These terms combine the prefix "iso" (meaning "same") with "tonic" (strength) and "plio" (more) with "metric" (distance). In "isotonic" exercises the force applied to the muscle does not change (while the length of the muscle decreases or increases) while in "plyometric" exercises the length of the http://www.fitandfearless.com/ muscle stretches and contracts rapidly to escalation the effectiveness output of a muscle.
Movements can become easier or harder depending on the angle of muscular arm relative to gravity; for example, a standard biceps curl becomes easier as the hand approaches the take on as enhanced of the load is taken by the arrangement of the elbow. Secure machines such as the Nautilus involve earmarked adaptations to keep resistance constant irrespective of the joint angle.