Cradle client’s calf across your forearm so that your hand is on the medial side of their knee. Have your client try to externally rotate their femur (bring the foot medially) at 10% of their strength against your resistance for 10 seconds. Then have your client try to internally rotate their femur (bring their foot laterally) against your resistance.
To stretch the other five of the deep six muscles abduct your clients thigh 30 degrees and repeat the procedure in this position. This pulls the greater trochanter of the femur away from the ischium and gives a better stretch to these deeper muscles.
External and internal rotation may seem confusing from this position. Think of which way the femur is turning from anatomical position. Another way to think of it is to consider the calf as an arrow, with the knee being the tip of the arrow, then see which way the arrow is pointing.
You can also do a PNF stretch to the piriformis with the client supine. See the Piriformis PNF Stretch from the Sports Massage DVD.