An exercise machine applicable for strengthening a user's abdominal muscles is described. The exercise machine can include a track frame having a higher end and a lower end. Kneeling assemblies may be mounted on parallel gliding rails of the track frame to receive the knees from a user to make movements up and down between the lower end and the higher end of the track frame. Each kneeling assembly may be engaged with a pair of the gliding rails. A rotationally adjustable armrest holder may be adjustable rotationally mounted on the upper end of the track frame. The armrest holder can be oriented towards a direction having an angular relationship with an axis of the track frame between the lower end and the higher end. The armrest holder may capable of supporting the arms of the user making the movements with a twisting effect according to the angular relationship.