Specific modulation of presynaptic and recurrent inhibition of the soleus muscle during lengthening and shortening submaximal and maximal contractions.

The study analyzed neural mechanisms mediating spinal excitability modulation during eccentric (ECC) movement (passive muscle lengthening, submaximal, and maximal ECC contractions) as compared with concentric (CON) conditions. Twenty-two healthy subjects participated in three experiments. Experiment A (n = 13) examined D1 presynaptic inhibition (D1 PI) and recurrent inhibition (RI) modulation during passive muscle lengthening and shortening, by conditioning the soleus (SOL) H-reflex with common peroneal nerve submaximal and tibial nerve maximal stimulation, respectively. Experiment B (n = 13) analyzed the effect of passive muscle lengthening on D1 PI and heteronymous Ia facilitation (HF, conditioning the SOL H-reflex by femoral stimulation). Experiment C (n = 13) focused on the effect of muscle contraction level (20%, 50%, and 100% of maximal voluntary contraction) on D1 PI and RI. Results showed a significantly higher level of D1 PI during passive muscle lengthening than shortening (P < 0.01), whereas RI and HF were not affected by passive muscle movement. D1 PI and RI were both higher during ECC as compared with CON contractions (P < 0.001). However, the amount of D1 PI was independent of the torque level, whereas RI was reduced as the torque level increased (P < 0.05). The decreased spinal excitability induced by muscle lengthening during both passive and active conditions is mainly attributed to D1 PI, whereas RI also plays a role in the control of the specific motoneuron output during ECC contractions. Both inhibitory mechanisms are centrally controlled, but the fact that they evolve differently with torque increases, suggests a distinct supraspinal control.NEW & NOTEWORTHY Presynaptic (PI) and recurrent inhibitions (RI) were studied during passive muscle lengthening and eccentric contractions. Results indicate that the increased PI during passive muscle lengthening accounts for the decreased spinal excitability at rest. During eccentric contraction both mechanisms contribute to spinal excitability modulation. The same amount of PI was observed during eccentric contractions, while RI decreased as developed torque increased. This distinct modulation according to torque level suggests a distinct supraspinal control of these mechanisms.

Static stretch and dynamic muscle activity induce acute similar increase in corticospinal excitability.

Even though the acute effects of pre-exercise static stretching and dynamic muscle activity on muscular and functional performance have been largely investigated, their effects on the corticospinal pathway are still unclear. For that reason, this study examined the acute effects of 5×20 s of static stretching, dynamic muscle activity and a control condition on spinal excitability, corticospinal excitability and plantar flexor neuromuscular properties. Fifteen volunteers were randomly tested on separate days. Transcranial magnetic stimulation was applied to investigate corticospinal excitability by recording the amplitude of the motor-evoked potential (MEP) and the duration of the cortical silent period (cSP). Peripheral nerve stimulation was applied to investigate (i) spinal excitability using the Hoffmann reflex (Hmax), and (ii) neuromuscular properties using the amplitude of the maximal M-wave (Mmax) and corresponding peak twitch torque. These measurements were performed with a background 30% of maximal voluntary isometric contraction. Finally, the maximal voluntary isometric contraction torque and the corresponding electromyography (EMG) from soleus, gastrocnemius medialis and gastrocnemius lateralis were recorded. These parameters were measured immediately before and 10 s after each conditioning activity of plantar flexors. Corticospinal excitability (MEP/Mmax) was significantly enhanced after static stretching in soleus (P = 0.001; ES = 0.54) and gastrocnemius lateralis (P<0.001; ES = 0.64), and after dynamic muscle activity in gastrocnemius lateralis (P = 0.003; ES = 0.53) only. On the other hand, spinal excitability (Hmax/Mmax), cSP duration, muscle activation (EMG/Mmax) as well as maximal voluntary and evoked torque remained unaltered after all pre-exercise interventions. These findings indicate the presence of facilitation of the corticospinal pathway without change in muscle function after both static stretching (particularly) and dynamic muscle activity.