I could cut to the chase and give the short answer, which is: “None,” but there is so much confusion over this that it is worth explaining why this is so, and why “activating” your hamstrings in leg recovery is a very poor idea.
The hamstrings consist of 3 posterior thigh muscles: the semitendinosus, the semimembranosus and the biceps femoris. All 3 are biarticular, which simply means that they cross 2 joints, the hip and knees joints (see:The Biarticular Muscles in Running). When the hip is stabilized, contracting the hamstrings will flex the knee, drawing the lower leg back. When the knee is stabilized, contraction of the hamstrings will extend the hip, drawing the entire leg back. Note that both of these actions are contrary to leg recovery.
When, however, the hamstrings are not stabilized at either knee or hip, they will respond to the pulls placed upon them by the weight of the body, and maintain tone, acting much like tendons. If the body is extended from a condition in which the 3 joints of the legs are flexed, either to jump or to run, and a leg is extended in line with the trunk, the hamstrings will maintain tone to deliver the work of the gluteus muscles to the lower leg, and, through the corresponding action of the similarly biarticular gastrocnemius, across the ankle and to the foot. Vertebrates, have, in essence, developed limbs to extend the action of muscles in the trunk, allowing for faster locomotion and extended grasp. Muscles close to the trunk (and, thus, to the heart) can still do the lion’s share of the work, and muscles further from the trunk can adapt to changing conditions, like small surface changes in road surface, without distracting the main motors from the task of locomotion. The biarticular muscles and tendons can alter their lengths when the need arises, (which the hamstrings do when they release to let the leg swing forward of the body in leg recovery).
After the running leg has completed its extension, and the foot is released from the ground, the hamstrings will maintain tone. If you lean forward far enough and extend a leg behind you such that the toes leave the ground, you can feel how the hamstrings keep their tone. Then, if the muscles stretched during extension, the ilio-psoas, are allowed to do their work on initiating leg recovery, we see that easy flexion of the hip will cause the knee to flex. There is no need to do this, it is simply a result of the hamstring maintaining light tone. Trying to activate the hamstrings in recovery to lift the heel will have the result of stabilizing the hip, and will create resistance to recovery. Not a good thing.
On the other hand, runners who insist on keeping their torsos vertical while running will defeat the biarticular function of the hamstrings, and will persistently shorten the hamstrings with each rearward movement of the leg – simple mechanics. This shortening will require that the hamstrings be allowed to lengthen during leg recovery, the very opposite of what is advocated by those recommending “activating” the hamstrings to initiate leg recovery.