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The possible peripheral and/or central origin in the mechanisms responsible for day-time fluctuation in maximal torque of the triceps surae muscle were investigated with a special emphasis on antagonist muscle coactivation. Eleven healthy male subjects (physical education students) took part in this investigation. The electromechanical properties of the plantar flexor muscles were recorded at two different times of day: between 06:00 h and 08:00 h in the morning and between 17:00 h and 19:00 h in the evening. To investigate peripheral mechanisms, the posterior tibial nerve was stimulated at rest, using percutaneous electrical stimuli, to evoke single twitch, double twitch, and maximal tetanic contraction (100 Hz). Maximal voluntary contraction of the plantar flexors was also assessed by means of the relative electromyographic activity of respective agonist and antagonist muscles (soleus, gastrocnemius medialis, gastrocnemius lateralis, and tibialis anterior). A double twitch was delivered during maximal voluntary plantar flexion to record muscle activation (i.e., interpolated twitch technique). The coactivation level of the tibialis anterior muscle during plantar flexion was calculated. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae in the evening as compared with the morning (-7.0 %; p < 0.05). Concerning the central command, when extrapolated by the twitch interpolation technique, the decrease in mean activation level of -6.8 % was consistent with the fluctuation in torque (-7.0 %). Soleus muscle electromyographic activity (normalized to the M-wave) showed a significant decline (21.6 %; p < 0.001). Moreover, individual changes in MVC percentage were significantly related to those of normalized electromyographic activity of the soleus muscle (r = 0.688; p < 0.01). Thus, it indicated that the subject's capacity to activate the soleus muscle was affected by the time of day. The coactivation level in the tibialis anterior muscle during plantar flexion did not change significantly in the evening. Concerning peripheral mechanisms, we observed a decrease in maximal M-wave amplitude for soleus and gastrocnemii, associated with unchanged single twitch and tetanus torque. To conclude, impairment in soleus muscle central command seemed to be the mechanism in the origin of torque failure. Such information would be of importance in the investigation of day-time fluctuations in complex motor task performances implicating the triceps surae muscle.

A mechanical device for studying changes in mechanical properties of human muscle as a result of spaceflight is presented. Its main capacities are to allow during a given experiment investigation of both contractile and visco-elastic properties of a musculo-articular complex using respectively isometric contractions, isokinetic movements, quick-release tests and sinusoidal perturbations. This device is a motor driven ergometer associated to an experimental protocol designed for pre- and post-flight experiments. As microgravity preferentially affects postural muscles, the apparatus was designed to test muscle groups crossing the ankle joint. Three subjects were tested during the Euromir '94 mission. Preliminary results obtained on the european astronaut are briefly reported. During the next two years the experiments will be performed during six missions.

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NMEind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (PtMmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value -10.7 +/- 5.5%, p < 0.05), whereas PtMmax was not significantly modified. NMEind (PtMmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 +/- 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (PtTmax), and the corresponding NME (NMEreflex = PtTmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 +/- 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NMEMvc was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 +/- 13.9%, p

The disynaptic Ia-alpha inhibition between the tibialis anterior and the soleus muscles is now well known. This neuronal organization has been established thanks to the analysis of the Hoffmann reflex (H reflex) changes following an electrical stimulation of the antagonist muscle nerve. In some cases, anatomical constraints impede the use of this classical technique for assessing a reciprocal Ia inhibition between muscles. Furthermore, an electrical stimulus solicits the primary spindle afferents in conditions which are very different from their natural stimulus: the muscle stretch. Thus we have undertaken to analyse the changes of a soleus H reflex following a rapid stretch of the ankle dorsiflexors and therefore of their prime-mover: the tibialis anterior. The mechanical perturbations were imposed with a strong initial acceleration and a limited angular displacement thanks to an original device including electromagnet and spring. This technique, derivated from the quick release technique, was applied on relaxed muscles to avoid a co-contraction phenomenon. 45 to 50 ms after the initiation of the mechanical perturbation an early reflex response was observed on the stretched tibialis anterior. The area of this first reflex response was related to the initial acceleration of the passive dorsiflexion. Several arguments are presented in favour of the myotatic origin of this reflex component. The passive dorsiflexion used as a conditioning stimulus led to a strong and longlasting inhibition of the test soleus H reflex. The early inhibition of the soleus H reflex was observed only about 18 ms after the conditioning stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

With reference to the concept that contraction of a muscle can be inhibited by contraction of its antagonists, the excitability of a bifunctional muscle, the soleus (plantarflexor and adductor) was studied during voluntary isometric contractions of the abductors in man. For this purpose, the soleus monosynaptic stretch reflex (MSR) was solicited by means of quick-release dorsiflexion movements which also induce inactivation of the tibialis anterior (unloading reflex). When different initial torques of dorsiflexion were developed (reference conditions) the amplitude of the soleus MSR was found to be related linearly to the maximal value of angular acceleration. When an abduction torque was superimposed (combined efforts), the soleus response was always depressed, but no significant reinforcement of this inhibition was observed when the abduction torque increased. From present results, and from classical data concerning reciprocal inhibition between soleus and tibialis anterior, it is concluded that the MSR of a bifunctional muscle is depressed when the antagonists of either one of its functions are voluntarily activated.

The activation of antagonistic muscles is studied during isometric combined contractions of bifunctional muscles. It is established that the co-contraction mechanism observed in such situations limits unwanted movements. Furthermore, when a co-contraction appears between two biantagonistic muscles, the phenomenon underlies a conflicting situation and, consequently, the muscular patterns encountered are in most cases the reverse of those classically described. In these circumstances the inhibition usually present between antagonistic muscles could be masked, due to convergence on the motoneurones of both inhibitory and excitatory discharges coming from homonymous and antagonistic muscles of a given bifunctional muscle.

The electrical activity of the biceps brachii and pronator teres muscles is studied through the prono-supination of the forearm in some anisometrical conditions (dynamic work) when the inertia of the mobile system and the elbow position are being varied. The subjects are required to perform pronation, supination and flexion movements, either isolated or combined. From the findings obtained when the integrated electrical activity (Q) is related to the mechanical work (W), one can conclude that a. the Q-W linear relationship seems to characterize the chief function of a muscle, b. the slope of the Q-W relationship depends on the elbow position, c. the pronator muscles do not inhibit in a selective manner the biceps supinating function. So a bifunctional muscle seems to act as a whole.

The electrical activity of the biceps brachii and pronator teres muscles is studied through the prono-supination of the forearm in some isometrical conditions (static work) with different loads and joint positions. If the pronator teres is always being active in pronation, this activity is a function of the load and of the wrist and elbow positions. The same phenomena can be observed for the biceps brachii but when in supination. From the curvilinear relationships between the integrated electrical activity and the load--observed on both muscles--some torque-angle relationships can be established for the biceps brachii which show that a bifunctional muscle seems to be characterized by a very and unique force-length relationship.