Final answer:
The sensation of increased comfort during a stretch, as mentioned by Eric, relates to muscle tissue's elasticity and the viscoelastic behavior, allowing muscles to lengthen and diminish tension, adapting to the stretch over time.
Step-by-step explanation:
When Eric tells his personal training clients that a stretch will start to feel more comfortable as they hold the position, he is referring to two processes related to the muscle tissue properties: elasticity and viscoelasticity. The muscle tissue exhibits elasticity, which allows it to return to its original length when not under tension due to elastic fibers. During a sustained stretch, a muscle's initial tension diminishes (strain-softening) and it lengthens, a response that is also related to the viscoelastic behavior of the muscle.
Additionally, the muscle tissue is covered by the endomysium, a loose, well-hydrated connective tissue that provides further flexibility and support. This sensation of increased comfort during a stretch likely occurs due to a combination of factors, including a reduction in passive tension within the muscle and connective tissue as well as the adaptation of sensory receptors such as muscle spindles to the new muscle length.