Muscle knots in the Serratus Anterior are a little unusual because this muscle is tucked away on the side of your rib cage, underneath your armpit and shoulder blade. These muscle knots often feels like a deep, persistent ache along the side of your ribs or even under your shoulder blade, making it uncomfortable to lift your arm or push forward. You might feel a sharp pain in your chest that could be confused with a lung or heart issue, especially when you try to take a full, deep breath.