False regarding protective co-contraction muscle splinting:
(A) Reduced mouth opening due to infection (B) Protective co-contraction is not a pathologic condition (C) There is no pain reported when the muscle is at rest, but use of the muscle will typically result in increased pain. (D) Altered sensory input or pain, antagonistic muscle groups fire during movement in an attempt to protect the injured part.
(A) Protective Co-contraction (Muscle Splinting) 
Protective co-contraction is the initial response of a muscle to altered sensory or proprioceptive input or injury (or threat of injury). This response has also been called protective muscle splinting or coactivation. It is not associated with reduced mouth opening due to infection.