Final answer:
Muscle spindles are stretched during the eccentric contraction phase of the stretch-shortening cycle, as the muscle lengthens under tension.
Step-by-step explanation:
The muscle spindles are stretched along with the noncontractile tissues within the muscle during the stretching phase of the stretch-shortening cycle (SSC), which is not specifically mentioned in the provided reference material. Muscle spindles are proprioceptive receptors within the muscle that respond to changes in muscle length and are involved in the regulation of muscle contraction via the stretch reflex.
The SSC is a natural and powerful movement pattern seen in various activities that is comprised of an eccentric contraction phase where the muscle lengthens, an amortization or transition phase, and a concentric contraction phase where the muscle shortens and generates force. The stretching of muscle spindles occurs during the eccentric contraction phase of the SSC, when the muscle is actively lengthening under tension.