Same word as strong:

able, able-bodied, active, athletic, big, capable, durable, enduring, energetic, firm, fixed, forceful, forcible, hale, hard as nails, hardy, hearty, heavy, heavy-duty, in fine feather, mighty, muscular, reinforced, robust, rugged, secure, sinewy, solid, sound, stable, stalwart, stark, staunch, steady, stout, strapping, sturdy, substantial, tenacious, tough, unyielding, vigorous, well-built, well-founded, well-made