Time on stance; recovery of elastic energy; and risk of stress fracture

On Jan 5th, I discussed the question of how long should be spent on stance, and reached the tentative conclusion that it is best to be fast enough off stance to avoid unnecessary waste of energy sustaining the isometric contraction of calf and quadriceps muscles, but not so fast as to cause dangerously high vertical ground reaction forces. Today, I want to return to this issue and attempt to make an estimate of the minimum time on stance to allow efficient and safe recovery of elastic energy. This requires a consideration of the processes by which elastic energy is distributed within the foot, and also the risks associated with metatarsal stress fracture when vertical ground reaction force (GRF) is high.

First, why do we need to spend any time in stance at all? The essence of running (in contrast to walking) is that when running we are airborne for part of the gait cycle. This allows a longer stride length and hence, for a given cadence, a faster pace. However, being airborne comes at a price. While airborne, we fall freely under the influence of gravity and therefore must use energy to recover the height lost while falling. Furthermore, if we let our bones absorb the impact force arising from the free-fall, we would produce heavy jarring and inevitable damage to bones and joints. Fortunately the human body has a well developed mechanism for absorbing energy of impact in muscles and ligaments in early stance, and then releasing this in late stance, thereby improving efficiency and lowering the risk of injury. I believe that one the major goals of developing an efficient running style is adjusting the time on stance to optimise this process of storing and recovering the energy of impact efficiently and safely.

 

Absorbtion of the energy of impact

If we land with slightly flexed knee and with the heel off the ground, impact will stretch the quadriceps and the calf muscles. If we are to avoid risk of tearing these muscles, and the tendons that attachments them to bone, the impact force must be absorbed gradually. Furthermore, to avoid too much stress on the Achilles tendon, it is almost certainly necessary to let the heal touch the ground in mid-stance so that part of the energy can be stored in the ligaments that maintain the longitudinal arch of the foot. Impact tends to occur on the outer edge of the foot, because of the inclination of the leg necessary to ensure the foot is near the mid-line at foot strike, so the first action after foot-strike is pronation, a rolling the foot so that the weight is transferred towards the inner edge and onto the longitudinal arch. Then the heel drops allowing the arch to take some of the load. How long does this take? The force plate data for mid-foot runners collected by Cavanagh and LaFortune (Journal of Biomechanics, 1980) indicates that this process takes somewhere in the order of 40-50 milliseconds.

Preparation for lift-off

How long does it take to recover this stored elastic energy in the latter part of stance? One way to answer this question is to invoke the principle that we need to maintain GRF as near to constant as is possible while on stance to avoid sharp and potentially injurious peaks in GRF. As the amount of upward impulse delivered in the latter part of the stance period must equal the downward impulse that follows foot-strike, if we want to maintain near constant GRF, then the time spent developing the impulse that promotes lifting-off should be similar to the time spent absorbing the energy at impact. (Impulse is given by the product of force by time. If force is to be near constant, the time interval should be similar). So the minimum time for the second phase of stance should also be around 40-50 milliseconds. This limit might be over-ridden by a very sharp pull from the hamstrings but if we do this, we might fail to use elastic recoil fully.

Ground reaction forces

So far, we have estimated that we should aim to spend a total of at least 80-100 milliseconds on stance (It should be noted that these estimates are only approximate, but they allow is us to explore the principles.) However, we must also consider the influence of time on stance on the average value of the vertical GRF. As discussed on my article on the mechanics of running (see the side bar) the average GRF over the entire gait cycle must be at least equal to the body weight, if the body is to be supported. Therefore, the average GRF over the time on stance must be greater than body weight by the ratio of total duration of the gait cycle to the time on stance. At a cadence of 180 strides per minute, the stride duration is 333 milliseconds, so a time on stance of only 100 milliseconds would generate an average GRF of over three times body weight, and the peak GRF might be somewhat higher unless the force was maintained at a very uniform level throughout stance.

Stress fracture

What are the likely consequences of such high values of GRF? One issue to consider is the risk of metatarsal stress fracture. Metatarsal stress fracture is a relatively common injury in army recruits (‘march fracture’), dancers and runners.

It is instructive to consider march fracture. Traditionally, this was observed in new recruits to the army who are required to march for long distance carrying a backpack. In a soldier marching with a back-pack of 50 lbs, his effective weight is increased by about one third. When marching, one foot is always on the ground, so peak GRF is unlikely to rise by more than about 30-50% compared with standing still. Therefore, overall GRF is probably no more than twice that when standing still on one leg without a back-pack. A force of this magnitude applied briefly on one occasion would not be expected to cause a fracture of a metatarsal. So why is the new recruit at risk of march fracture?

The major issue is that stress fracture occurs from repeated application of a force that is substantially less than that required to break the bone during single impact. In this respect, it is analogous to the metal fatigue that caused ships in the Atlantic convoys during World War Two to break up and sink in mid-Atlantic. It was the repeated application of the relatively minor forces associated with bucking over the ocean waves that did the damage. Bone also suffers fatigue and fracture after repeated relatively minor stress.

The first important conclusion is that GRF of no more than twice body weight might cause stress fracture, at least in new recruits, when it is applied repetitively during a long march. However, it is recognised in military circles, that it is the new recruits who are most at risk, not the seasoned veterans. It is probable that for new recruits, muscles such as the peroneal muscles became less able to sustain the arches of the foot as they become more tired, so the ability of the foot arch to distribute the load is diminished. Perhaps even more important is the fact that bone can adapt to repeated stress by redistribution of bone mass, thereby effectively providing internal struts that increase the capacity of the bone to bear weight. Thus it is probable that seasoned soldiers are less at risk from march fracture because they have developed greater muscle and bone strength.

This discussion of march fracture makes it clear that we would be very unwise to attempt to run long distance with GRF much greater than twice body weight until we have developed strong muscles and bones. At cadence 180 strides per minute, this would require a time on stance of about 165 milliseconds. This is at least 60 milliseconds longer than the minimum we estimated for the distribution and recovery of elastic energy in the foot, while on stance. During this extra 60 milliseconds, the quadriceps and calf muscles will consume energy maintaining isometric contraction, thereby decreasing efficiency. We could of course decrease GRF for a given duration on stance by increasing cadence, but maintaining a very high cadence fro a long period will require well developed muscles.

Conclusion

So where does this leave us. First of all, it is clear that aiming for times on stance of less than around 165 milliseconds for long distances is likely to create a substantial risk of metatarsal stress fracture unless muscular strength and bone strength have been developed. Maybe no sensible runner would attempt this, so this cautionary tale might appear unnecessary. However, at this stage, we know very little of how much training is required to develop adequate muscle and bone strength to sustain repeated impacts of three times body weight or more over long distances. Almost certainly we can afford to spend less than 165 milliseconds on stance once we have acquired reasonable fitness. What target should we aim for?

A reasonable goal for Pose Method runners is sometimes suggested to be around 132 milliseconds (4 video frames at 30 frames per second). As the members of the PoseTech forum (http://posetech..com) will be aware, one of the most experienced Pose coaches who advises on that forum suffered a stress fracture despite several years of substantial training and drilling. Maybe in this case there was some incidental factor that is unknown to me. However, this instance, together with the fact that metatarsal stress fractures are recognised to be a relatively common injury amongst runners would suggest that we should be cautious about aiming for time on stance less than 130 milliseconds when running long distances, unless we are confident that we have very well developed strength in muscles and bones. We might pay a small price in extra energy consumed in maintaining isometric contraction in quads and calf muscles, but in my opinion, this price is probably worth paying. Furthermore, it should be borne in mind that if time on stance is much shorter than this, we might suffer inefficiency due to failure to fully recover stored elastic through recoil. In my view, a simple recommendation to spend as little time on stance as possible without consideration of these issues is misleading and potentially dangerous.

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3 Responses to “Time on stance; recovery of elastic energy; and risk of stress fracture”

  1. Simon Says:

    I agree completely with your underlying calculations for GRF as you probably know, but I think your conclusions are skewed a little heavily towards injury prevention rather than performance in running. The aim is to be as efficient as possible in my view; so 2 frames (66ms) of absorption and 2 frames to release seems good to me for distance running in terms of elastic stored strain energy. Many runners can do this for marathon distance without suffering stress fractures.
    In running, it is the responsibility of the runner to allow sensible time for full recovery and to stop running when exhausted. I suspect the army recruits would be fine if the marches were not forced to the point of exhaustion and they had suitable rest between marches.
    Where particular care should be taken is when the runner suspects defects in form or in bone strength.

  2. canute1 Says:

    Simon, Thanks for your comment. I think we agree not only about the calculations of GRF but also about the advice. In my article, I recommend 130 ms on stance (whereas you recommend 2×66 milliseconds) as a reasonable target for a fit runner. If a relatively unfit runner wanted to run a marathon (as happens in some of the big city marathons) I would recommend that they aimed for 165 milliseconds on stance.

    I also agree that it is the responsibility of the runner to avoid over-stretching him or herself. However, it is the duty of the coach to point out when giving advice such as ‘spend as little time on stance as possible’ that this statement should be interpreted in light of the potential risks associated with high GRF .

  3. Average Weight » Time on stance; recovery of elastic energy; and risk of stress … Says:

    […] Here’s another interesting post I read today by canute1 […]

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