In Anjaneyasana (Low Lunge), keep your front knee over your front foot, and choose the arm position that feels best for you.
Athletes often appear in my classes complaining of tight hamstrings that they think limit their performance. Sure, their hamstrings are tight, but they aren't tight and short--they're tight and long. The real culprit is the state of their hip flexors, which work in opposition to the hamstrings. When your hip flexors grow short and tense, whether through training actions like pulling up during the pedal stroke or just lifting knees while running, or by sitting for too long each day, they pull the pelvis into a forward tilt, overstretching the hamstrings and holding them in this too-long position. By releasing the hip flexors, we can start to find the right balance between the front and the back of the hip, creating the correct partnership between the hip flexors the hamstrings. When this balance is re-established you'll feel more fluid and be more efficient.
Anjaneyasana (Low Lunge) is a great pose for feeling the synergy between these two groups of muscles. The pose builds the range of motion required for a fluid running stride, too. Investigate your experience in the pose to see how your hip flexors and your hamstrings relate. You might find that your hip flexors are much tighter than your hamstrings. If so, make sure to include the pose in your regular practice and you'll find more balance. At the same time, you'll be shoring up the muscles that support your knees.
Let's go through the pose together. Come onto your mat in a lunge position, your front knee directly over your front ankle. (If your knee rolls in or out, that reflects tightness in the inner thigh or outer hip, a subject for another post.) Drop your back knee to the mat, padding it if you need to, and slide it back until you feel a pleasant stretch in the front of that leg. You can stay here with your hands framing your front foot, lift your hands to your quads, or take your hands overhead.
In any of these variations, note where you feel the stretch. Is it in your front-leg hamstrings? Your back-leg hip flexors? Does your front foot wobble as you try to hold steady? All of this information is useful for your sport. If your ankle seems weak, start to include one-legged balance poses into your practice to build strength. If your front-leg hamstrings feel tight, focus on gentle reclined forward folds to help them release. If your back-leg quadriceps and hip flexors are tight, Anjaneyasana is a good pose to practice. You can also include some supported backbends to help the hip flexors open--Setu Bandha Sarvangasana (Bridge Pose) resting the back of the pelvis on a block positioned horizontally is a good choice.