Objective: 

After an high-intensity exercise, the accumulation of lactate in the blood is associated to an increase of lactate extractions by the CNS. However, no information has been yet provided about the influences of blood lactate levels on excitability of the spinal cord, in particular, of the spinal motoneurons. The aim of the present study was to examine the effects of high blood lactate levels, induced with a maximal cycling or with an intravenous infusion, on motoneuronal excitability.

Materials and Methods: 

The study was carried out on 23 male athletes from the Track and Field Team of the University. Seventeen subject performed a maximal cycling test on a mechanically braked cycloergometer, whereas 6 of them were submitted to the intravenous infusion of a lactate solution (3 mg/kg in 1 min). Before the exercise or the injection, at the end as well as 5 and 10 minutes after the conclusion, blood lactate was measured and excitability of the motor cortex of each subject were evaluated by studying the H reflex of the first dorsal interosseus (FDI) muscle.

Results: 

In these experimental conditions, it was observed that, after an exhaustive exercise, the H/M ratio (amplitude of maximal H response vs. that of the maximal M response) showed a small increase, whereas an injection of lactate did not change the H/M ratio, i.e. the motoneuronal excitability.

Discussion: 

The main result of this study were that, after an acute maximal exercise, there is a small but significant improvement of motoneuronal excitability, evaluated with the H reflex technique. Iintravenous infusion of lactate, in subjects not performing any exercises, showed that blood lactate increase is not associated with a significant change of spinal motoneurons' threshold.

Conclusion: 

It can be concluded that the raise of blood lactate levels during a maximal exercise is not per se capable to modify the excitability of spinal motoneurons.