Forcetime Principle

The Force-Time Principle for applying bio-mechanics is not the same as the Force-Time Relationship of muscle mechanics. The Force-Time Principle states that the time available for force application is as important as the size of the forces used to create or modify movement. So the Force-Time Principle is concerned with the temporal strategy of force application in movements, while the Force-Time Relationship (electromechanical delay) states a fact that the tension build-up of muscle takes time. The electromechanical delay is clearly related to how a person selects the appropriate timing of force application. The Force-Time Principle will be illustrated in using forces to slow down an external object, and to project or strike an object.

Movers can apply forces in the opposite direction of the motion of an object to gradually slow down the object. Movements like catching a ball or landing from a jump (Figure 4.16) employ primarily eccentric muscle actions to gradually slow down a mass over some period of time. Positioning the body to intercept the object early allows the mover to maximize the time the object can be slowed down. How does a gymnast maximize the time of force application to cushion the landing from a dismount from a high apparatus? Near complete extension of the lower extremities at touchdown on the mat allows near maximal joint range of motion to flex the joints and more time to bring the body to a stop.

The primary biomechanical advantage of this longer time of force application is safety, because the peak force experienced by the body (and consequently the stress in tissues) will be lower than during a short application of force. Moving the body and reaching with the extremities to maximize the time of catching also has strategic advantages in many sports. A team handball player intercepting the ball early not only has a higher chance of a successful catch, but they may prevent an opponent from intercepting. The distance and time the ball is in the air before contacting the catcher's hands is decreased with good arm extension, so there is less chance of an opponent intercepting the pass.

Imagine you are a track coach whose observations of a discus thrower indicate they are rushing their motion across the ring. The Force-Motion Principle and the force-velocity relationship make you think that slowing the increase in speed on the turns and motion across the circle might allow for longer throws. There is likely a limit to the benefit of increasing the time to apply because maximizing discus speed in an appropriate angle at release is the objective of the event. Are there timing data for elite discus throwers available to help with this athlete, or is there a little art in the application of this principle? Are you aware of other sports or activities where coaches focus on a controlled build-up in speed or un-rushed rhythm?

So there are sometimes limits to the benefit of increasing the time of force application. In movements with high demands on timing accuracy (baseball batting or a tennis forehand), the athlete should not

Figure 4.16. The extension of the limbs before contact and increasing flexion in landing increases the time that forces can be applied to slow down the body. In chapter 6 we will see that this decreases the peak force on the body and decreases the risk of injury.

maximize the time of force application because extra speed is of lower importance than temporal accuracy. If a tennis player preferred a large loop backswing where they used a large amount of time and the force of gravity to create racket head speed, the player will be vulnerable to fast and unpredictable strokes from an opponent. The wise opponent would mix up shot placements, spin, and increase time pressure to make it difficult for the player to get their long stroke in.

Suppose a patient rehabilitating from surgery is having difficulty using even the smallest weights in the clinic. How could a therapist use the Force-Time Principle to provide a therapeutic muscular overload? If bodyweight or assistive devices were available, could the therapist have the patient progressively increase the time they isometrically hold various positions? While this approach would tend to benefit muscular endurance more so than muscular strength, these two variables are related and tend to improve the other. Increasing the time of muscle activity in isometric actions or by modifying the cadence of dynamic exercises is a common training device used in rehabilitation and strength training. Timing is an important aspect of the application of force in all human movement.

In studying the kinetics of human movement (chapters 6 and 7), we will see several examples of how the human body creates forces over time. There will be many examples where temporal and other biomechanical factors make it a poor strategy to increase the time to apply force. In sprinting, each foot contact has to remain short (about 100 ms), so increasing rate of force development (Force-Motion Principle) is more appropriate than increasing the time of force application. It is important to realize that applying a force over a long time period can be a useful principle to apply, but it must be weighed with the other biomechanical principles, the environment, and subject characteristics that interact with the purpose of the movement.

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Responses

  • Tauno
    What is forcetime principle?
    8 years ago

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