Amuscle contraction usually begins when

For a contraction to occur there must first be a stimulation of the muscle in the form of an impulse (action potential) from a motor neuron (nerve that connects to muscle). Note that one motor neuron does not stimulate the entire muscle but only a number of muscle fibres within a muscle.

Explanation:

scientific consensus is : a general agreement among scientists about a scientific idea.

even though consensus implied general agreement, it doesn't mean unanimity. for example, scientists generally agree that meteor caused dinosaur's extinction. though this may not be proven yet, but it considered a consensus.

hope it