Newton’s third law is commonly called action– reaction, as in ‘For every action in movement there is an equal and opposite reaction.’ In other words, for every force exerted on an object there is an equal, opposite and simul- taneous force. When you apply a force to an object, you don’t have to wait around for the reactive force; it happens simultaneously.
For example, to generate acceleration in running, the athlete pushes his or her foot into the ground; the ground reaction force is
equal and simultaneous. Newton’s first two laws explain the need to apply large forces in minimal periods for acceleration; the greater the force applied in the necessary ground contact period, the greater the acceleration. Newton’s third law indicates the importance of technique in a movement, because technique determines the direction of the resultant force and therefore the success of the skill attempt.
The importance of this law can be seen through analysis of the required pattern for forces generated during linear running, from the acceleration phase through a transition phase and into high-speed (maximum veloc- ity) running mechanics. During this time, the magnitude of the impulse needs to send the athlete back into flight (i.e., working against gravity to achieve an unsupported phase of a stride cycle, with neither foot in contact with the floor) for sufficient time to reposition the limbs (this aspect is put into context by the technical models presented in chapter 8) and to overcome horizontal forces (e.g., wind resistance and friction forces from the ground).
During the acceleration phase of running, the athlete tries to keep the centre of mass in front of the base of support by generating forces into the ground; the reactive force from the ground pushes the body forwards. The posterior chain muscles (gluteals, hamstrings) extend the hips, the quadriceps extend the knees, and the gastrocsoleus muscles extend the ankles to push fully into the ground. The reactive force from the ground accelerates the centre of mass forward along the line of direc- tion of the ground reaction force.
This action drives the athlete into a fully extended (trunk, hip, knee, ankle) position illustrated in figure 4.6. The other leg moves into the triple-flexed position in preparation for the next ground contact with the shins at the same angle so that the pushing force will be in the same direction. The effect of this move- ment, and the actions of the arms, is explored later in this chapter with the consideration of rotational forces.
In acceleration, the athlete has a longer contact time relative to maximum velocity running, so there is more time to reach peak force. Large components of this force can be E5649/Brewer/fig 04.05/542734/HR/mh-R2 Athlete A Athlete B RFD Athlete A Athlete B Peak force Athlete A Athlete B Force applied in skill execution Athlete A Athlete B Explosive strength deficit Time Time constrained application of force F or ce
distributed to generating a significant hori- zontal force (the reaction force in figure 4.6). The centre of mass will accelerate forwards with each step so that the athlete’s body gains momentum, which is important in track accel- eration and in other sport contexts.
During acceleration, vertical forces will be smaller than they are in top-speed running because of the decreased falling pattern of the centre of mass and a much shorter flight phase. Imagine an athlete performing a wall sprint, an acceleration technique drill explained in chapter 8. The aim of this drill is to complete a driving action with the legs so that the shins of both legs are at the same angle when one leg is fully flexed and the other leg is fully extended. When the shins are at different angles, the vertical component of the resultant ground reaction forces is greater than the horizontal compo- nent. This less-than-optimal positioning causes the athlete to march up the wall during the wall sprint drill (i.e., the feet move closer with each successive action to the increased vertical forces). In a running action, the athlete would adopt a position that is too upright too early in the action, decreasing horizontal momentum and deviating from the optimal technique.
As the athlete’s velocity increases, he or she transitions into maximum velocity. At this phase, the athlete’s centre of mass has a lot of
horizontal momentum propelling the body for- wards. At maximum velocity, the body position is much more upright and the ground reaction forces are predominantly vertical (figure 4.7). The athlete has very short ground contact times (less than 0.08 seconds in an elite sprinter), and all force production is used to overcome the downward force of gravity. The athlete can therefore keep the centre of mass high and maximize stride length, the distance between the centre of mass at the point of touchdown on one foot to the position of the centre of mass at the point of touchdown on the other foot.
Faster sprinters do this by applying higher peak forces during a shorter contact period (i.e., the same total impulse is generated but in a shorter time). The front leg does not reach, and the ankle joint does not push off in this phase, because the centre of mass needs to be above the foot at ground contact. Trying to perform either of these actions results in over- striding, which slows the horizontal velocity of the centre of mass because the foot contacts too far forwards. Stride length is therefore effectively determined by the athlete’s ability to generate vertical impulse (practical application of Newton’s second and third laws) and the athlete’s limb length.
For example, when he broke the world 100-metre record (9.58 seconds) Usain Bolt E5649/Brewer/fig 04.06/542736/mh-R2 Vertical Horizontal Time F orce
Parallel shin angle Action force: Push into the blocks
Reaction force
Resultant line of f
orce application
Figure 4.6 The shin angle in sprinting acceleration determines the line of action in reactive force. When
the shin angles are correct, the shin of the unsupported leg is prepared to drive into the floor at the same angle as the driving leg.
was able to maximize his ground contact time without increasing it beyond a critical period to generate sufficient vertical ground reaction forces to propel his centre of mass forward with an average stride length of 2.47 metres per step. (At 93 kilograms and 1.96 metres tall, Bolt is both heavy and tall for a sprinter.) This average stride length masks the effective maximal stride length; in the 41 steps that he took in the race, those taken during the acceleration phases were smaller (1.43 metres for 0 to 10 metres; 2.38 metres for 10 to 20 metres) than at the later stages of the race (2.94 metres for 80 to 90 metres). Throughout the race Bolt had an average step frequency (cadence) of 4.23 steps per second.5
His maximum stride length of 2.94 metres is a product of his long legs and his ability to generate high vertical impulses in ground contact.