Rectus Abdominis & Hamstrings
Acting as a group, these muscles extend the innominate (nutation) [1, 2] p409-424, with the long head of the biceps femoris providing some position dependent sacral extension (counternutation). The short head of the biceps femoris may promote counternutation.
Rectus Abdominis: Nutation
Superior Attachment: xiphoid process
Inferior Attachment: crest of pubis
- flexes the trunk
- extends the pelvis
- counters shear stress on lumbar spine
- compresses abdominal contents
- causes nutation by bringing the pubis anteriorly and superiorly, causing the ilium to go posteriorly and inferiorly
Bicep Femoris – two heads
Short head of Biceps Femoris: Counternutation (not shown)
Origin: linea aspera
Insertion: head of the fibula and the lateral side of the head of the fibula.
- flexes and laterally rotates the knee joint
- observation reveals that it may participate in counternutation by pulling superiorly on the fibular head, assisting in rebound during shock absorption.
Long head of Biceps Femoris: Nutation and Counternutation
Origin: ischial tuberosity, with some fibers originating at the sacrotuberous ligament
Insertion: head of the fibula, lateral collateral ligament, and lateral tibial condyle
- flexes and laterally rotates the knee
- extends and laterally rotates the femur
- observation reveals that it may participate in counternutation by pulling
superiorly on the fibular head, assisting in shock absorption
- extends the innominate (nutation) through its attachment at the ischial tuberosity
- extends the sacrum (counternutation) through its attachment at the sacrotuberous ligament.
There is evidence that the long head of the biceps femoris has a dual function: it can promote both nutation and counternutation. Along with the other hamstrings attaching to the ischium, it rotates the ilia posteriorly into nutation. It is also connected to the sacrotuberous ligament and, through it, to the caudal part of the sacrum where its action would cause counternutation by pulling the sacral apex inferiorly and anteriorly .
This seeming contradiction was explained by Wingerden, et al.  when he showed that the tension developed in the sacrotuberous ligament was dependent of body position; a high percentage of the force on this ligament was transferred in a flexed, stooped position. They reasoned that, because larger forces are acting on the sacrum in a flexed position (inducing nutation) extra counterbalancing muscular force is needed. The counterbalancing muscular force may also be provided by other muscles that attach to the sacrotuberous ligament such as the gluteus maximus and sacral part of the erector spinae which, like the biceps femoris, promote nutation and which, in a flexed position, may promote counternutation. This is an example of position dependent function that is not rare in our musculoskeletal system. For example, the piriformis is an external rotator of the femur when the hip is straight and slightly flexed but, when the hip is flexed close to 90 degrees, it becomes an internal rotator.
Further, Wingerden et al.  combined Color Echo Doppler with an oscillation device and found that the biceps femoris produced a significant increase in sacroiliac joint stiffness when the subject is prone. In this case, biceps femoris activation may have promoted nutation by bringing the sacrum and iliac surfaces closer together through its action on the ischium. However, in a flexed, stooped position, where there is a lot of weight on the sacrum and the limits of nutation are reached, the biceps femoris acts to assist in limiting nutation by pulling the sacral base posteriorly, into counternutation, through its pull on the sacral apex, via the sacrotuberous ligament .
According to Vleeming , the long dorsal sacroiliac ligament is tensed during counternutation and slackened during nutation. Loading of the gluteus maximus decreased tension on the ligament, demonstrating that the gluteus maximus promoted nutation. However, in this study, loading the biceps femoris neither increased nor decreased tension in the long dorsal sacroiliac ligament, showing no effect on nutation or counternutation; but, there may be other factors that influenced this study. In other studies, Vleeming  and Van Wingerden , suggested that the long head of the biceps femoris prevented nutation by tensing the sacrotuberous ligament (thus inducing counternutation).
Origin: medial and posterior aspect of the ischial tuberosity
Insertion: proximal medial surface of the tibia just posterior to the attachment of the sartorius
Function: pulls the ischium inferiorly and anteriorly, rotating the iliac crest posteriorly
Origin: lateral and posterior aspect of ischial tuberosity
Insertion: posterior aspect of the medial condyle of tibia, medial collateral ligament, oblique popliteal ligament and popliteus muscle
Function: pulls the ischium inferiorly and anteriorly, rotating the iliac crest posteriorly
Gray’s Anatomy p570-571stated that the biceps femoris is a lateral rotator when the knee is semiflexed and when the hip is extended. However, the semitendinosus and semimembranosus, both nutators, are medial femoral rotators in these positions. This relationship helps associate medial rotation with nutation and lateral rotation with counternutation, as demonstrated by other counternutation muscles that induce lateral rotation, such as the piriformis and obturator externus.
Posterior Adductor Magnus: Nutation
Origin: ischial tuberosity
Insertion: adductor tubercle on the femur.
Function: pelvic extension (nutation).
Why are the Hamstrings Usually Tight?
It is evident that they both the Rectus Abdominis and Hamstrings induce pelvic extension, aka nutation, so we would expect them both to be inhibited and relatively flaccid. While the Rectus Abdominus is relatively straightforward, the Hamstrings are much more complicated. As nutation muscles, we would expect to see them inhibited and relatively flaccid but, we usually see them tight. Why? Because, during running, when we plant our heel on the ground to pull ourselves forward, being inhibited, they are not prepared for the force, and they strain, which is followed by a stretch reaction that holds it in a contracted state; this is a secondary reaction, or a compensation to a compensation. The sequence may be expressed as: ligament sprain – inhibited muscle – delayed activation – muscle strains – stretch reaction – tight muscle. The muscles, and ligaments, are protecting themselves from the trauma that just occurred, and from future trauma.
So, instead of stretching them, we should strengthen them, using low force, high repetition exercises to train the proprioceptive system to maintain a state of relaxed tension and, better yet, stabilize the SIJ. For more information, please see Rehab Principles But, whether stretching or strengthening, we can stress the SIJ without proper stabilization. The funny thing is that this results in inhibition to the hamstrings, which causes relaxation but sets it up for another stretch reaction in a vicious cycle.
There are other considerations for the hamstrings, as well. There is a slip of muscle coming off the long head of the biceps femoris which attaches to the sacrotuberous ligament. This slip induces counternutation by pulling the sacral apex anteriorly. This is part of the mechanism that helps provide stability and extra force in the SIJ while raising the pelvis as a unit from a position of hip flexion.
- Wingerden, J.v., A functional-anatomical approach to the spine-pelvic mechanism: interaction between the biceps femoris muscle and the sacrotuberous ligament. European Spine Journal, 1993. 2: p. 140-144.
- Vleeming, A., et al., The role of the sacroiliac joints in coupling between spine, pelvis, legs and arms., in Movement, Stability, and Low Back Pain, A. Vleeming, et al., Editors. 1997, Churchill Livingstone. p. 53-71.
- Neumann, D., Kinesiology of the Musculoskeletal System. Foundations for Physical Medicine. 2002: Mosby.
- Wingerden, J.v., et al. Muscular Contribution to Force Closure: Sacroiliac Joint Stabilization In Vivo. in 4th Interdisciplinary World Congress on Low Back & Pelvic Pain. 2001.
- Wingerden, J.v., The role of the hamstrings in pelvic and spinal function, in Movement, Stability, and Low Back Pain, A. Vleeming, et al., Editors. 1997, Churchill Livingstone: Edinburgh.
- Vleeming, A., et al., The function of the long dorsal sacroiliac ligament: its implication for understanding low back pain. Spine, 1996. 21(5): p. 556-62.
- Vleeming, A., et al. Load Application to the Sacrotuberous Ligament: Influences on Sacroiliac Joint Mechanics. in Proceedings of the First Interdisciplinary World Congress on Low Back Pain And its Relation to the Sacroiliac Joint. 1992. Rotterdam: ECO.
- Van Wingerden, J.P., et al. The Spine-Pelvis-Leg Mechanism: With a Study of the Sacrotuberous Ligament. in Proceedings of the 1st Interdisciplinary World Congress on Low Back Pain and its Relation to the Sacroiliac Joint. 1992. Rotterdam: ECO.
- Warwick, R. and P.L. Williams, eds. Gray’s Anatomy. 35 ed., ed. R. Warwick and P.L. Williams. 1973, W.B. Saunders Company: New York.