When performing abdominal exercises, knee support can also be used to support the feet, although the difficulty of the movement is different, but there is no difference in the training effect of the abdominal muscles. Therefore, for most trainers, because the foot support is too difficult, knee support is recommended. Because the support capacity of the shoulder joint is different, so beginners only need to push to the farthest end of their own support, do not have to pursue the lowest point, try to expand the abdomen to the limit, the abdominal muscles slowly elongate, so as to avoid injury. Elbow flat, in addition to the need to lower the support force (mainly or abdominal force), basically do not use elbow strength. Training should be gradual and safe to avoid injury.