Limiting mechanisms of force production after repetitive dynamic contractions in human triceps surae.

The influence of repetitive dynamic fatiguing contractions on the neuromuscular characteristics of the human triceps surae was investigated in 10 subjects. The load was 50% of the torque produced during a maximal voluntary contraction, and the exercise ended when the ankle range of motion declined to 50% of control. The maximal torque of the triceps surae and the electromyographic (EMG) activities of the soleus and medial gastrocnemius were studied in response to voluntary and electrically induced contractions before and after the fatiguing task and after 5 min of recovery. Reflex activities were also tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. The results indicated that whereas the maximal voluntary contraction torque, tested in isometric conditions, was reduced to a greater extent (P < 0.05) at 20 degrees of plantar flexion (-33%) compared with the neutral position (-23%) of the ankle joint, the EMG activity of both muscles was not significantly reduced after fatigue. Muscle activation, tested by the interpolated-twitch method or the ratio of the voluntary EMG to the amplitude of the muscle action potential (M-wave), as well as the neuromuscular transmission and sarcolemmal excitation, tested by the M-wave amplitude, did not change significantly after the fatiguing exercise. Although the H and T reflexes declined slightly (10-13%; P < 0.05) after fatigue, these adjustments did not appear to have a direct deleterious effect on muscle activation. In contrast, alterations in the mechanical twitch time course and postactivation potentiation indicated that intracellular Ca(2+)-controlled excitation-contraction coupling processes most likely played a major role in the force decrease after dynamic fatiguing contractions performed for short duration.

Mechanisms of decreased motoneurone excitation during passive muscle stretching.

The effect of pre- versus postsynaptic mechanisms in the decrease in spinal reflex response during passive muscle stretching was studied. The change in the electromyographic (EMG) responses of two reflex pathways sharing a common pool of motoneurones, with (Hoffmann or H reflex) or without (exteroceptive or E reflex) a presynaptic inhibitory mechanism, was compared. The EMG activities were recorded in the soleus muscle in response to the electrical stimulation of the tibial nerve at the popliteal fossa (H reflex), and at the ankle (E reflex) for different dorsiflexion angles of the ankle. The compound muscle action potential (M wave) in the soleus and the abductor hallucis was recorded in order to control the stability of the electrical stimulation during stretching. The results indicate that in the case of small-amplitude muscle stretching (10 degrees of dorsiflexion), a significant reduction (-25%; P < 0.05) in the Hmax/Mmax ratio was present without any significant change in the Emax/Mmax ratio. At a greater stretching amplitude (20 degrees of dorsiflexion), the E reflex was found to be reduced (-54.6%; P < 0.001) to a similar extent as the H reflex (-54.2%). As soon as the ankle joint returned to the neutral position (ankle at 90 degrees), the two reflex responses recovered their initial values. In additional experiments, motor-evoked potential (MEP) induced by the magnetic stimulation of the motor cortex was recorded and showed a similar type of behaviour to that observed in the E reflex. These results indicate that reduced motoneurone excitation during stretching is caused by pre- and postsynaptic mechanisms. Whereas premotoneuronal mechanisms are mainly involved in the case of small stretching amplitude, postsynaptic ones play a dominant role in the reflex inhibition when larger stretching amplitude is performed.

Effect of time of day on force variation in a human muscle.

The effect of time of day on the neural activation and contractile properties of the human adductor pollicis muscle was investigated in 13 healthy subjects. Two different times of day were chosen, corresponding to the minimum (7 h) and maximum (18 h) levels of strength. The force produced was compared with the associated electromyographic (EMG) activity during voluntary and electrically induced contractions in order to determine whether peripheral or central mechanisms play a dominant role in diurnal force fluctuation. The results indicated that the force produced during a maximum voluntary contraction (MVC) was significantly higher (+8.9%) in the evening than the morning. Since the increase in force of the MVC and the tetanic contraction (100 Hz) were similar, it is suggested that peripheral mechanisms are responsible for diurnal fluctuations in force. This conclusion is supported by the observation that central activation, tested by the interpolated twitch method during an MVC, did not change, and that the EMG was less per unit force in the evening. In addition to the increase in maximum twitch and tetanus force, significant changes in muscle contractile kinetics were also observed. The maximum rate of tension development and the relaxation of the twitch and tetanus increased in the evening, and the twitch contraction time (CT) and the time to half-relaxation (TR(1/2)) were reduced. Because the mean range of variation in skin temperature (2. 6 degrees C) observed over the course of the day was very low, this change cannot entirely explain those observed in muscle contractile properties.

EMG and mechanical changes during sprint starts at different front block obliquities.

The effect of decreased front block obliquity on start velocity was studied during sprint starts. The electromyographic (EMG) activity of the medial gastrocnemius (MG), the soleus (Sol), and the vastus medialis (VM) was recorded and analyzed at a 70 degrees, a 50 degrees, and a 30 degrees angle between the foot plate surface and the horizontal. Integrated EMGs (IEMG) were compared with muscle length changes in the MG and Sol in relation to foot and knee movements. The results indicate that decreasing front block obliquity significantly (P < 0.05) increases the start velocity without any change to the total duration of the pushing phase and the overall EMG activity. This improvement in sprint start performance is associated with the enhanced contribution of the MG during eccentric and concentric phases of calf muscles contraction. In the "set position" the initial length of MG and Sol is increased at 50 degrees and 30 degrees as compared with 70 degrees. The subsequent stretch-shortening cycle is improved and contributes more effectively to the speed of the muscle shortening. Moreover, lengthening these muscles during the eccentric phase stretches the muscle spindles, and the reflex activities that contribute to the observed increase in the MG IEMG, are present when the slope of the block is reduced. The results indicate that decreasing front block obliquity induces neural and mechanical modifications that contribute to increasing the sprint start velocity without any increase in the duration of the pushing phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease of motoneuron excitability during stretching of the human soleus.

Changes of motoneuron (MN) excitability was studied during the basic modalities of slow or static stretching of the human soleus muscle (Sol). Tendon (T) and Hofmann (H) reflexes were recorded during slow passive (assisted) stretching (SS). The H response was analysed in SS, in SS preceded by a maximal isometric contraction of the muscle or contraction-relaxation (CR) and during stretching of the muscle by contracting the antagonistic muscle (AC). The maximal joint mobilization during SS, CR and AC appears to be closely related to the decrease of the H response. It is suggested that the joint mobilization during slow stretching is closely controlled by the level of inhibition of the MN pool excitability, which lasts only as long as the stretching manoeuvre is maintained.

Muscle stretching and motoneuron excitability.

Change of motoneuron excitability has been studied during the three basic modalities of slow or static stretching of the human soleus muscle. Tendon (T) and Hoffmann (H) reflexes were analyzed during static stretching (SS). The H response was compared in SS, in SS preceded by a maximal isometric contraction of the muscle or contraction-relaxation (CR) and during stretching of the muscle by contracting the antagonistic muscles (AC). During progressive dorsiflexion of the foot there is a significant difference (p less than 0.05) between T and H reflexes during SS, although the amplitude of direct motor (M) response, evoked by a maximal stimulation of the motor nerve, is not changed. The maximal joint mobilization during SS, CR and AC modalities appears to be closely related to the decrease in the H response during stretching. This decrease is significantly (p less than 0.05) smaller in SS than in AC or CR. In this last method, the duration of the maximal isometric contraction does not affect the results. In these three basic stretching procedures, the H reflex quickly recovers as soon as the manoeuvre is interrupted. It is suggested that changes in muscle motoneuron pool excitability closely control joint mobilization during slow or static stretching. The inhibition of the motoneurons observed during SS, CR and AC modalities is limited to the duration of the stretching manoeuvre.