Force

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Force is the term used to describe interactions between objects causing changes in velocity. Forces are either repulsive (pushing) or attractive (pulling). There are four fundamental forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. 

Other forces that can be observed are functions of these fundamental forces. In biomechanics, these include the normal force, friction, and elasticity.

Newton’s three laws describe motion as follows: (1) objects remain at rest or continue moving at constant velocity unless a force acts upon them, (2) the rate of change of velocity of an object as a result of an applied force is proportional to its mass, and (3) when an object applies a force to a second object, the second object applies an equal and opposite force to the first.

Ground reaction forces are normal forces that are produced when objects come into contact with the ground. They are exerted by the ground in response to the action forces exerted by the object.

Force plates or force platforms measure the ground reaction forces in response to the action forces exerted by the feet of individuals who stand on them, jump onto them, or step on them while walking, running or sprinting. The forces are recorded as components in the vertical, horizontal and medio-lateral directions.

Force treadmills are treadmills with integrated force plates or force platforms that measure the ground reaction forces in response to the action forces exerted by the feet of individuals who step on them while walking, running or sprinting.

Despite concerns, force treadmills can be used to measure horizontal ground reaction forces reliably, as the internal forces within the treadmill cancel each other out, so long as the whole treadmill is mounted upon the force transducers (as in most common models).

Treadmill running involves a lower oxygen cost than overground running because there is no wind resistance. Using a gradient of 1% equates the oxygen costs of running on a treadmill and running overground.

When a familiarisation period of >6 minutes is used, joint angle movements are very similar between running at a constant speed on a treadmill and running overground, as theoretical analysis using a moving frame of reference would suggest.

Since treadmill running does not involve changes in whole-body inertia, accelerating running on a treadmill and overground are not the same and involve substantial differences in joint angle movements at the trunk, hip, knee and ankle.


CONTENTS

  1. Background
  2. Ground reaction forces
  3. References
  4. Contributors
  5. Provide feedback


BACKGROUND

FUNDAMENTAL FORCES

Force is the term used to describe interactions between objects that cause changes in velocity. Forces can be either repulsive (pushing) or attractive (pulling). Researchers working in the field of physics have identified that there are four fundamental forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. Gravitational force (also known as gravity) is an attractive force between objects that have mass. Electromagnetic force can be either attractive or repulsive (depending on the charge) and acts between electrically charged ions. The strong and weak nuclear forces describe the interactions between subatomic particles.

DERIVED FORCES

Other forces that can be observed in nature are simply functions of these fundamental forces and these can be described as derived forces. In biomechanics, there are many important forces that are derived from these fundamental forces, including the normal force, friction, and elasticity. The normal force is extremely important in biomechanics, as it describes the interaction between objects that come into contact with one another. In this context, “normal” refers to the direction of force, which is directly opposing the direction of approach of the two objects. Normal forces act when atomic particles come within range of one another and are repulsive. They therefore act in order to repel other objects directly away from them. However, they are only active within a very short range. Friction acts at right angles (perpendicular) to the normal force and describes the interaction between objects that move adjacent to one another. Again, this force opposes the prevailing direction of motion. Elasticity in biomechanics often describes a simple phenomenon that is made up of a complex set of interactions. Elastic tissues, such as muscles and tendons, display a tendency to return to their starting length after being elongated, which is the property of elasticity. However, there are many different strings of molecules involved in this process, all of which contribute to the overall elastic property of the tissue, and different materials have different bases for their elastic properties.

DESCRIBING MOTION

As explained above, forces are those interactions that influence objects with mass to change velocity. Newton’s three laws of motion together describe the way in which objects interact with one another and change velocity on account of the normal force, friction, and (most of the time) gravity. The normal force is the force that is exerted when one object comes into contact with another. Newton’s three laws of motion can be stated as follows: (1) objects remain at rest or continue moving at constant velocity unless a force acts upon them, (2) the rate of change of velocity of an object as a result of an applied force is proportional to its mass, and (3) when an object applies a force to a second object, the second object applies an equal and opposite force to the first. From these laws, a range of equations can be deduced that relate force, mass, acceleration, velocity, time, and displacement.

DIRECTION OF FORCE

In addition to Newton’s laws, it is also important to recall that since forces, acceleration and velocity are all vectors, they have a direction as well as a magnitude. For ease of calculation, forces can be broken down into vertical, horizontal and medio-lateral components using trigonometry. The overall effect of these components is known as the “resultant” force.

SECTION CONCLUSIONS

Force is the term used to describe interactions between objects that cause changes in velocity. Forces can be either repulsive (pushing) or attractive (pulling). There are four fundamental forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. 

Other forces that can be observed are functions of these fundamental forces. In biomechanics, these include the normal force, friction, and elasticity.

Newton’s three laws describe motion as follows: (1) objects remain at rest or continue moving at constant velocity unless a force acts upon them, (2) the rate of change of velocity of an object as a result of an applied force is proportional to its mass, and (3) when an object applies a force to a second object, the second object applies an equal and opposite force to the first.

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GROUND REACTION FORCES

INTRODUCTION

Ground reaction forces are normal forces that are produced when objects come into contact with the ground. They are exerted by the ground in response to the action forces exerted by the object (Lees & Lake, 2007).

FORCE PLATE

Introduction

Force plates or force platforms measure the ground reaction forces in response to the action forces exerted by the feet of individuals who stand on them, jump onto them, or step on them while walking, running or sprinting (Lees & Lake, 2007). The forces are recorded as components in the vertical, horizontal and medio-lateral directions. Generally, a force plate has four load cells, one positioned at each corner, that are constructed either using piezo-electric technology or with strain gauges (Lees & Lake, 2007). When the external action force is applied as an individual steps onto the force plate or performs an exercise such as a back squat or hip thrust while upon the platform, a reaction force is exerted by the strain gauge or piezo-electric device at each corner in order that the whole force platform remains in place on the surface of the ground (Lees & Lake, 2007).

Force plate

Components

When standing quietly, the force platform only measures small ground reaction forces in the horizontal and medio-lateral directions and the value recorded for vertical ground reaction force is essentially a measurement of bodyweight (in Newtons). When moving forwards (as during walking or running), the force platform measures larger ground reaction forces in the horizontal direction. These ground reaction forces have two different directions depending on the point in the stance phase when they are measured. In the initial part of the stance phase, there is a negative horizontal ground reaction force (called a braking force), which acts to slow the athlete down. In the latter part of the stance phase, there is a positive ground reaction force (called a propulsive force), which is where the athlete accelerates away into the next stride.

Sampling rate

Most force plates sample ground reaction force data at very high rates (between 500 – 2,000Hz), which is more than sufficient to record the information about forces produced when athletes jump, land, or run over the device. Sprint running is known to involve some of the shortest ground contact times of any athletic activity. Taylor and Beneke (2012) reported data for the 3 top-finishing athletes in the 100m World Athletics Championship final of 2009. They noted that the mean ground contact times for Usain Bolt, Tyson Gay, and Asafa Powell were: 0.091 ± 0.001s, 0.070 ± 0.001s, and 0.080 ± 0.001s. In other words, ground contact time ranged between 70 – 90ms in these athletes in this event. Sampling at a rate of 1000Hz would mean that 70 – 90 data points would be collected for these athletes per footfall if they ran over a standard force plate during a 100m sprint.

Ground contact time

The total ground contact time can be measured using a force plate, although this does not relate directly to the forces produced. Rather, it simply describes the window of time in which the individual is in contact with the ground and force is exerted. Ground contact times are thought to be important when analysing the biomechanics of sprint running, as they tend to reduce markedly with increasing running speed (Kyröläinen et al. 1999; Kyröläinen et al. 2001; Kivi et al. 2002; Kuitunen et al. 2002). Thus, the elite sprint athlete must be able to produce forces in a progressively shorter and shorter period of time, as running speeds increase and the ground contact time window reduces.

Derived variables

IMPULSE

When recording ground reaction forces over the ground contact time period, several additional variables can be calculated. By calculating the integral (also known as the area under the curve) of the data for a component of ground reaction force over the ground contact time, the ground reaction impulse can be derived. This is very commonly done when measuring horizontal ground reaction forces in running, as it allows very informative measures of the total amount of force exerted throughout each of the braking and propulsive phases of the stance phase to be produced (called braking and propulsive impulses) (Lees & Lake, 2007).

MOVEMENTS

In addition to impulse, where body mass is also known, Newton’s laws of motion can be used to calculate the expected acceleration that results from the forces applied during the ground contact period, the maximal velocity, and the displacement. This method of calculating movement-related variables is often performed when investigating vertical jumping. This is partly because the forces act in only one principal direction (vertical), which simplifies calculations and partly because where the flight time is also measured, this provides a cross-check for the calculation of displacement.

FORCE TREADMILL

Introduction

Force treadmills are simply treadmills with integrated force plates or force platforms that measure the ground reaction forces in response to the action forces exerted by the feet of individuals who step on them while walking, running or sprinting. As with force plates and force platforms, the forces are recorded as components in the vertical, horizontal and medio-lateral directions. Early versions of force treadmills simply mounted existing force plates or force platforms underneath the treadmill belt (Kram & Powell, 1989; Dingwell et al. 1996) but these devices were only suitable for measuring vertical ground reaction forces. Later versions of force treadmills mounted the entire treadmill upon a force platform structure (Kram et al. 1996). These treadmills found that ground reaction forces recorded in vertical, horizontal and medio-lateral directions were more similar to those recorded during overground running (Kram et al. 1996).

Force treadmill variations

There are many different types of force treadmill available, which have different features. Perhaps most importantly, some types of force treadmills are able to replicate overground running biomechanics to a greater extent than others (McKenna & Riches, 2007). Force treadmills can be either totally motorised, as in the case of most early versions (Dingwell et al. 1996; Kram et al. 1998) partially motorized and referred to as torque treadmills (Morin et al. 2010; Morin & Sève, 2011) or entirely non-motorised (Lakomy, 1987; Tong et al. 2001; Franks et al. 2012). The use of non-motorised treadmills generally leads to a lower running velocity than fully motorised or partially motorised treadmills, because of the additional resistance involved in driving the belt (McKenna & Riches, 2007). Additionally, treadmills can either have a single belt for both legs or use a split belt, where each leg runs on a separate belt that has its own set of force sensors attached (Tesio & Rota, 2008). Finally, where force treadmills are used to measure ground reaction forces during high speed running or sprinting, they often make use of a harness and tether to help the subjects using them to remain in place throughout the running or sprinting efforts. Guidelines are available for how to calibrate such devices for maximizing the reliability of the equipment (Padulo et al. 2014).

Validity for measuring ground reaction forces

Ever since they were developed, there has been concern over the validity of force treadmills for recording horizontal ground reaction forces correctly. Moreover, such concern persists even to this day, although much of this concern is now unfounded (Willems & Gosseye, 2013). Early models were unable to measure horizontal ground reaction forces correctly because of the impact of belt friction (Kram & Powell, 1989) and even though later models reported more sensible results because of methodological improvements (Kram et al. 1998), the values reported were still different from those measured during overground running (Riley et al. 2008). On this basis, it has been suggested that force treadmills still cannot measure horizontal ground reaction forces because of fundamental methodological principles inherent in using a treadmill. In order to assuage these concerns, Willems & Gosseye (2013) constructed a model of a treadmill showing that force transducers placed under the body of the treadmill (as is the case with all modern force treadmills) do in fact measure the ground reaction forces correctly during treadmill running, although noting that there is still a need to use filtering of the signal to remove noise generated by vibrations and by the inertial properties of the treadmill itself (Kram et al. 1998; Willems & Gosseye, 2013).

Differences between treadmill and overground running

ENERGY COST

Treadmill and overground running differs predominantly in the softness and/or elasticity of the surface used. However, it also differs slightly insofar as there is no wind resistance during treadmill running, which reduces the energy costs slightly. It has therefore been suggested that using a small gradient might be necessary during treadmill running to equate the energy cost of running overground. Jones & Doust (1996) investigated this question by exploring the oxygen cost of running overground and on a treadmill using a range of gradients from 1% – 3% at a range of submaximal running speeds. They found that a gradient of 1% very closely approximated the oxygen cost of running overground at the same speeds.

CONSTANT SPEED RUNNING

Overall, athletes tend to display similar biomechanics when running on a treadmill and when running overground at constant speeds. Indeed, excepting the nature of the surface itself, which may in many cases be softer than a typical overground surface, there should theoretically be no differences in the biomechanics of running on either surface if a moving frame of reference is used to analyze the data (Van Ingen, 1979; Willems & Gosseye, 2013). Some early studies found some differences between overground and treadmill running (Elliott & Blanksby, 1975; Frishberg, 1982; Nigg et al. 1995). Later studies have generally concluded that there are few differences of any importance (Schache et al. 2001; Riley et al. 2008). Such differences appear to be found mostly in relation to the knee joint angle in the support leg during the stance phase. Riley et al. (2008) compared overground and treadmill running at constant speeds of 3.5 – 3.8m/s in recreational runners. They found that joint angle movements were very similar in both running conditions, with only peak knee angle being slightly greater during overground running compared to treadmill running (110 vs. 104 degrees) and minimum knee angle being slightly smaller during overground running compared to treadmill running (8 vs. 10 degrees). These differences may have arisen because of insufficient familiarisation. Matsas et al. (2000) reported similar differences in knee joint angle movements between treadmill and overground walking that disappeared after a familiarisation period of 6 minutes. Lavanska et al. (2005) later confirmed that changes in joint angle movements occurred during running on a treadmill during a familiarisation period of 6 minutes. It is now generally recommended to provide familiarisation period of this length or longer for subjects when making use of a treadmill for taking measurements of joint angle movements (Padulo et al. 2014).

ACCELERATING RUNNING

In contrast to constant speed running, researchers have concluded that treadmill and overground running biomechanics differ substantially when athletes are accelerating, leading to substantial differences in joint angle movements at the trunk, hip, knee and ankle (Van Caekenberghe et al. 2013b). It is thought that the difference arises because there is no change in whole-body linear inertia when the athlete accelerates on a treadmill, whereas there is a change when the athlete accelerates overground (Van Caekenberghe et al. 2013a). This difference is thought to lead to reduced horizontal propulsive ground reaction forces during accelerating treadmill running compared with accelerating overground running (Van Caekenberghe et al. 2013a; 2013b). These differences in joint angle movements are likely reduced by using torque treadmills, which are partially motorised and drive the belt sufficiently to overcome friction but still require subjects to accelerate the belt themselves (McKenna & Riches, 2007; Morin et al. 2010; Van Caekenberghe et al. 2013b).

SECTION CONCLUSIONS

Ground reaction forces are normal forces that are produced when objects come into contact with the ground. They are exerted by the ground in response to the action forces exerted by the object.

Force plates or force platforms measure the ground reaction forces in response to the action forces exerted by the feet of individuals who stand on them, jump onto them, or step on them while walking, running or sprinting. The forces are recorded as components in the vertical, horizontal and medio-lateral directions.

Force treadmills are treadmills with integrated force plates or force platforms that measure the ground reaction forces in response to the action forces exerted by the feet of individuals who step on them while walking, running or sprinting.

Despite concerns, force treadmills can be used to measure horizontal ground reaction forces reliably, as the internal forces within the treadmill cancel each other out, so long as the whole treadmill is mounted upon the force transducers (as in most common models).

Treadmill running involves a lower oxygen cost than overground running because there is no wind resistance. Using a gradient of 1% equates the oxygen costs of running on a treadmill and running overground.

When a familiarisation period of >6 minutes is used, joint angle movements are very similar between running at a constant speed on a treadmill and running overground, as theoretical analysis using a moving frame of reference would suggest.

Since treadmill running does not involve changes in whole-body inertia, accelerating running on a treadmill and overground are not the same and involve substantial differences in joint angle movements at the trunk, hip, knee and ankle.

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REFERENCES

  1. Dingwell, J. B., Davis, B. L., & Frazier, D. M. (1996). Use of an instrumented treadmill for real-time gait symmetry evaluation and feedback in normal and trans-tibial amputee subjects. Prosthetics and orthotics international, 20(2), 101-110.[PubMed]
  2. Elliott, B. C., & Blanksby, B. A. (1975). A cinematographic analysis of overground and treadmill running by males and females. Medicine and science in sports, 8(2), 84-87.[PubMed]
  3. Franks, K. A., Brown, L. E., Coburn, J. W., Kersey, R. D., & Bottaro, M. (2012). Effects of motorized vs non-motorized treadmill training on hamstring/quadriceps strength ratios. Journal of sports science & medicine, 11(1), 71.[PubMed]
  4. Frishberg, B. A. (1983). An analysis of overground and treadmill sprinting. Medicine and science in sports and exercise, 15(6), 478.[PubMed]
  5. Funato, K., Yanagiya, T., & Fukunaga, T. (2001). Ergometry for estimation of mechanical power output in sprinting in humans using a newly developed self-driven treadmill. European journal of applied physiology, 84(3), 169-173.[PubMed]
  6. Kivi, D. M., Maraj, B. K., & Gervais, P. (2002). A kinematic analysis of high-speed treadmill sprinting over a range of velocities. Medicine & Science in Sports & Exercise34(4), 662-666.[PubMed]
  7. Jones, A. M., & Doust, J. H. (1996). A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. Journal of sports sciences, 14(4), 321-327.[PubMed]
  8. Kram, R., & Powell, A. J. (1989). A treadmill-mounted force platform. Journal of Applied Physiology, 67(4), 1692-1698.[PubMed]
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  10. Kuitunen, S., Komi, P. V., & Kyröläinen, H. (2002). Knee and ankle joint stiffness in sprint running. Medicine and science in sports and exercise34(1), 166.[PubMed]
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  14. Lavcanska, V., Taylor, N. F., & Schache, A. G. (2005). Familiarization to treadmill running in young unimpaired adults. Human Movement Science, 24(4), 544-557.[PubMed]
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  17. McKenna, M., & Riches, P. E. (2007). A comparison of sprinting kinematics on two types of treadmill and over‐ground. Scandinavian journal of medicine & science in sports, 17(6), 649-655.[PubMed]
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  21. Padulo, J., Chamari, K., & Ardigò, L. P. (2014). Walking and running on treadmill: the standard criteria for kinematics studies. Muscles, ligaments and tendons journal, 4(2), 159.[PubMed]
  22. Riley, P. O., Dicharry, J., Franz, J., & Croce, U. D. (2008). A kinematics and kinetic comparison of overground and treadmill running. Med Sci Sports Exerc, 40, 1093-1100.[PubMed]
  23. Schache, A. G., Blanch, P. D., Rath, D. A., Wrigley, T. V., Starr, R., & Bennell, K. L. (2001). A comparison of overground and treadmill running for measuring the three-dimensional kinematics of the lumbo–pelvic–hip complex. Clinical Biomechanics, 16(8), 667-680.[PubMed]
  24. Taylor, M. J. D., & Beneke, R. (2012). Spring Mass Characteristics of the Fastest Men on Earth. International journal of sports medicine, 33(8), 667. [PubMed]
  25. Tesio, L., & Rota, V. (2008). Gait analysis on split-belt force treadmills: validation of an instrument. American journal of physical medicine & rehabilitation, 87(7), 515-526.[PubMed]
  26. Tong, R. J., Bell, W., Ball, G., & Winter, E. M. (2001). Reliability of power output measurements during repeated treadmill sprinting in rugby players. Journal of sports sciences, 19(4), 289-297.[PubMed]
  27. Van Caekenberghe, I., Segers, V., Willems, P., Gosseye, T., Aerts, P., & De Clercq, D. (2013). Mechanics of overground accelerated running vs. running on an accelerated treadmill. Gait & posture, 38(1), 125.[PubMed]
  28. Van Caekenberghe, I., Segers, V., Aerts, P., Willems, P., & De Clercq, D. (2013b). Joint kinematics and kinetics of overground accelerated running versus running on an accelerated treadmill. Journal of The Royal Society Interface, 10(84), 20130222.[PubMed]
  29. Van Ingen, S. G. (1979). Some fundamental aspects of the biomechanics of overground versus treadmill locomotion. Medicine and Science in Sports and Exercise, 12(4), 257-261.[PubMed]
  30. Willems, P. A., & Gosseye, T. P. (2013). Does an instrumented treadmill correctly measure the ground reaction forces?. Biology Open, 2(12), 1421. [PubMed]


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CONTRIBUTORS

Chris Beardsley performed the literature reviews, wrote the first draft of this page and was the primary author.


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