feedback method, the ankle joint angle effect on the agonist and antagonist torques in plantar- (PF) and dorsi-flexion (DF). The isometric PF and DF maximal voluntary contractions (MVCs) torques were measured simultaneously with surface EMG activity of triceps surae (TS) and tibialis anterior (TA) muscles in 12 young adults (mean age 27) at five different ankle joint angles. Our results showed that: (i) The coactivation level does not properly reflect the mechanical effect of the antagonist muscle, (ii) TS antagonist torque significantly altered the DF MVC-angle relationship, whereas TA antagonist torque did not influence this MVC-angle relationship in PF. The alteration of the MVC with angular position was due, in part, to the coactivation phenomenon in DF, but not in PF. Thenceforth, when investigating the torque at the ankle joint, it is necessary to take into account both agonist and antagonist torque modifications with ankle joint angle.

Pointing to double-step visual stimuli from a standing position: very short latency (express) corrections are observed in upper and lower limbs and may not require cortical involvement.

How fast can we correct a planned movement following an unexpected target jump? Subjects, starting in an upright standing position, were required to point to a target that randomly and unexpectedly jumps forward to a constant spatial location. Rapid motor corrections in the upper and lower limbs, with latency responses of less than 100 ms, were revealed by contrasting electromyographic activities in perturbed and unperturbed trials. The earliest responses were observed primarily in the anterior section of the deltoïdus anterior (shoulder) and the tibialis anterior (leg) muscles. Our findings indicate that visual on-going movement corrections may be accomplished via fast loops at the level of the upper and lower limbs and may not require cortical involvement.

The influence of ageing on the force-velocity-power characteristics of human elbow flexor muscles.

The purpose of this study was to quantify the effects of ageing on the maximal power (P(max)) of the elbow flexor muscles and to determine the impact of velocity on the loss of power in older people. Sixteen elderly subjects (7 men and 9 women, age range 61-78 years) and 17 young subjects (11 men and 6 women, age range 18-27 years) participated in this study. Maximal elbow flexions were performed against increasing inertia. The maximal force (F(max)), maximal shortening velocity (V(max)), P(max), dynamic constants (a, b and a/F(max)), optimal force (F(opt)), optimal velocity (V(opt)) and V(opt)/V(max) were determined from Hill's equation. Myoelectrical activity (EMG) of the biceps and triceps muscles was quantified as an root mean square (RMS) value. F(max), V(max), P(max), F(opt), and V(opt) were significantly lower in elderly than in young subjects (28, 31, 45, 24 and 28% lower, respectively; p<0.05), whereas a/F(max) and V(opt)/V(max) were not different between the two age groups. In women, the greater decrease in P(max) appears to be more dependent on V(opt) than F(opt). In addition, V(max) decreased with age in women but not in men. The absence of significant differences between age groups in normalised RMS values indicates that P(max) and V(max) loss with increasing age could result more from changes in the properties of contractile element than from changes in muscular activity.

Mechanisms contributing to knee extensor strength loss after prolonged running exercise.

The aim of this study was to identify the mechanisms that contribute to the decline in knee extensor (KE) muscles strength after a prolonged running exercise. During the 2 days preceding a 30-km running race [duration 188.7 +/- 27.0 (SD) min] and immediately after the race, maximal percutaneous electrical stimulations (single twitch, 0.5-s tetanus at 20 and 80 Hz) were applied to the femoral nerve of 12 trained runners. Superimposed twitches were also delivered during isometric maximal voluntary contraction (MVC) to determine the level of voluntary activation (%VA). The vastus lateralis electromyogram was recorded. KE MVC decreased from pre- to postexercise (from 188.1 +/- 25.2 to 142.7 +/- 29.7 N x m; P < 0.001) as did %VA (from 98.8 +/- 1.8 to 91.3 +/- 10.7%; P < 0.05). The changes from pre- to postexercise in these two variables were highly correlated (R = 0.88; P < 0.001). The modifications in the mechanical response after the 80-Hz stimulation and M-wave peak-to-peak amplitude were also significant (P < 0.001 and P < 0.05, respectively). It can be concluded that 1) central fatigue, neuromuscular propagation, and muscular factors are involved in the 23.5 +/- 14.9% reduction in MVC after a prolonged running bout at racing pace and 2) runners with the greatest KE strength loss experience large activation deficit.

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.

Short-term changes in the series elastic component after an acute eccentric exercise of the elbow flexors.

We have studied the effect of a unique eccentric exercise session on the series elastic component (SEC) properties of human elbow flexors. Ten active females performed five sets of ten maximal eccentric contractions on an isokinetic ergometer. Maximal isometric (MVC) torque, and the corresponding myoelectrical activity (RMS) of the biceps brachii and the triceps brachii muscles were recorded before, immediately after, 48 h after, and 1 week after the exercise session. SEC compliance was also measured, using the in situ quick-release technique. Maximal eccentric torque declined significantly among the five sets of the exercise session (P < 0.01). The exercise session induced a significant decrease of the MVC over the post-exercise 48-h period. MVC returned to its control value 1 week after the exercise session. The neuromuscular efficiency calculated from the MVC torque:RMS ratio did not change throughout the experimental period, suggesting an optimisation of the neural drive to muscle force production capacity. The compliance values increased when force values decreased, but the slope of the relationship between both compliance and force logarithmic values was not significantly modified by the experimental procedure, indicating that the intrinsic properties of the SEC were not affected by the eccentric exercise session. The contractility-elasticity coupling was also maintained at a constant level during the recovery period. In other words, after an acute eccentric exercise, the evolution of SEC compliance was closely linked to force changes.

[Effects of an eccentric exercise session short-term recovery of muscle contractility]. / Effets d'une séance d'exercices excentriques sur la récupération à court terme de la contractilité musculaire.

Short term effects of 5 sets of 10 maximal eccentric contractions of the elbow flexors, performed using an isokinetic ergometer, were studied. Maximal eccentric, isometric, concentric torque, myoelectrical activity of biceps and triceps brachii, voluntary activation, M-wave amplitude, as well as twitch and maximal contraction and relaxation velocities were measured before (Control), 2 minutes after (Post), 24 hours (Post24 h) and 48 hours (Post48 h) after the exercise session. Torque significantly decreased over the recovery period, whatever the contraction type, excepted concentric torque assessed at 240 degrees.s-1 which recovered its Control value at Post48 h. Activation level significantly decreased at Post (p < 0.05) and returned to its Control value at Post24 h. Twitch, as well as maximal contraction and relaxation velocities had significantly declined among the experimental procedure (p < 0.01). M-wave amplitude was not modified after the exercise. These results indicate that, over a 48 hour rest period, torque decrement following a maximal eccentric exercise session should mainly be due to a failure of the peripheral part of the neuromuscular system, and force recovery should closely be linked to the developed force value.