Work Done by Muscle
W=Fd *Work is force over a distance
Muscles provide the ability for a body to do work. A muscle contraction, F produces movement, d and the muscle does dynamic work. A concentric movement gives positive dynamic work while a eccentric movement gives negative dynamic work.
(courtesy of google images)
To climb this flight of stairs, the work required from the leg muscles would equal the force X distance; where force is the mass of the cat X the acceleration of gravity, and distance is the height of the stairs.
Longer muscles do more work even when the force of contraction is equal because the distance the muscle moves increases.
*If muscle contracts without movement, an isometric contraction, no real physics work is done, but it is said to be doing static work.
("resistance training")