Final answer:
During the eccentric contraction phase of the Stretch-Shortening Cycle (SSC), muscle spindles are stretched along with the noncontractile tissues within the muscle, triggering a reflexive muscle contraction.
Step-by-step explanation:
The muscle spindles are stretched along with the noncontractile tissues within the muscle during the stretch-reflex, specifically during the eccentric contraction phase of the Stretch-Shortening Cycle (SSC). Muscle spindles, which are proprioceptive stretch receptors within a muscle, are activated when the muscle is lengthened. In response to this stretching, muscle spindles send signals to the spinal cord, which then trigger a reflexive contraction of the muscle to prevent overstretching and injury. This reflexive action contributes to the rapid transition from muscle lengthening to muscle contracting, which is a characteristic of the SSC, often seen in activities like jumping and running.