Janu Sirsasana is one of my favorite poses, stretching a smörgåsbord of dorsal muscles: lats, hamstrings, gastrocnemius (muscle at the back of your calf), glutes and spinal extensors — just to name a few.
Although seemingly simple, an added level of awareness can help to deepen the stretch.
Engaging your biceps and quads can help you move further forward.
You can also adjust the “set length” of your muscles — which is the point that feels like you cannot stretch beyond, without encountering some sort of injury.
First, you want to stretch your muscles out to their set length.
Next, softly contract the muscle you are trying to stretch out more. Here, that would be hamstrings and gastrocnemius (muscle at the back of your calf). It’s a lot to focus on two muscles, so you may want to only focus on the hamstrings to start.
When you relax your muscle, you should be able to go more deeply into the stretch.
You can also adjust what’s called the “insertion point” of a muscle in order to lengthen a stretch.
For example, the gastrocnemius has insertion points at the base of the femur and at the heel. By bending your knee and lifting the heel slightly, the muscle can move with more freedom, manipulating the insertion point to a spot on the heel farther away form the femur. That in turn, lengthens the muscle.
Likewise, tilting the pelvis forward can manipulate the insertion point of the hamstrings at the pelvis, thereby lengthening the muscle.