Critical illness polyneuromyopathy (CIP) leads to major muscle weakness correlated with peripheral nerve and/or muscle alterations. Because sepsis seems to be the main factor, we used an experimental model of chronic sepsis in rats to study the localization of the first alterations on isolated motor units of soleus muscle. Seven days of chronic sepsis leads to a decrease in muscle force and an increase in muscle fatigability. Muscle twitch contraction time is also slower and all the motor units exhibit a slow profile in septic rats. Motor axon conduction velocity remains normal. We observed a significant increase in the latency between nerve and muscle action potentials but no modifications in the electromechanical delay. The first action of sepsis on motor units seems to be a delayed trigger of muscle action potential along with a muscle weakness but without nerve conduction impairment.