Running, ‘the classical athletic sport’, can be considered both simple and difficult: simple, because it is an instinctive, natural skill performed at some time by all but the most unfortunate; difficult in its mechanical complexity.
No two athletes run in precisely the same way, for people vary in their anatomic structure and body proportion, in strength and flexibility, in posture (often influenced by characteristics of personality) and in their interpretation of some fundamental phase of running action. Moreover, the emphasis on particular aspects of form changes from one running event to another. The ancient Greeks were aware of this; Philostratus speaks of sprinters ‘moving their legs with their arms to achieve speed, as if winged by their hands’; and Aristotle analyses aspects of running in his On the Gait of Animals.
Yet all the many kinds of running, from the shortest sprint to races over the longest distances, share certain basic mechanical principles, a knowledge of which is helpful not only to an understanding of running itself, but in the analysis of other track and field events and sports. For many skills are derived from or are influenced by these innate running movements.
Running movement is brought about by a combination of forces: internally, muscular force, producing a change in ground reaction, as well as overcoming resistance due to muscle viscosity, the tensions of fascia, ligaments, tendons, etc.; externally, the force of gravity, the resistance of the air and the forces exerted by the ground on the runner’s shoe (to ensure that the ground can exert a maximum forward force, the runner wears spiked shoes).
Good running calls for a co-ordinated action of the entire body. However, for the purpose of movement analysis it is convenient to consider it in various parts and phases.
Compared with the locomotion of modern machines, human locomotion is cumbersome and inefficient, for it depends upon the rotation of legs and arms, i.e. approximately 47 per cent of the body’s total mass, and their moments of inertia necessitate the use of tremendous muscular force and expenditure of energy to start, retard, stop or reverse limb movement. In terms of effort economy a wheel mechanism is far better; but Nature uses only rods and levers: arms and legs.
The human body is designed for accuracy of control rather than mere mechanical efficiency, however, and its combined adaptiveness, elasticity and strength have never been equalled by machines. In particular, the three leg levers, articulated on the pelvis, adapt themselves admirably to an enormous variety of postures, efforts and movements, of which running is only one.
Running speed is the product of length and frequency of stride, their ratio changing from one phase of a race to another and from athlete to athlete. Yet these two factors are always interdependent, and maximum running efficiency exists only when they are in correct proportion, depending, mainly, on the weight, build, strength, flexibility and coordination of the runner.
Leg movement, i.e. over two strides. It divides naturally into phases of recovery, when the leg swings from the hip with its foot clear of the ground, and drive, when the foot is in contact with the ground. Both are finely co-ordinated.
Recovery phase. The instant the toes leave the ground the foot, which had been brought momentarily to rest, undergoes an acceleration, when the leg flexes at the hip, knee and ankle joints. This appears to be the result of (i) a reflex mechanism which prevents over- extension and (ii) the forward motion of the thigh causing (iii) a transference of angular momentum to the fore leg. These flexions are marked, particularly, in sprinting.
In this way the mass of the leg is brought closer to the hip axis, reducing the leg’s moment of inertia and increasing angular velocity. The work of pulling the leg mass forward and upward (mainly that of the hip flexors, supported by the abdominals) is also reduced because of the tapering of leg mass; for the arrangement of the muscles ensures that the comparatively light calf and foot, distant from the hip, are easier to accelerate (this principle also applies to the arms).
The co-ordination of both legs is so timed that the flexion of the recovery leg is greatest, and its back-kick highest, fractionally after the front foot meets the ground. The swinging thigh then begins forward-upward movement of great importance to the runner’s drive; in particular, the acceleration of this thigh increases the forward force exerted by the ground, thus increasing the speed with which the Centre of Gravity is moved away from the supporting foot.
At the limit of its forward swing (coinciding with the completion of rear leg drive, thigh movement is reversed, the leg extends from the knee joint and the foot accelerates first forward and then backward. The reversal of thigh movement (brought about primarily by the glutei and hamstring muscles) produces a transference of angular momentum and flail-like action in the fore leg; yet the movement is smooth and neuromuscularly controlled.
It is important to note that in efficient running the leading foot is never stretched grotesquely for a longer stride; stride length is the product of a driving forward of the entire body.
The recovery phase is much more the result of muscular force and control than a pendulum action. It takes longer than the driving phase and for about half the time on each stride, when both feet are off the ground, the legs recover simultaneously. This is in contrast to a walking action, in which the legs swing for not more than half the stride duration, never more than one at a time and contact with the ground is unbroken.
Driving phase. The foot lands: (i) First, on the outside edge, and with the toes pointing slightly outward. It then takes the full weight of the body at a point which varies with the runner’s speed; in sprinting, well up on the ball of the foot , almost flat-footed at very low speeds. As the body passes over the foot, the heel touches the ground lightly. These movements are an instinctive, natural phenomenon. (ii) With its leg flexed at the knee joint, ‘giving’ on impact ; knee and ankle axes are parallel. (iii) Ideally, with a backward speed, relative to the runner’s Centre of Gravity, at least equal to his forward speed over the ground. (iv) In front of the Centre of Gravity; the distance diminishes with an increase in speed.
The optimum position of the feet in running is one in which their inner borders fall approximately along a single straight line. When one foot is placed directly in front of the other, lateral balance is impaired. Too wide a spacing (sometimes due to large thighs or knock knees) encourages a ‘weaving’ running action.
Experts are not agreed on whether the leg’s backward movement can be used to ‘pull’ the runner over the foot before the ‘push’ of the drive. Some say it is impossible, while others rate it purely incidental to grounding the foot quickly for another drive. Still others recommend a deliberate pulling action.
Mechanically, pulling and pushing forces are equally efficient: e.g. it is all the same whether a railway engine pushes or pulls; but with the human machine pushing forces are much stronger. The formation of the leg is unsuited to a pulling force, for the resultant of the ground’s reaction to such a force and the upthrust of the ground on the foot cannot pass through the runner’s Centre of Gravity. Hence, to attempt to accelerate the Centre of Gravity by this means is a waste of effort. Distance runners (who, in conserving a little energy on each stride, save a great deal over the full distance) should therefore reject the ‘pulling’ theory on grounds of effort economy.
As for sprinters, they can obtain and benefit from this second impulse provided the front foot has backward motion, relative to the ground, as touchdown occurs. Obviously, such a method of acceleration becomes progressively more difficult as the athlete gathers forward speed; and at top speed there can be little—if any—relative motion as this foot contacts the track. However, whereas the distance runner permits the lower leg to swing naturally as it approaches the gound, the sprinters need for a rapid striding cadence (at a rate, in good sprinting of about four and a half to five strides per second—it quickens very slightly with a progressive reduction in the stride-impulse) demands an emphasis on getting the front leg quickly under the body for a more immediate leg drive.
As the body moves over the foot the thigh’s backward motion (relative to the hips) is momentarily retarded, while flexion increases at the knee and ankle and the heel drops to touch the ground lightly. This gives the foot more time in which to apply force against the ground and stretches the extensor leg muscles, strengthening and improving their range. The lowering of body weight, also, reduces the athlete’s moment of inertia about the supporting foot, making it easier and quicker to pivot over and beyond.
The supporting leg’s accelerating effect on the runner increases progressively as he moves forward, and the leg is able to direct more of its driving force towards the body. However, it may be that the forward force reaches a maximum and then tapers off before contact with the ground is broken. In good running, the vertical component of this force , is never more than necessary to counteract the pull of gravity on each stride; but it usually exceeds the horizontal thrust component— even in acceleration.
The gluteus maximus, hamstring, quadricep, gastrocnemius and hallucis longus muscles bear the burden of the movement. Extension originates in the stronger but slower muscles surrounding the runner’s Centre of Gravity, and is taken up at the knee, ankle and foot, in that order. All extensions end together with the driving leg ‘athletically straight’, the foot well behind the body —again pointing slightly outward, with the runner breaking contact with the inside front edge.
Eccentric movement. In a normal standing position, a man’s Centre of Gravity is situated approximately at the level of the upper third of the sacrum and, with the raising of various parts of the body, in running at times it is even higher. Relative to the athlete’s Centre of Gravity, leg movement is eccentric, i.e. ‘Off centre’.
Thus, while the force of reac tion to the effective leg drive tran slates, or projects, the body’s
Centre of Gravity, its upward thrust tends to lift, and its for ward thrust pushes forward, the corresponding hip. Likewise, in recovery, the forward-upward swing encourages a retarding and dropping of the hip on the same side of the body. In fact, hip action tends to reduce the thrust received from it or delivered to it. The greater the effective leg drive, the more powerful these tendencies are; and as, in the course of the leg drive, the direction of the thrust or lift changes, so does the proportion of these horizontal and vertical reactions.
However, this hip movement is prevented by muscles and ligaments; e.g. as the body is first supported and then driven forward by one leg, the corresponding gluteus medius and minimus muscles prevent a sagging of the opposite hip. On the contrary, the hips follow the legs.
The reaction cannot be denied, however ; if the pelvis will not assimilate the by-product of eccentric leg thrust, then another part of the body, and/or the ground, must.
Arm and shoulder action
Because of the muscular connections between the pelvis and the upper trunk (e.g. internal oblique and latissimus dorsi muscles) most of this reaction in running is absorbed by the upper body, which can be seen to twist rhythmically in opposition to the leg movement. However, a little is taken up internally and by the ground, and is therefore not apparent.
We have seen that the location of body reaction can be controlled to some limited extent. In different types of running the reaction to the horizontal and vertical components of the eccentric leg thrust is absorbed by: (i) vigorous, but properly directed, arm action ; (ii) the shoulders and arms twisting en bloc, without a pivoting of the arms about the shoulders. This action was exemplified in the style of Emil Zatopek; (iii) a combination of (i) and (ii) above, as is most common to all but the short sprinting events.
Sprinting. In sprinting the accelerations of leg movement required for a striding cadence of four and a half to five times per second (the frequency in top-class competition) and for a powerful leg thrust, are possible only when the shoulders are kept steady about the trunk’s longitudinal axis; because the trunk, with its great inertia, cannot twist and untwist with sufficient rapidity.
In good sprinting the reaction to the horizontal (i.e. twisting) component of the leg thrust is absorbed by the more easily controlled arms, and the shoulders remain steady. However, to ‘take up’ this angular momentum, the arms have to operate with sufficient force and, primarily, in a sagittal (i.e. backward-forward) plane. Force of action is indicated by their radius and angular acceleration, while their range about the shoulders denotes the time/distance of force application. (Arm action will tend to be more effective in absorbing ‘twist’ the greater its distance from the body’s longitudinal axis.)
Both forward and backward arm movements are part of a clockwise or counter-clockwise upper body twist; they work in sympathy with each other, not in opposition. During their forward swings they are kept flexed at about a right angle, giving great angular velocity and co-ordinating with the quick recovery action of the forward-swinging leg. The forward arm movement sets up a backward reaction on the corresponding shoulder, ‘absorbing’ the forward twisting which would otherwise ensue. Of particular importance here is the upper-arm movement in a sagittal plane; a slight cross-body swing of the lower arm is both natural and desirable.
The backward phase of arm action tends to thrust the corresponding shoulder forward. The arm’s effect is at first strengthened and prolonged by a natural straightening at the elbow, corresponding with the longer leverage of the driving leg on the opposite side. But towards the end of its backward movement the arm bends and speeds up again, to match the final, fast stages of leg drive.
The range of arm movement in sprinting is about as much in front of as behind the shoulder axis. It varies with the individual (e.g. thin, small arms might move through a greater arc) and, to a certain extent, from one phase of a sprint to another (emphasised, especially at the start); but, usually, the hands swing no higher than shoulder level to the front, nor more than a foot behind the hip-line to the rear.
While, in all forms of running, the primary function of the upper body is to ‘take up’ reaction to the eccentric leg drive, ‘counterbalancing’ and ‘following’ leg action, in sprinting particularly, the arms may be used to spur on the legs, which speed up and consequently add to their horizontal component of drive; for action and reaction arc interchangeable factors.
Since both arms accelerate upwards and downwards simultaneously (and, in sprinting, with values greater than gravitational acceleration) their upward movement adds to the vertical component of drive; and their downward acceleration coinciding with touch down, lessens the impact between the ground and front foot.
Moreover, by losing upward speed fractionally before the completion of leg drive, they ease the compression of the thrusting leg—and so permit more forceful and freer use of its foot and ankle. Hopper writes: ‘It is in this connection that the vital importance of timing becomes obvious; and one wonders how many pulled muscles and other troubles are due to temporary lack of exact co-ordination between leg and arm action.’
Longer distances. While wishing to maintain as high a speed as possible, those who run longer distances must conserve energy by reducing their effort and frequency of striding. Their weaker leg drive and slower leg swing in recovery develop less angular momentum than in sprinting, and a reduced striding frequency gives the trunk time to take up the reaction to this angular momentum without recourse to forceful and tiring arm movement. Arm action is ‘quieter’ and does not fully compensate; hence the relaxed flowing shoulder-twist and gentle arm movement typical of the distance runner.
Trunk and head positions
Running movement can give maximum efficiency only when the athlete is properly balanced, which depends considerably upon the correct angling of the trunk. The following are relevant factors:
The force of the leg drive and the proportion of its horizontal and vertical components. As previously maintained , for balanced running the moments of the vertical and horizontal components of drive must be adjusted about the runner’s Centre of Gravity, and, at uniform speed, can be considered constant: posture is almost erect though, when a good runner is viewed from the side, there is an illusion of a pronounced forward trunk lean when his driving leg is fully extended ; a fairer view is obtained when he is in mid-stride.
However, in acceleration (i.e. in the gaining or losing of speed) the problem of balance is complicated because of variation in the horizontal component of leg drive. In positive acceleration, for example, the faster a man runs the more difficult it is for him to exert a large force against the ground, which to him, seems to be receding rapidly; he is unable to move his feet fast enough. Thus, the force he exerts and its duration (i.e. the impulse) are successively reduced. (0n leaving the blocks a sprinter will be in contact with the ground for approximately twice as long as when both feet are off the ground. After about ten strides, however, the times will be equal; and, thereafter, will attain a ratio of between 1:1-3 and 1:1-5. Thereafter, at a maximum or near-maximum speed, while he is in contact with the ground in one unit of time he has to counteract the effect of gravity during 13 to 1-5 units—requiring an additional upward thrust of 1-3 to 1-5 times body weight. His vertical component of drive must at all times keep him off the ground for sufficient time for his legs to get into position for the next stride.)
For balance in varying accelerations a runner has constantly to alter the lever-arms of the force components by adjusting the position of his Centre of Gravity in relation to his supporting foot; this he achieves by changing the angle of his trunk. In a phase of great positive acceleration (the result of a large horizontal component of force), as in the first stride from the blocks , a sprinter needs a pronounced forward lean; hence the main justification for a crouched start. But later in his race, with a reduction in the force he can exert, he has to assume a more erect position to avoid toppling forward. (Note: Here it is a vertical component of leg-drive in excess of body weight on each stride which raises the sprinter’s Centre of Gravity progressively towards a normal running position).
By the same token, balanced negative acceleration calls for a backward lean. An exaggerated lean either way reduces the stride length and places an unnecessary strain on the muscles of the trunk.
Rotation in a sagittal plane. So far, it has been convenient to assume the line of thrust from the ground reaction on each stride always to pass through a runner’s Centre of Gravity. Certainly, the resultant effect is as if it did so, and this approach to balance in running is recommended as most practical.
As Hopper has shown, however, ground reaction, besides supporting and propelling the runner, creates angular momentum in a sagittal plane; for when the foot first meets the track both vertical and horizontal components of reaction act in front of his Centre of Gravity, tending to rotate him backwards. But later, when a large vertical component acts behind his Centre of Gravity, the tendency is for the trunk to be rotated forward’, and (particularly in acceleration), to be rotated backward again just before the foot breaks contact, when the vertical component has greatly diminished.
Hopper suggests that, to maintain the trunk in an efficient running position, the legs and arms ‘take up’ these angular momenta. Thus, with the line of thrust from the ground in front of the runner’s Centre of Gravity , foreleg and forearm movements! possess a counter clockwise angular momentum; and with this ground reaction behind, their effect is reversed. Finally, this ‘absorbing’ process is reversed yet again.
But Hopper says, ‘… the transmission of a big force from the ground to the body of the athlete will take place only when the line of its action passes close to both hip and the man’s Centre of Gravity; so it is not surprising that the maximum ground reaction developed… in running does not occur until these conditions are fulfilled.’ position of the Centre of Gravity to the force and direction of ground reaction.
Posture. Hip mobility is of special influence in determining the angle at which the trunk is held in running, because unusual flexibility in these joints enables an athlete to adjust his balance while maintaining a more upright position.
Other postural idiosyncracics can give an illusory, as opposed to real, trunk angle. Round shoulders and a tendency to stoop create an appearance of pronounced forward lean in running , while pigeon-chested or hollow-backed athletes appear to run with an upright carriage.
Posture, a product of heredity, environment and self-expression, is acceptable in an athlete if it permits the proper functioning of respiration, circulation, digestion, etc., and involves no unnecessary tensions or restrictions. In considering its relation to running efficiency these should be the only criteria.
Air resistance. As an athlete runs the resistance of the air not only requires him to do work which, in consequence, restricts his speed; it can also impair his running position; in particular, at top sprinting speed or when running into a strong head wind, air resistance tends to straighten the trunk. Under such circumstances, therefore, he maintains balance by shifting his Centre of Gravity sufficiently far forward to counteract the tendency to rotate backwards. He leans well forward into a strong head wind and is more upright with a following wind. The need for greater emphasis on horizontal force when air resistance is increased has been mentioned already and is illustrated diagrammatically.
Head. By virtue of its weight (approximately a fourteenth of the total mass of the body in an adult) and position on the spine, movements of the head can have considerable effect on other parts of the body. Hence the expression ‘The head is the rudder of the body’.
As a general rule it is better for the balance of the runner for the head to be kept in natural alignment with the shoulders, with the eyes directed to that end. However, a twisting of the head from side to side (i.e. in a horizontal plane) need not upset balance and may even be necessary, sometimes, in middle- and long-distance running.
The effects of a poor head position are often to be seen towards the end of a race, when runners are tiring; throwing it back straightens the trunk and shortens the stride.
Expenditure of energy
For practical coaching purposes the techniques of athletics are best studied through the concept of momentum , since accurate measurement of total mechanical work in athletics is always difficult and is frequently impossible. For this reason, physiologists prefer to analyse in terms of energy, calculating directly from the amount of heat produced during exercise, or indirectly from oxygen consumption and carbon dioxide elimination. Although physiological techniques do not fall within the scope of this article none the less, they are closely related to the mechanics of running; the following information should be useful to coaches and teachers as a background to their study of man, the running machine.
As energy expenditure in running is directly proportional to the square of the speed, there can be no optimum running speed. It has been calculated that the rate of oxygen usage in an average sprinter running ‘all out’, provides energy at the rate of 13 horse-power.
In all forms of running some energy is spent on working against air resistance; as a man runs he drives part of the air to one side, and either carries along or pushes more of it in front of him. This requires work, diminishing his kinetic energy and, therefore, his speed. The force of this air resistence varies as the square of the runner’s speed and is therefore greatest in sprinting. It has been estimated that, in still air, at a speed of 35 ft per second (the top speed of a good sprinter) the force of air resistance is about 3-58 lb.
When an athlete runs, his Centre of Gravity undulates continuously. Off the ground it moves up and down, and while he is in contact energy is used to stop the downward movement of the Centre of Gravity and to give it upward movement again. Because the foot makes contact with the ground almost directly below the Centre of Gravity, the retardation of downward movement can be expected to exceed the following upward acceleration, which occurs when the Centre of Gravity is in front of the foot. If this is so, the time spent in retardation is less than that of acceleration.
When the athlete is not in contact with the ground the vertical movement of his Centre of Gravity is, of course, regulated by the force of gravity. Here, again, the periods spent in upward and downward movement are most probably unequal.
Much more total energy is expended in producing and destroying the kinetic energy in the limbs. Each foot is brought to rest about every sixteen feet and the remainder of each leg is slowed down and speeded up in a continuous cycle of movement. In addition, as the movement is mainly rotary, the direction is continually changing. This means that there is a continual change in the momentum of the legs, and this also applies to the arms.
While the athlete has contact with the ground, part of this total change in momentum is produced by the work done by the driving leg, and accounts for much of the energy expended by this leg. However, when there is no contact with the ground, all change is produced by the transference of momentum from one part of the body to another, at the expense of energy; for there is always a loss of energy in any transference of momentum, because some is transformed into heat.
In consequence, if the work done by the driving leg is not equal to the expenditure of energy during all the movements of a running stride, the runner will slow down. This should serve to emphasise the danger of wasting energy through unnecessary movement and lack of proper relaxation.
Although reference has been made, already, to an average striding cadence of four and a half to five times per second some sprinters have used even greater frequencies. Clearly, an athlete striding only 6 ft at top speed must take as many as six strides in covering 36 ft in one second. Is such a high frequency possible without a reduction in stride length? Have we reached a stage where we can expect faster sprint times only from athletes with longer strides than
Tolan’s? Certainly, the tendency in modern sprinting is for the world’s best to stride between 7 ft and 8 ½ ft.
Leg speed in sprinting is not limited by the neuromuscular mechanism. The mass of the leg, its moment of inertia in recovery, the rate of development of kinetic energy, internal viscosity, the weight of the athlete and the angle of propulsion—these are the limiting factors.
At very high running speeds, especially, muscle movement is extremely uneconomical ; in fact, the efficiency of muscular contraction approaches only 25 per cent. By its very nature, therefore, human locomotion must be wasteful. Yet, through superior balance, relaxation and timing, trained runners can undoubtedly conserve energy and transfer momentum from one part of the body to another, so improving their efficiency. Indeed, the skill accomplished in many sports consists largely of concentrating momentum where it is wanted without unnecessary waste of kinetic energy.
In contradiction to Professor A. V. Hill’s original hypothesis that the fastest time for a given middle or long distance could be attained by running at a constant speed, some physiologists now suggest that the second half of, for example, a mile, should be run faster than the first, with the athlete conserving his anaerobic (i.e. oxygen debt) reserves until comparatively late in the race. And this latter opinion appears to have been substantiated, practically, in the running of a majority of sub four-minute miles so far.
From the point of view of energy expenditure, the writer thinks it very probable that Professor Hill’s conclusion is still valid—but that running with a uniformly accumulating load of oxygen debt, and its chemical consequences, has a more deleterious effect on performance than a less efficient use of energy.