How fast off stance? – a comparison of Pirie and Pose

What is the problem?
Many runners believe that they should minimize the time spent with a foot stationary on the ground. One hears statements along the lines of ‘you are going no-where when you are on stance’. In fact that statement is very misleading and needs to be re-examined. Your grounded foot might be going nowhere, but your torso continues forwards while the foot is on the ground at approximately the same speed as while you are airborne. The race is won (or at least finished) when the torso crosses the finish line, not the trailing foot. Clearly we shouldn’t stay fixed on stance for ever, but neither should we be airborne for too long either, as free-fall under the influence of gravity is inevitable while airborne, and free-fall is very wasteful of energy. The calculations section in the side bar of this blog illustrates why the airborne period should be short but frequent. Frequent airborne periods are achieved by running with high cadence (at least 180 strides per minute). High cadence also places a very tight limit on how long we can remain on stance, but it leaves unanswered the question of what proportion of each stride should be spent on stance.

What does Pirie propose?
This issue was addressed explicitly by Gordon Pirie in chapter 3 of his book ‘Running Fast and Injury free’ (http://www.gordonpirie.com). Pirie is a very strong advocate of high cadence, but he also recommends staying on stance for a substantial period. He states: ‘This low running posture allows me to stay in contact with the ground longer, and makes it possible for me to generate more power during each contact power-phase with the ground.’ (p21).

What does Pose propose?
In contrast to Pirie, some of the other schools of thought on efficient running recommend getting off stance as quickly as possible. In particular, proponents of the Pose technique developed by Dr Nicholas Romanov (http://www.posetech.com/) emphasize the importance of attempting to get off stance very rapidly, by means of a rapid pull. This pull is facilitated by the unbalancing of the body as the centre of gravity (COG) passes over the point of support. The pull itself is executed mainly by a rapid contraction of the hamstrings.

Features shared by Pirie and Pose, and differences between them
Pirie and Pose share many features, especially the emphasis on high cadence and also the emphasis on landing with the foot travelling backwards relative to the COG so that at point of contact with the ground it is stationary relative to the ground. This minimizes wasteful and potentially injurious braking. However, it appears that Pirie and Pose make opposite recommendations regarding the proportion of the gait cycle which should be spent on stance, so it is worthwhile making a close examination of what they each recommend and evaluating these recommendations in light of the physics and physiology of running. But before doing that, it is worth considering the evidence regarding the efficiency of these two styles.

The evidence for efficacy of Pirie style
The evidence regarding the efficacy of the Pirie style is largely anecdotal evidence based on Pirie’s own achievements. He was undoubtedly fast. He broke 5 world records in his lifetime, including a spectacular breaking of the 5000m and 3000m records in 1956. In ‘Running Fast and Injury Free’ he claims to have run 240,000 miles during 45 years, with very few injuries. In 1998, The Guinness Book of Records credited him with running 216,000 miles in 40 years and described this as the greatest mileage by a human being (http://www.gordonpirie.com). Thus, anecdotal evidence, confirmed by world-record breaking performances, provides good grounds for accepting that he achieved what is claimed in the title of his book: he ran both fast and (relatively) injury free.

The evidence for efficacy of Pose
In the case of Pose, there is evidence about efficiency from scientific trials, but that evidence is equivocal. The only published study of running efficiency with Pose is a study by Dallam and colleagues published in Journal of Sports Science in 2004 (volume 23, pages 757-764). In that study 8 sub-elite triathletes, who were trained using the Pose method for twelve weeks, were compared with a matched sample of 8 sub-elite triathletes, who continued training with their usual running style. Running efficiency was assessed by measuring oxygen requirements at a particular running speed, on the assumption that a more efficient style would require less oxygen. After twelve weeks the athletes trained using the Pose style had suffered a significant decrease in their efficiency, whereas those who continued to train using their usual style do not exhibit a change in efficiency. Were the athletes properly trained in authentic Pose style? One of the investigators in this study was Dr Romanov himself, so it is hard to imagine that the Pose training could have been any more authentic. Was the impaired efficiency due to unfamiliarity with the new style? Maybe, but if there was still a problem with unfamiliarity after 12 weeks, that in itself is a problem for the practicality of learning the Pose style.

With regard to risk of injury, the evidence is indirect but it is more encouraging. Several studies, including the Arendse study performed in Tim Noakes’ laboratory in Cape Town, and published in Medicine & Science in Sports & Exercise in 2004 (Volume 36. pages 272-277), demonstrate significant reduction in stressful forces at the knee joint when running Pose style compared with either mid-foot running or heel-striking. In contrast, stress at the ankle was found to be greater, raising the possibility of higher risk of injury of the Achilles tendon and calf muscles. Again Dr Romanov was a co-investigator in this study, so it is likely that the athletes received authentic Pose training. However, the average time spent learning Pose technique was only 7.5 hours and it is possible that with more training the athletes might have learned to avoid extra stress at the ankle by allowing the heel to rest briefly on the ground, so that stress is absorbed by the longitudinal arch of the foot in the latter part of the stance phase.

So, overall, the evidence does provide some support for reduced risk of knee injury with Pose, but the evidence regarding improved efficiency as assessed by oxygen consumption is disappointing.

Ground reaction forces
The first issue to consider in deciding on the optimum proportion of the gait cycle to spend on stance is the requirement that the weight of the body be supported. Body weight pressing down through the feet elicits an upwards directed ground reaction force (GRF) that supports the body. The average value of the vertical component of the GRF during the gait cycle must be equal to the body weight. When the body is airborne there is no GRF, so the average GRF during stance must be greater than the body weight by the ratio of total stride duration to time on stance. If the body is on stance for half of the stride duration, the average GRF during stance must be twice the body weight and the peak GRF might be somewhat greater. So if the time on stance is too small a proportion of the stride duration, the forces on the foot will be intolerable. Pirie’s recommendation for a relatively long time on stance will protect the foot against excessive GRF.

What can be achieved while on stance?
Pirie argues that running consists of brief bursts of activity, while on stance, interspersed with periods of relaxation while airborne. He states: ‘Correct running should feel like a series of very quick but powerful pulses, with the arms and legs working in unison, followed by a period of relaxed flying between each power phase’ (page 24). He does not state explicitly what the brief period of activity during stance involves though his use the term ‘powerful pulses’ suggests the generation of upwards and forwards forces. Some of this might be derived from recoil of the quadriceps and calf muscles and tendons, thereby recovering energy that was stored in these structures during impact at foot fall. Pirie’s choice of the word ‘power phase’ possibly implies an even more active process, and perhaps that active process was what gave him the edge in his world-record breaking performances.

Although Pose style aims for a relatively brief time on stance, Dr Romanov argues that the destabilization of the body when the COG is forward of the point of support generates a gravitational torque that promotes lift-off from stance and helps propel the body forwards. Thus Pose also proposes utilizing the time on support, but mainly to achieve unbalancing of the body. Like Pirie, Dr Romanov also emphasizes relaxation during the airborne period.

One thing that is clear about the time stance is that once the COG is ahead of the point of support the force exerted by the foot on the ground will have a backwards directed component, so there will be a forwards directed component of the GRF that tends to push the foot forwards. Thus, the latter part of the time on stance can be used to generate force in a forwards direction.

Penalties of time on stance
The gravitational torque proposed by Dr Romanov will produce acceleration of rotation in a ‘head forwards and downwards’ direction, and if this is not corrected at some other point in the gait cycle, a face-down crash will occur after a few strides. I am not sure how or when in this occurs (see my blog of 2 Jan 08). However, irrespective of how or when it occurs, reversing of this forward and downwards acceleration will consume energy, and hence, is a potential drain on efficiency. The longer the runner remains on stance after the COG has passed over the point of support, the greater the effect of gravitational torque and therefore, the greater the energy required to prevent a face-down crash.

If the footfall occurs ahead of the COG and lift-off occurs at a similar distance behind the COG, the net gravitational torque will be zero and no energy will be required to reverse the rotational acceleration. However, landing in front of the COG will result in braking that consumes energy and is also likely to increase risk of injury.

An additional issue to consider is that, as discussed in my article on the mechanics of efficient running (see side bar), the foot appears to have a very elegant mechanism for absorbing the energy of impact and conserving it so that it can drive elastic recoil. It is likely that a time of at least 50-70 milliseconds will be required for this mechanism to be engaged, so time on stance should be adequate to allow this. However, the energy can only be conserved while the calf muscles are contracted, so too long on stance will result in inefficient energy consumption by the calf, and eventual exhaustion of the muscles.

Conclusion
The evidence suggests that the proportion of time on stance should not be very short or GRF forces will be intolerable. On the other hand, it should not be too long either, or face-down crash will ensue, unless we are prepared to risk the penalties of landing in front of the COG. The one lesson that is very clear from consideration of the mechanics of running is that cadence must be high so that the absolute value of both time on stance and airborne time in each stride are small. In this respect, both Pirie and Pose are in agreement.

We still require a better understanding of the benefits and penalties of time spent on stance. However, despite the arguments in support of Pirie’s view that time on stance should be substantial, my intuitive sense is in accord with Pose. When I run I still attempt to get the foot off the ground as quickly as possible. I think the main reason why I favour a relatively short time on stance is that this allows recovery of the stored energy in Achilles and quadriceps tendon by elastic recoil at lift-off, with minimal exhaustion of the calf and quadriceps muscles as they sustain the stored energy. However, this speculation is not directly supported by evidence

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2 Responses to “How fast off stance? – a comparison of Pirie and Pose”

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