Baby Sit-ups!!! 💖 I love this exercise for strengthening the anterior muscles including the neck flexors and abdominal muscles. This is also a test for head lag-checking to see if your baby can keep their head in line with their body when pulled to sit….which they ideally should be able to do by 3-4 months of age. 💖 As you can see above, the little 3 month old cutie is not able to pull to sit using his arms. He exhibits poor neck and trunk control. He did not help me at all when I first tried.