I can't complete 1 'Tuck Up' without swinging or timing the movement and even then it's a struggle to get 3. I have never been able to complete 1 sit up without having my ankles held. Once they are held, no problems cranking out 50+. It feels like i'm lacking any strength in whatever muscle it is that enables both sit ups (unheld ankles) and 'Tuck Up'.
Can anyone suggest an isolation exercise to assist with strengthening this muscle....whatever it maybe?