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The effects of exercise training on mechanical properties and fibre type transitions have been investigated in rat soleus muscles. The exercise was a repetition of stretch-shortening cycles. A method of dual controlled releases was applied to obtain tension/extension curves, which characterize the elastic behaviour of the series elastic component (SEC), and the force/velocity relationship characterizing the contractile elements. Other contractile measurements included: contraction time (tc), half-relaxation time (t1/2) and twitch/tetanus ratio (Pt/Po). Additionally, the muscle fibre type composition was determined by a classical histochemical method. A 12-week period of training induced a significantly higher percentage of fast-twitch fibres and a lower percentage of slow-twitch fibres (P < 0.01). This fibre adaptation led to a significant (P < 0.01) decrease in tc and an increase in maximum shortening velocity (Vmax). An increase in compliance of the SEC was also observed. This elastic adaptation is interpreted in terms of modification of the active components of the SEC. All the histochemical and mechanical data presented in this study show that rat soleus muscles trained by stretch-shortening cycles acquired faster characteristics. Thus the results confirm that a slow-twitch to a fast-twitch fibre transition is obtainable in mature rats.

At rest, common peroneal nerve stimulation induced weak H responses in the peroneus longus (PL) and/or in the tibialis anterior (TA) muscles in half the subjects. The genuine H response of each muscle had to be proved since cross-talk could be suspected. PL and TA H responses were identified since (i) the reflex was often observed in only one muscle; (ii) H and M recruitment curves of each muscle were not systematically correlated; (iii) weak voluntary contractions induced opposite reflex changes in the antagonist PL and TA; (iv) direct stimulation of one of the muscles did not induce any cross-talk in the other provided that the EMG was of low voltage. It is concluded that H reflexes of close muscles can be studied independently.

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)

Rat soleus muscles were overloaded with intent to induce a relative increase in fast fibres and modifications in muscular stiffness. The overloading technique was a training period consisting of an 11-week vertical jump programme. The method of controlled releases was used to obtain tension/extension curves characterizing the elastic behaviour of the soleus. Fibre typing was made by myofibrillar adenosine 5'-triphosphatase staining. With regard to a control group, training resulted in a relative decrease in type I fibres for the benefit of type II fibres. Training also induced a decrease in muscle stiffness as attested notably by significant differences in maximal extension. These results are interpreted in terms of modifications occurring in the active fraction of the so-called series elastic component.

Tendon (T-) and Hoffmann (H-) responses in the soleus muscle were quantified either separately or in association to compare the mononeurons activated and to study their changes after a period of endurance training. In a first experiment T- and H-responses of the same amplitude were compared: the electrical stimulus (inducing the H-response) and the Achilles tendon tap (inducing the T-response) were associated so that the T-response firstly was concomitant with the H-response, and secondly shifted 10 ms forward or back compared to the H-response. From the study of these combined reflexes we would suggest that the same motoneurons are involved in T- or H-responses of the same amplitude. In a second experiment the maximal H-responses, the T-responses and maximal aerobic power (Waer,max) were measured on 20 subjects before and after a period of endurance training. For 75% of the subjects the Waer,max and the reflex parameters (T or H) varied in the same direction: most of them exhibited higher values of both Waer,max and reflex amplitudes while the others had Waer,max and reflex values hardly modified or decreased. The different effects of the training period could reflect the heterogeneity of the subject's status and involvement in sport. In most cases the T:Hmax ratios were also influenced, reflecting the fact that T- and H-responses were not identically affected by training. Thus it is suggested that an endurance training programme can influence not only the excitability of the motoneurons but also the response of the muscle receptors to stretch. An interpretation in terms of a change of spindle receptivity and/or a change in their recruitment due to a greater stiffness of the trained muscles is suggested.

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.

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.