Sorry I'm a little late on the topic...
I'm concerned that exercises such as Jefferson Curls could end up stretching ligaments rather than muscles (especially at my age of 15, where my bones are growing faster than my tendons and ligaments), resulting in "loose jointedness", which can cause osteoarthritis. Thomas Kurz, the author of Stretching Scientifically, is wary even of any lower back stretches that are performed from a standing position because the weight of the upper body can put too much stress on the ligaments. To Coach Sommer (or anyone with experience regarding the issue): In your experience, is putting strain on the ligaments of a joint harmful, and if so, how does one distinguish between maintaining healthy mobility and potentially damaging a joint?
Daniel