Hey Charles,
Jcurls are great. We load the spine in flexion all the time with movement. We just don’t always reach for the Jcurl first. We tend to see people lack hip flexion and then use their spines instead. An old Rolfing skill is to sit on a chair and curl down unloaded. We also warm up with flexion everyday like in Yoga. Fold over/round then go flat from that rounded shape. Even teaching kipping is great, or rolling pistols. Or short lever hip flexion like deep relaxed squatting. As for “loaded” stretches, it’s one of the reasons we use weights/bars and bands on nearly everything. Adding a vector load makes it loaded (ant band distraction on hip for ex). Also, ANY isometric with body weight is loaded. Kelly