The Steps of the Dance: 2. Stance

Preamble

The early articles in this series examined the constraints that Newton’s laws of motion place on the way in which we run. The most recent article, posted on 31st March examined the question of how we should orient the joints and tension the muscles at foot fall in order to capture the energy of impact as elastic potential energy, while minimizing the risk of damage to muscles and other tissues. This article examines the actions that occur during stance.

STANCE

Following foot fall the foot remains stationary on the ground, anchored by friction, while the COG passes over the point of support. Then as the COG continues forwards the hip extends until lift-off. We will use the term early stance to describe the period from foot fall to point where COG passes over point of support, and late stance for the period from the passing of the COG over point of support to lift off.

The preceding article in this series discussed the mechanisms by which a substantial portion of the energy of impact at footfall is converted to elastic potential energy, stored in quadriceps, calf muscles and the connective tissues of the foot. During late stance the elastic potential energy is recovered and contributes to the generation of the impulse required to get airborne at lift off.

 

While the main task that must be performed during stance is the generation of the upwards impulse required to get airborne, there are also a number of important subsidiary actions, including:

– generation of horizontal GRF.

– subjecting the hip flexors to eccentric contraction preparing them for the task of accelerating the leg forwards in early swing phase

– subjecting the hip rotators to eccentric loading in preparation for the rotation around the long axis of the body during swing phase

– preventing the unsupported hip from dropping and tilting the pelvis.

These actions are mostly performed automatically when running, but understanding them is important to allow us to develop strength and resilience in the muscles and other connective tissues, and to identify faults that might cause injuries. In particular, it is important to identify the muscles that undergo a large change in length during the gait cycle, as flexibility exercises might profitably be employed to maintain as these muscles and their tendons in a pliant state. In contrast, those muscles which exert large forces over relatively short distances are generally better maintained in a stiffer state.

 

Generation of the upwards impulse.
The only upwards directed external force acting on the body is the vertical component of Ground Reaction Force (vGRF) and hence this force is responsible for lifting the body. vGRF is a reaction by the ground as it resists compression by a downwards directed force exerted though the foot. It should be noted that pulling the foot towards the hip (like pulling on ones own boot-straps) cannot lift the body.

About 50% of the energy required to become airborne might be derived from elastic recoil of quads and calf muscles. The knee had been initially slightly flexed at footfall and then flexed even more as the impact energy was absorbed in early stance; as this flexion occurs the quadriceps undergo a moderate eccentric contraction , then in late stance, recoil augmented by the concentric contraction of the quadriceps straightens the leg, and imparts an upwards impulse to the body.

Similarly, the recoil in the calf abolishes the mild dorsiflexion of the ankle that had developed by midstance and re-establishes the mild degree of plantar flexion present at footfall. The pronation of the foot in mid-stance is replaced by slight supination.

 

Generation of horizontal impulse
As the hip extends in late stance, the predominantly downwards directed forces exerted by the leg on the ground inevitably have a small backward directed component, which is resisted by friction, thereby generating a forward directed horizontal GRF. This will propel the body forwards against wind resistance – but unless there is a strong head wind, this forward propulsion is usually excessive and the excess must be matched by braking in early stance (as discussed when we addressed the question of where the foot must land at footfall). Apart from the contribution that overcomes wind resistance, the impulse due to horizontal GRF does not achieving any useful purpose. Therefore this impulse should be minimized as far as is feasible by lift-off from stance which is as rapid as possible (bearing in mind that stance must be long enough to allow the generation of adequate vertical impulse without necessitating very high and potentially damaging vGRF

 

Preloading the hip flexors
Immediately after foot fall the hip extensors (mainly gluteus maximus and the hamstrings) will undergo a degree of eccentric contraction as impact forces are absorbed. In mid and late stance, the elastic energy stored during eccentric contraction will be released in conjunction with moderate concentric contraction of the hip extensors. In addition there will be a strong impetus to passive extension of the hip as momentum carries the COG forwards beyond the anchored foot. Thus there is an almost effortless extension of the hip which stretches the hip flexors, priming them for a powerful concentric contraction in early swing phase that will help propel the foot and leg forwards to overtake the torso. Because the hip flexors, predominantly psoas undergo a major change in length, these muscles should be maintained in flexible, pliant state. The hip flexors are perhaps the most important muscles for a runner to maintain in a flexible, pliant state.

Preloading the hip rotators
In late stance, as the torso moves forwards leaving the support foot behind, not only is there passive extension of the hip but there is also a passive (external) rotation of the hip about the long axis of the body because the hip on the supported side is tethered via the leg to the ground while the other hip is free to move with the torso. Thus the internal rotators of the hip are subjected to an eccentric contraction which primes them for an internal rotation after lift-off that will help increase stride length.

 

Supporting the pelvis
The unsupported hip tends to drop, causing the pelvis to tend swing inwards towards the supporting leg. This adduction of the hip must be opposed by the hip abductor muscles of the supporting leg. The main abductors are gluteus medius and gluteus minimus, assisted by tensor fascia lata (TFL), a long muscle running down the outside aspect of the thigh from the rim of the pelvis to attach to the tibia via the iliotibial band (ITB). If gluteus medius is weak, TFL is called upon to bear too much of the load in prevent the pelvis from tilting. Excessive tension of the iliotibial band creates a risk of friction at the point where the ITB passes adjacent to the bony protrusion (femoral condyle) on the lateral aspect of the knee. Excessive eversion of the ankle, which tilts the tibia (shin bone) outwards at the knee also increases the friction on ITB. Increased friction may lead to painful inflammation (ITB syndrome).

 

 

In summary

The quads and calf muscles undergo eccentric contraction in early stance thereby storing much of the energy of impact as elastic energy. In late stance, recoil of these muscles, aided by moderate concentric contraction provides the impulse required to accelerate the body upwards against gravity. The other major muscle action during stance is hip extension. Some of the impact energy is absorbed in an eccentric contraction of the hip extensors (gluteus maximus and hamstrings) which subsequently recoil while undergoing concentric contraction in late stance. Hip extension is further promoted by passive extension generated by the momentum of the torso. The resulting hip extension pre-loads the hip flexors preparing them for a powerful hip flexion to accelerate the leg forwards after lift-off. Other important actions are external rotation of the hip that preloads the internal rotators, and hip abduction that prevents tilt of the pelvis.

In the next article in this series we will examine the actions occurring at lift-off and during swing phase.

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