- leg extension at the hip joint
- leg flexion at the knee
- rotation of the lower leg at the knee joint
- stabilization of the pelvis when bending forward
If we do not gain (or improve) control in all four movement functions, one or more of those movement habits will dominate control of the other movement(s).
In addition, the hamstrings of one leg work alternately with those of the other -- as in walking; when the hamstrings of one leg are bending or stabilizing the knee, the hamstrings of the other leg are extending or stabilizing the other leg at the hip. In those movements, the hamstrings coordinate with the hip flexors and psoas muscles. (Co-contraction of hamstrings and hip flexors/psoas muscles leads to hip joint and ilio-sacral (SI) joint compression.) So our approach (being movement-based) must take those relationships into account. Otherwise, we never develop the feeling of free hamstrings in their familiar movements.
LEG EXTENSION AT THE HIP JOINT
That's the "leg backward" movement of walking. The hamstrings are aided by the gluteal (butt) muscles, but only in a stabilizing capacity. The major work is done by the hamstrings. In this movement, the hamstrings, inner and outer, work together in tandem.
LEG FLEXION AT THE KNEE JOINT
That's the "getting ready to kick" movement and also the "pawing the ground" movement. In these movements, the hamstrings, inner and outer, also work together in tandem (same movement).
To the anatomist and kinesiologist, it may seem incomprehensible ("paradoxical") that the hamstrings are involved in both movements -- leg forward and leg backward -- but that's how it is. Though the hamstrings are involved in both cases, different movements cause a different feel.
LOWER LEG ROTATION AT THE KNEE
That's the turning movement used in skating and in turning a corner. In this movement, the inner hamstrings (semi-membranosis and semi-tendinosis) relax and lengthen as the outer hamstring (biceps femoris) tighten to turn toes-out and the inner hamstrings tighten to turn toes-in as the outer hamstring relaxes and lengthens.
STABILIZATION OF THE PELVIS WHEN BENDING FORWARD
The hamstrings anchor the pelvis at the sitbones (ischial tuberosities) deep to the 'smile' creases beneath the buttocks (not the crack), so one can bend forward in a controlled way, instead of flopping forward at the hips like a marionette. In this movement, the hamstrings coordinate with the front belly muscles (rectus abdominis).
In most people, either the rectus or hamstrings dominates the other in a chronic state of excessive tension, so freeing and coordinating the hamstrings involves coordinating and matching the efforts of the two muscle groups. When the hamstrings dominate, we see swayback; when the rectus muscles dominate, we see flat ribs.
TRAINING HAMSTRING CONTROL
In training hamstring control, it's convenient to start with the less complicated movement, first. That's the anchoring movement that stabilizes bowing in a standing position. (See first video, above.)
After we cultivate control of "in tandem" hamstring movements, we cultivate control of "alternating" hamstring movements. (See second video, above.)
By cultivating control of "in tandem" and "alternating" movements, we fulfill the requirements of functions (1.), (2.), and (4.). The exercise linked in the paragraph above indirectly addresses function (3.) (lower leg rotation at the knee). Other exercises that have this effect exist in the somatic exercise programs, "The Cat Stretch" and "Free Your Psoas", for which previews exist through the preceding links.