Antonyms of supple:
awkward, inflexible, dull, inactive, lazy, slow, heavy, indolent, quiescent, sluggish, idle, inert, quiet, stupid, inelastic, nonmalleable, unyielding, breakable, brittle, fragile, rigid, stiff, stiffened, compact, firm, hard, solid, crisp, crumbly, flaky, friable, short, nonelastic.
Examples of usage:
  1. He saw Boucher begin to play and as the latter held his cards, noticed again his thick and strong, but supple wrists.
    "The Hunters of the Hills", Joseph Altsheler.
  2. The man moved on as he spoke, and the Doctor moved with him, for his arm was still clasped by the stranger's strong supple fingers.
    "A Bachelor's Dream", Mrs. Hungerford.
  3. I'm supple enough from the waist up.
    "Vane of the Timberlands", Harold Bindloss.