I recently overhead a new CrossFit trainee mention that the kipping pull-up he was being taught was "kind of cheating." This is a very common response that shows that many people are unaware that functional movements often require contributions of eccentric (lengthening), isometric (static), and concentric (shortening) muscle actions and that one very common power movement uses a stretch immediately prior to the muscle shortening. This pattern is called the stretch-shortening cycle, as the muscle is lengthened (while actively working) prior to shortening. Rather than cheating, kipping is just one example of an athlete utilizing this natural mechanical response. Cutting from right to left when playing a sport or performing a drop-down counter-movement before jumping are also examples of stretch-shortening cycles. Maybe I should quickly review some terminology. When a muscle is active but lengthening, the muscle action is called eccentric ("away from the center"). This is different from trying to lengthen a muscle while doing a stretch. In the latter case, the muscle is not actively trying to shorten; it is trying to relax. The opposite movement--the work of a muscle actively shortening, or contracting-- is called a concentric ("toward the center") contraction. When a muscle engages (tries to shorten) but does not change length (or produce motion) it is called an isometric contraction.
When you lower yourself slowly into a chair, your hip, knee, and ankle joints flex. Does this mean that your hip flexors, knee flexors (e.g., hamstrings), and ankle flexors (tibialis anterior) are contracting to produce this movement?