Thursday, 13 December 2012

Hip Extension

Do you have adequate hip extension?

Running is an integrated and skilled movement. We have discussed motor patterns and programming in previous posts. Today we briefly discuss hip extension mechanisms (or lack of) and resultant forces in the lower limb.

It is common to see running technique with a torso/ trunk that is behind the centre of mass (COM), particularly when fatigue sets in. At the centre of this is the position of the pelvis and the musculature acting upon this.

The driving force behind this is almost always poor pelvic position. This occurs most commonly due to posterior pelvic tilt. Another position may exist where an anterior pelvic tilt is observed with overly tight (short) hip flexors disrupting full extension of the hip. For now we will focus on lack of hip extension in  the following two forms:

·      Posterior pelvic tilt (the pelvis is tucked under)
·      Anterior displacement of the pelvis (essentially a posterior pelvic tilt with the pelvis displaced anteriorly fostering a host of biomechanical faults)

These postures create a nutated (‘tucked under’) sacrum jamming the ability of the boney innominates to rotate around the sacrum disrupting full hip extension. A lack of full hip extension will result in a posterior lean during the stance phase of gait to accommodate lack of range of motion and resultant weakness. This position decreases demand on the hip extensors by reducing hip flexion. The result is an increased work on the knee due to increased knee flexion moments. In this case (reduced hip extension) the glute max, hamstring and adductor magnus and a host of deep glute stabilisers decrease their activity due to length tension relationships within the pelvis.

The increased knee moment loads the patella-femoral joint. Larger forces across the knee joint are due to the eccentric loading as the quadriceps slows the increased knee moments. These larger angles have been associated with patella-femoral pain, a leading cause of over use injury in distance runners.

Lack of hip extension increases overstriding.  Overstriding is seen when we land in front of the bodies COM essentially creating a rear foot strike increasing collision forces. Dan Lieberman (2010) found this in his correlation between habitually shod and barefoot runners. The piece of information we don’t have conclusive evidence for is whether this increased collision force increases injury risk. The more important factor here is probably not how you land but ‘where ‘ you land. More research in this area is needed.

Landing outside the COM also puts the hamstring at a biomechanical disadvantage attempting to decelerate the limb at peak knee extension just before stance phase. In this position there is a decreased pre stretch which loses the ability to fully use the stretch shortening cycle (SSC). This is often termed ‘free energy’ as the serial elastic components store potential energy in the tendinous structures and myofascial system (predominantly the ‘slings’* through out the body)

A decrease hip extension may also change the toe off phase placing increase loads on the calf musculature as it attempts to push off in a shortened position. Essentially there are larger contractile forces on the calf with less hip extension.

The typical runners body observed here is one with large quads and calves and lazy glutes and hamstrings (they have ‘no bum’). These are injuries waiting to happen in the calves and lumbo-pelvic area.

A further set back of decreased hip extension is lack of stability and loading through the lumbar spine. A posterior tilt ‘opens’ up the lumbar spine lengthening the multifidus and deep spinal stabilisers (rotatores, transversarri, spinales). These protect and stabilise the lumbar spine. When the length tension relationship is disrupted – the stabilisers cannot contract, as they do not have enough muscle fibre overlap – they are too long and weak in this position. Changing the pelvic position towards neutral and some basic re-education usually helps address this however, depending on the health of your back it may take some time to recover from this.

On a very practical level this position of running creates a run that ‘muscles’ and ‘fights’ it’s way through the gait cycle. There is no free fall or use of the SSC every step accelerates and decelerates more than it needs to. Increased amount of rotation will find their way into the gait. This results in a quick onset of fatigue.

If you have the range to get into good hip extension when running but you don’t do it, it’s probably a biomechanical education scenario. In other words you need some education on how to clean up your biomechanics. If you don’t have the range then get your pelvis in the correct position

Try this; Go for a run and tuck your pelvis under. What you’ll find is that it decreases your hip extension, creates large rear foot loading and tightens up the hamtrings. Because your pelvis is in a sense in front of you, a lean back is the most natural thing to do.

In closing start with the pelvis and you’ll be surprised with the changes it can achieve!

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