This study investigates the effect of preparation warning on the functional connectivity between forearm muscles during movement's initiation. The Normalized Mutual Information (NMI) is the used indicator. It is calculated for muscle pairs activated during a handgrip exercice. They are innervated by the same nerve or by different nerves. Results show that: i) NMI is more important when muscles pairs are stimulated by the same nerve during both preparation and motor activities. ii) NMI is higher during preparation comparing to motor activity. iii) The functional connectivity during contraction's beginning increases when a preparation warning is given. We conclude that muscles put together their effort to initiate the movement especially when participants prepare in advance their activity.