An OT Practitioner Is Testing Thumb Interphalangeal (IP)flexion Strength

An OT practitioner is testing thumb interphalangeal (IP) flexion strength.The therapist definitely observes flexion of the IP joint but cannot palpate a contraction of the flexor pollicis longus (FPL) .What is the most likely reason?

A) The FPL may be too weak to palpate.
B) The client is giving less than maximal effort.
C) The flexor pollicis brevis may be substituting.
D) The extensor pollicis longus may be substituting.