Mechanisms of shock attenuation via the lower extremity during running

Clinical

Biomechanics

1989;

4: 51-57

Review Paper Mechanisms of shock attenuation via the lower extremity during running D J Pratt, Orthotics

PhD

and Disability

Research

Centre,

Derbyshire

Royal Infirmary,

UK.

Summary Due to the increase in popularity of running or jogging as a pastime there has been an associated increase in injuries. Evidence of a causative link between excessive shock during running and these injuries has prompted a great deal of research interest centred around the reduction of the shock. This paper outlines the various ways in which shock is both produced and attenuated by the body. These include both active (proprioception, joint position, muscle tone) and passive (elasticity of bone, cartilage, synovial fluid and soft tissues) mechanisms and both are affected by the style of running. Artificial shock attenuation via footwear and/or insoles is discussed and the balance between shock attenuation and rearfoot control mentioned. It is found that as shock attenuation capacity increases, rearfoot control decreases, and this causes other injuries due to the lack of control. Therefore a balance has to be found between the two and this is suggested as an area for further research. The design of running shoes is examined and the sole shape and material is discussed with regard to improving running performance and reduction of injury. Key words: Running, shock, attenuation,

footwear,

insoles

Introduction Shock is described in the 0.vfor.d English Dictionary as a ‘violent concussion or impact’ and has been considered to be the cause of many kinds of injuries from running. The rapid rise in interest in running by a large number of people has produced much pleasure and many benefits, particularly in terms of cardiorespiratory fitness. Unfortunately, it is also responsible for a large number of orthopaedic problems, many of which are notoriously difficult to treat’-“. This paper does not intend to examine these injuries in detail, but will mention them as appropriate during its discussion of shock during running and the effects of various parameters upon its mediation. Running differs from walking in that there is no period of double support. This has profound effects on the biomechanics of locomotion beyond simply enabling us to move faster. For this reason, running and walking at the same speed feel totally different. The runner’s foot hits the ground between 800 and 2000 times a mile, and at each impact a force of 2.5-8 times body weight is transmitted to the body4. This comSuhmirred: 19 May 1988 Correspondence and reprint

requests to: Dr DJ Pratt. Orthotics

Disability Research Centre, Derbyshire Road. Derby DE1 2QY. UK.

Royal

0 1989 Butterworth & Co (Publishers) Ltd 0268-0033/89/01051-07.SO3.00

Infirmary,

London

and

pares with about 1.25 times body weight transmitted during walking5. It is easy to appreciate that under such conditions minor anatomical or biomechanical abnormalities that would not trouble a walking person can cause problems to the runner. When walking the stance phase begins when the posterior part of the heel makes contact with the floor (heel strike). The picture is not so clear when running is considered, as studies have show+’ that foot contact can take place at any point on the shoe from heel to toe. Rather than try to score the position of the centre of pressure at initial contact as a means of classifying foot strike, it is suggested that a division into heel and toe strike should be used. It is conceded that this division is equally fallacious; what is called a toe strike could possibly have a centre of pressure at impact that lies in the midfoot region. However, the terms are meant to describe the functional characteristics of the foot strike rather than simply describe the position of the centre of pressure at impact. One feature of foot strike that is not in dispute is that it occurs on the lateral border of the foot. This is because most people place their left and right feet along a straight line when running in the best position to support and propel the body. Since the legs are separated proximally at the pelvis, they must be adducted to enable a midline foot strike. This causes a functional varus of the foot,

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bringing the lateral border of the foot nearer to the ground prior to foot strike. This effect is seen to a greater degree in female runners, as they generally have shorter legs and wider pelvis. Foot strike occurs on the lateral border because the foot is supinated prior to contact (supination is a complex movement of the foot involving plantarflexion at the subtalar joint, adduction of the forefoot and inversion of the heel). Functionally, this causes the toe and lateral border of the foot to be brought closer to the ground which encourages foot strike to occur on the lateral border. The supinated foot prior to foot strike is thought to act as a rigid suppon at impact’. Following impact, the foot assumes a flat position by pivoting about the lateral border of the foot. The medial border is lowered to the ground by a combination of heel eversion and pronation’ (pronation is the opposite of supination, namely dorsiflexion at the subtalar joint, abduction of the forefoot and eversion of the heel). The pronated foot is thought to act as a ‘mobile adaptor’, able to adjust to irregularities in the terrain and to compensate for any anatomical deformities such as a permanent varus or valgus angulation of the leg or footxDespite its beneficial role in running, pronation has been blamed for a great number of running injuries. Excessive pronation is said to be common in runners with tibia varum or tight gastrocnemius/soleus muscle groups’ or a direct cause of chondromalacia patellae”‘. Bates et al.’ point out that pronation of the foot and flexion of the knee both cause internal rotation, whereas supination of the foot and knee extension cause external tibia1 rotation. During running the period of maximum knee flexion coincides with the period of maximum foot pronation, and this synchronous action of the knee and foot joint is particularly important. If pronation and knee flexion do not occur together, reciprocal stress is placed on both joints via the tibia. If this functional antagony is prolonged a disorder of one of these joints is likely to occur. It has been proposed in the past that heel striking is typical of running slowly and that toe striking is typical of sprinting(‘. Contemporary authors, however, have challenged this view, pointing out that at a given speed one individual might toe strike while another may heel strike. The author believes that heel strike occurs at slower speeds, and that all heel strikers go on to toe strike at faster speeds, although the speed at which toe strike is adopted depends on the individual. Natural heel strikers at low speeds often find toe striking at these speeds to be quite tiring on the calf muscles. The effect on shock transmission from both foot striking styles will be discussed later; however, the energy cost of the two styles is of interest. Nigg et al.” have studied the variation in the position of foot strike with running speed. A subject ran on a treadmill, the speed of the treadmill being increased, and the oxygen consumption and lateral shoe angle were measured. The results indicated that the runner’s oxygen consumption falls off just as he begins to change his style of foot strike. They conclude that for a faster speed, a toe strike is less energy demanding and that foot strike style is influenced by

energy consumption. It is the author’s opinion that toe/ heel strike may influence energy consumption but not \G? IV=IXU;toe strike is primarily adopted to enable the runner to run faster, as is clearly indicated by the experiment.

Mechanisms of shock absorption in the body Shock attenuation in the body occurs by two types of process, namely active and passive mechanisms. Active mechanisms include proprioception, joint position and muscle tone. Passive mechanisms are elasticity of bone, cartilage and synovial fluid, and elasticity of soft tissue, e.g. capsule, intervertebral discs and menisci. Active

mecxhartisms

Using our joints, we are able to turn our body from a rigid dead weight into a flexible system capable of attenuating the shock of impact. Using a toe strike in running is a method of active shock attenuation. As the forefoot strikes the ground, the calf muscles undergo eccentric contraction to prevent the heel hitting the floor. Some workers’? believe that an eccentrically contracting muscle is able to absorb work which can be stored and used later. An eccentric contraction also enables the absorption of shock, especially its low frequency components. As the forefoot strikes the ground and is decelerated abruptly, the rest of the body continues to descend slowly towards the ground under the control of the calf muscles. Thus instead of coming to an abrupt halt the body is decelerated more slowly, allowing the force to be dissipated over a longer period and thus the shock is attenuated. It is this shock absorbing function of the calf muscles, coupled with their role at push off, which makes the process of toe striking so tiring (Figure la). It would seem that running with a heel strike gives the foot less opportunity to actively absorb the energy, but even during heel strike the foot is able to attenuate shock. When the foot strikes the ground it is supinated and the lateral border of the foot makes initial contact with the ground. Immediately after impact the whole foot begins to pronate, which also has the effect of lowering the leg towards the ground. Pronation therefore increases the time over which the energy can be absorbed and thus contributes to shock attenuation during heel strike (Figure lb). At heel strike, however, the impact of landing is transmitted directly through the heel into the body and the calf muscles cannot act as shock absorbers. This conservation of energy takes place at the expense of shock absorption, but it seems naturally better suited to long distance running where the demands on the muscles are already great. Sprint running is different, however, as it is not a type of running that has to be sustained for long periods. The major reason for running on the toes in a sprint is to enable one to run as quickly as possible, and the additional benefit of improved shock absorption thus achieved is probably coincidental. It is possible that the increased activity of the

Pratt:

a

?$ir--g-“’

large

b Hc .

-

Hf

small

1

HcHf

Figure 1. Diagram illustrating the differences in ability of the two contact styles to lower the body mass slowly and thus reduce shock. a Lateral view. At toe strike (dotted outline) the initial height of the body mass off the ground is H, and this reduces to l-$ once the foot is flat; H - H, IS large. b Posterior view. At heel strike (dotted outline) the foot is supinated and is much smaller than in foot the contact height, H strike. This reduces to %iF once the foot is flat; H, - H, is small. Therefore, less active shock attenuation is possible on heel strike compared to toe strike.

pretibial muscles as well as knee and ankle motion occurring at heel strike serve as shock absorbing mechanisms that lessen the impact of initial floor contact. Simon et al.’ argue that the reaction time of a muscle to a stimulus (approximately 75 ms13) is too slow to enable them to react rapidly enough to absorb an appreciable amount of energy. Nigg et al.” state that muscle latency is 30 ms and therefore muscle aCEiOn can only be expected to attenuate forces applied at less than 30 Hz. Loads applied at less than 30 Hz are therefore classified as active, whereas loads applied above 30 Hz are passive. There is evidencei that the contraction of the calf muscles may occur immediately upon impact in a preprogrammed manner, allowing some shock absorption to occur. McMahon and GreenI argue that our reflexes are too slow to respond to impact and cannot be expected to participate in muscular control in the first quarter of the support phase, the leg muscles being principally under the control of higher centres at this time. Light et aLIs go on to hypothesize that if we are fatigued this higher control will be less efficient, leading to reduced shock attenuation at impact. Evidence for the role of muscle in reducing shock can be deduced from the work of Winter16. He analysed the power generation and absorption at the hip, knee and ankle of joggers and found peak absorption occurring in

of shock

attenuation

53

the knee and ankle just after heel strike. Much of this energy absorption has the effect of slowing the rotation of the body segments; however, some might also contribute to shock attenuation. Passilv

Hc Hf

Mechanisms

mec~hanisms

As well as the active mechanisms already mentioned, body tissues act as passive shock attenuating devices. Slow motion film of a runner shows the amount of soft tissue distortion that occurs following foot strike. Another example is that following impact the adult femur can lose up to 1 cm in length as a result of bowing and elastic deformation3 One much talked about property of the body’s tissues is their viscoelasticity. In other words, they simultaneously exhibit both viscous and elastic properties (Figure 2). The practical consequence of this phenomenon is that the physical response of the tissues to a given load is affected by the rate of application of the load, with less deformation as the loading rate increases. Radin et al.” compared the abilities of both cancellous bone and cartilage to absorb shock in vitro, and point out that although cancellous bone is considerably stiffer than cartilage it is still able to absorb shock. They also showed that although synovial fluid is markedly viscoelastic, joints contain such extremely small quantities (sufficient to provide lubrication) it is unable to aid shock attenuation. Surprisingly, the removal of cartilage from the joint did not seem to greatly affect its shock attenuating properties either”. Paul et al.” splinted rabbit hind legs to remove the possibility of joint movement from contributing to shock attenuation and the legs were impacted at a range of frequencies. Minimal shock attenuation was found to occur between 3 and I8 Hz but increased gradually up to almost total attenuation over a range of X0-3000 Hz. The experiment was repeated with dead and anaesthetized animals and similar results were obtained, suggesting that muscle contraction played no role in the attenuation demonstrated. Removal of the heel pad caused a 20-28% reduction in shock attenuation over the full range of frequencies. They concluded that active shock attenuation.

Figure 2. A 1 degree of freedom representation of the mechanical properties of body tissues, particularly for the heel fat pad.

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C/in. Biomech. 1989; 4: No 1

such as the movement at joints and the contraction of muscles, is active at low frequencies and that the passive deformation of the body’s soft tissues is responsible for the attenuation of shock at higher frequencies. The heel pad has been studied in detail by Cavanagh et al.“. It lies between the calcaneus and the skin, protecting the skeleton from excessive shock at heel strike. The fatty tissue in the heel pad differs from adipose deposits at other sites in the body, because it has a positive compartment pressure. Histologically the subcalcaneal fat pad is a highly specialized structure characterized by a matrix of elastic fibrous connective tissue arranged in cells?’ . septa containing closely fat packed Blechschmidt*? describes ‘U’-shaped arcades of connective tissue with their jaws embedded in the calcaneus. Transverse and diagonal strands of elastic fibres complete a compartment that is full of fat cells and it has a mean thickness in Caucasian males of 17.8 mm23. Cavanagh et al.*O investigated the mechanical properties of the heel pad in viva with a loaded pendulum recorded on high speed film. Forces of between 338 and 676 N were applied and the heel pad was found to deform by up to 1 cm. It was found that about 85-90% of the

impulse was attenuated by the leg but they were unable to say how much of this attenuation occurred directly as a result of deformation of the heel pad. About 70% of the deformation of the heel pad was not recovered immediately and this was classed as ‘permanent deformation’. When the experiment was repeated 25 times at 0.3 Hz, no significant change in attenuation or maximum deformation occurred between the 1st and 25th impact. This rate of impact is much less than would occur in running, but we can assume that the heel pad is able to maintain its shock absorbing capacity over a long period of time. Shock attenuation

and footwear

Sports physicians are unanimous in their assertion that the proper running shoe can reduce running injuries. Inappropriate footwear is seen as a major cause of running injury and the provision of a good running shoe is one of the best ways to treat these injuriesZ4. Poor shoes are blamed for patellofemoral pain, Achilles peritendonitis, and tibialis posterior tendinitisZs. There is evidence that the use of shock attenuating materials does in fact reduce injuryzh. Falsetti et al.”

Figure 3. Photographs of the shock meter showing the ankle mounted accelerometer and the Fourier analysis system worn on a belt. A digital reading of a ‘shock factor’ is produced based upon the relative areas of parts of the frequency spectrum33.

Pratt: Mechanisms studied the haematological disturbances in 23 long distance runners who were randomly assigned to a group using either a firm soled running shoe or an air cushion shoe with superior shock absorbing qualities. A full blood count was carried out before and after a 15 mile run. Runners wearing the air cushioned shoes demonstrated less haematological disturbance after the race than did the runners who had worn the firm soled shoes. Evidence for the role of heel strike transients in the pathogenesis of osteoarthritis comes from the work of Murray and Duncan’*, who showed an increase in the incidence of degenerative joint disease of the hip in adults who reported a significant level of athletic activity in adolescence. Recently a similar study” compared the incidence of osteoarthritis of the hip in adults who were either runners or swimmers at university. No difference was found between the two groups. Many workers who have evaluated the performance of running shoes have done so on the basis of rearfoot shock absorption alonejO. Cavanagh, in the annual Runners World survey”‘, tests a number of other properties of a running shoe but agrees that the shock attenuating capacity is probably the most important quality. It is not merely under the heel that shock absorption is so important?; with a large number of runners impacting with a toe strike, cushioning under the forefoot is equally important. Cavanagh, like some othersj?, evaluates shock attenuation in the laboratory by means of an impact test, where a weighted missile with an accelerometer mounted in its head is dropped onto the area of the shoe to be evaluated. However, newer techniques for gait assessment of shoes and insoles offer advantages”3 (Figure 3). Many runners choose a light, firm pair of shoes for racing as they feel that shoes with high shock attenuation slow them down in a race, while others simple do not like the feel of running on well cushioned shoes. Light et al.lT have proposed that because skeletal transients travel through the body so much faster than nerve impulses, they could form a type of feedback mechanism. This feedback of information could be disturbed by using a layer of cushioning beneath the sole that distorts and attenuates the signal. Another reason why shoes with high shock attenuation are unpopular with certain runners could be that these shoes lead to instability of the rear-foot, disturbing the support phase of the running cycle. Two major roles of a running shoe are the absorption of shock and the provision of rearfoot stability, both properties being thought essential to reduce running injuryJ4. Paradoxically, when a number of shoes were tested for these two qualities it was found that a shoe that was good at shock attenuation was poor at rearfoot control and vice tIersa, this being supported by other studies’. A more fundamental reason why runners choose special shoes for racing could be that too much shock attenuation actually slows a runner down. McMahon and GreenI have shown that the stiffness of a running track can affect the performance of a runner. They measured the spring stiffness of the legs of a number of volunteers. A special board track was constructed which was de-

of shock attenuation

55

signed so that its stiffness could be altered. Very compliant surfaces with a spring stiffness much less than that of a man were responsible for a marked reduction in a runner’s performance. According to their calculations the optimum track stiffness that would enable the highest speeds to be attained was a track with a stiffness of about three times that of a man’s legs. This represents a running surface of quite springy boards, considerably more springy than surfaces used at present. A tuned board track of three times man’s spring stiffness has been built at Harvard University and initial results show that runners are able to improve their time over a mile by an average of 2%. The runners report that the track is also very comfortable to run on and a low incidence of injury also appears to be a feature. The midsoles of a running shoe should have good shock attenuating properties. yet also be light and wear well. The material must be able to absorb rapidly applied loads and then recover quickly prior to the next foot strike. A particular problem with early materials has been the tendency to undergo compression set”5. Cavanagh’ studied how the shock attenuating properties of running shoes were maintained after 350 miles of running. Rearfoot cushioning had remained virtually unaffected by the wear but the forefoot cushioning has been reduced by.20%. but both were made from the same materials. A possible explanation for this is that during the support phase the impact on the heel is a transient occurrence but the forefoot is loaded for a much longer part of the support phase and considerable forces (up to three times body weight) are taken through this area of sole. The heel of the shoe is also considerably thicker than the forefoot, giving a much greater reserve of shock attenuation and possibly masking any deterioration in this area. These data are substantiated by other workJ”. The design of the heel itself has also attracted much attention. All modern running shoes have a raised heel wedge to allow the introduction of additional shock attenuating material. If the foot is in a plantarflexed position relative to the ground prior to foot strike, the effect of building up the heel is to reduce the angle subtended between the ground and the sole. This causes the position of impact to be more posterior than it would otherwise have been. Therefore wedging the heel encourages a heel strike which is intrinsically less efficient at absorbing shock. Raising the heel also has another effect; a thick, compliant heel will deform to a large degree and this will reduce rearfoot stability, causing injury. Other authors see a raised heel as having intrinsic merit; Bates et al.’ sees a high heel wedge as limiting the range of pronation in habitual hyperpronators and Cavanagh? sees a raised heel as limiting the stress placed on the Achilles tendon, reducing the incidence of Achilles tendonitis. The amount of shock attenuation under the forefoot in a running shoe is much less that at the rearfoot; furthermore, it has been shown that the forefoot attenuation quickly diminishes with time’. With a large number of toe strikers having been identified, surely modem shoes are not best suited to them and a shoe with better

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attenuation under the forefoot would be of benefit.

Such a shoe would require a thicker forefoot midsole and would therefore lose the overall wedge shape of its sole. further encouraging toe strike. Bearing in mind the lower

skeletal transients achieved by toe striking, might there not be a case for making all shoes in such a way? Drezjh recommends that the heel base should not be greater than 3 inches, as this increases the moment arm at foot strike, causing pronation to occur too rapidly. Perhaps there is something to be said for bevelling the lateral border of the heel, allowing pronation to occur more slowly over a shorter moment arm. There is some evidence, recently produced, to suggest that midsole hardness has no effect upon external impact forces or loading rate”‘. Further studies have suggested that no systematic variation in kinetic or kinematic variables is produced by the use of viscoelastic insoles3’. These results perhaps have to be viewed remembering that the insoles were used in good running shoes that already demonstrated high shock attenuating behaviour. It is known that the addition of good shock attenuating material to such shoes has much less effect than when added to poor shock attenuating footwear”‘. Further work is needed to formulate the precise requirements for shock attenuation and its balance with foot stability during running. More consideration needs to be given to shoe design for toe strike runners and to rigorously determine the long term effects of higher levels of shock attenuation on the body. Reducing skeletal shock excessively may have a mixture of adverse effects, so the optimum level needs to be established.

Acknowledgements The author would like to thank Mrs A M Lees for typing

the manuscript, and the Department of Medical Illustration at Derbyshire Royal Infirmary for the figures.

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