Force

1. Force causes changes in motion, not motion itself.

Newton proposed that rather than force being the cause of an object’s velocity, rather it was the cause of the change in the object’s velocity.

Newton vs. Aristotle. Our intuition often aligns with Aristotle, suggesting force is needed to keep things moving. However, Newton's First Law states an object in motion stays in motion unless acted upon by a force. Forces are the cause of acceleration (changes in velocity), not velocity itself.

Everyday examples. When we push a car, it accelerates. When we stop pushing, it decelerates due to friction and air resistance (forces). If those forces weren't present, it would continue moving indefinitely. This distinction is fundamental to understanding biomechanics in sport.

Training implication. Understanding this means training should focus on improving the ability to change motion rapidly – accelerating, decelerating, or changing direction – which directly relates to applying forces effectively. It's not about just moving, but about changing how you move.

2. Impulse, or 'total force' over time, is the key to changing velocity in sport.

This means that in many cases we can explain differences in explosive physical performances in terms of impulse generation.

Impulse defined. Impulse is the area under the force-time curve, representing the accumulation of force over a period. It's essentially the 'total force' applied during a movement. Unlike instantaneous force, impulse directly relates to the magnitude of velocity change.

Impulse-momentum. The impulse-momentum theorem states that the impulse applied to an object equals its change in momentum (mass x velocity). Since mass is constant for an athlete, more impulse means a greater change in velocity.

• Vertical jump height depends on take-off velocity (change from zero).
• Throwing distance depends on implement velocity at release.
• Sprint acceleration depends on increasing horizontal velocity.

Performance driver. For many explosive actions, the goal is to maximize the change in velocity. Therefore, maximizing impulse production during the relevant phase of movement is often the most critical performance variable, more so than peak force or power alone.

3. Maximizing impulse involves trade-offs between peak force, RFD, and duration.

What is important to realise is that all of the methods for increasing impulse can affect each other, and so an increase in one variable does not necessarily mean that there will be an overall increase in impulse.

Three ways to increase impulse. Impulse (area under the force-time curve) can be increased by:

• Increasing peak or average force magnitude.
• Increasing the rate of force development (RFD), reaching higher forces faster.
• Increasing the duration of force application.

Context matters. The optimal strategy depends on the sport's demands. Sprinting has very short ground contact times (~0.1s), so increasing peak force and RFD is crucial. Volleyball players jumping for a block might have more time, allowing a longer force application phase.

Trade-offs exist. Improving one factor doesn't guarantee increased impulse. An athlete might increase peak force but shorten the time they can apply it, resulting in no net gain or even a loss of impulse. Measuring impulse directly is often superior to tracking isolated variables.

4. Gravity is a constant force; net impulse above body weight drives vertical motion.

If we want to propel ourselves away from the Earth we have to exert a force that is at least in excess of our weight.

Gravity's constant pull. Gravity exerts a constant downward force on us, equal to our body weight (mass x acceleration due to gravity, ~9.81 m/s²). This force is always present and must be accounted for in movement analysis.

Net impulse. When jumping or lifting, the force we apply against the ground or object must exceed gravity's pull to cause upward acceleration. The effective impulse causing upward motion is the net impulse – the total upward impulse minus the downward impulse from gravity (represented by body weight over time).

Jump height calculation. After leaving the ground, an athlete is a projectile subject only to gravity. Jump height can be calculated from take-off velocity (determined by net impulse) using constant acceleration equations, often measured via flight time on a timing mat.

5. Training specificity means matching mechanical demands, not just movement patterns.

Instead, in many cases sport specific training can still be focussed on very general qualities that are then applied in a specific way in the sporting arena.

SAID principle. The body adapts specifically to the demands placed upon it (Specific Adaptation to Imposed Demands). Effective training must challenge the capabilities needed in the sport. However, 'specific' is often misinterpreted as only mimicking sport movements.

Dynamic Correspondence. A better approach is Dynamic Correspondence, evaluating exercises based on mechanical similarity:

• Amplitude/direction of movement (kinematics)
• Region of accentuated force production (joint angles)
• Dynamics of the effort (overload relative to sport)
• Rate and time for force production
• Regime of muscular work (contraction type)

Overload and variation. Training needs progressive overload and variation to stimulate adaptation. Exercises don't need to match all criteria; they can target specific aspects of force production (e.g., high peak force, high RFD) to overload capabilities in ways sport practice alone cannot, ensuring transfer.

6. Force direction matters relative to the athlete, not just the ground.

The claim for correspondence is made by switching the coordinate frame in question for the two activities in order to fit the narrative.

Coordinate frames. Forces can be described relative to the Earth (world-fixed/global frame) or relative to the athlete's body (body-fixed/local frame). These frames are often misaligned, especially during dynamic movements like sprinting.

Sprint acceleration example. During sprint acceleration, the ground reaction force is directed forward and upward relative to the world. However, relative to the athlete's leaning body, the force is primarily directed along the line of the leg, which is mostly superior (upwards relative to the athlete's torso).

Critique of 'Force-Vector Theory'. Some argue exercises should match the force direction relative to the world (e.g., hip thrusts for horizontal force). This is flawed because it often switches frames – hip thrust force is horizontal relative to the athlete, while sprint force is horizontal relative to the world. Matching force direction relative to the athlete's body position is often more relevant for training transfer.

7. Ground reaction forces are generated through push, pull, bounce, or block strategies.

In the majority of normal movements I would say there are essentially just four fundamental strategies which we employ to exert force on the ground with our lower limb: push, pull, bounce or block.

Fundamental strategies. While movements are complex, force generation against the ground often relies on four core strategies:

• Push (Squat): Coordinated extension of ankle, knee, and hip (e.g., vertical jump, squat). Primarily active muscle force.
• Pull (Hinge): Primarily hip extension/flexion with minimal knee movement (e.g., Romanian deadlift). Primarily active muscle force.
• Bounce: Utilizing elastic recoil of tissues (muscle-tendon unit) after rapid stretch (e.g., running, plyometrics). Combines active and passive force.
• Block: Rapidly stopping momentum against a rigid limb to generate high peak force (e.g., javelin throw final step). Primarily passive force from impact.

Strategy influences training. Each strategy relies on different muscles and mechanisms. Identifying the dominant strategy in a sport skill helps select training exercises that target those specific force production methods and structures.

Continuum and blend. Real movements often blend these strategies. Athletes may also show individual preferences (e.g., more 'bouncy' vs. 'pushing' jumpers), which should inform training choices.

8. Focus on the critical force production phase, not just the end position like 'triple extension'.

Essentially, it is the momentum that the athlete accrued in the earlier part of the jump that carries them through to the fully extended position.

Observability bias. Coaches often focus on what's easily visible, like the fully extended position in jumps or lifts ('triple extension'). However, this end position is often a result of momentum generated earlier, not where peak force occurs.

Follow-through. In movements like jumping or Olympic lifting, peak force is typically produced when the joints are still somewhat flexed. The body continues extending due to the velocity gained during the main force application phase. The fully extended position is follow-through.

Misguided cueing. Cueing athletes to 'hit' full extension can be counterproductive, potentially causing them to delay peak effort or adopt inefficient strategies (e.g., slowing down to get under the bar in weightlifting). Training should emphasize maximizing force/impulse during the propulsive phase.

9. Internal forces within the body are much larger than external forces applied.

The magnitude of the forces in our muscles is generally much greater than the external force that we are able to express.

Levers and moments. Bones act as levers, joints as pivots. Muscles exert linear forces that create rotational forces, or moments (Force x perpendicular distance from pivot). Muscle attachment points are often close to the joint, giving muscles a mechanical disadvantage.

Force magnification. Due to this mechanical disadvantage, the force required from a muscle to produce a given external force is often much larger. Additionally, muscles pulling across a joint contribute to joint compression forces.

Injury implications. While a vertical jump might involve ground reaction forces 2-3 times body weight, internal forces (e.g., in the knee joint or ACL) can be 7-9 times body weight or more. Understanding these large internal loads is crucial for injury prevention and rehabilitation.

10. Muscle force production depends on length, velocity, time, and contraction type.

The one big take away from the material presented in this and the previous chapter is therefore that contraction regime is important when considering the specificity of training.

Muscle mechanics. Muscle force is generated by cross-bridge cycling between actin and myosin filaments. The amount of force depends on:

• Force-Length: Optimal force at optimal sarcomere length (overlap of actin/myosin).
• Force-Velocity: Higher force potential at slower shortening velocities (more time for cross-bridges). Highest force during lengthening (eccentric) contractions.
• Time: More time allows more cross-bridges to form.
• Contraction Regime: Eccentric > Isometric > Concentric force capacity.

Specificity of contraction. Training adaptations are specific to the type of muscle action (concentric, eccentric, isometric) and potentially the velocity and length at which force is produced. This reinforces the need for varied training methods that match sport demands.

Muscle-tendon unit. Muscle works with passive elastic tissues (tendons, connective tissue). The stretch-shortening cycle (eccentric stretch followed by concentric shortening) utilizes elastic energy storage and neural reflexes to enhance force and RFD, highlighting the importance of training the entire unit.

11. The 'force-velocity relationship' in training often confuses load, peak values, and impulse.

The point of this analysis is to show, at length, that the presentation of data showing a linear relationship between the peak force and peak velocities seen in a complete movement, where each data point is a repetition of the movement performed with a different load, is, categorically, not evidence of an instantaneous force-velocity relationship.

Common confusion. A common graph in training shows peak force vs. peak velocity for a movement performed with different loads, often presented as a 'force-velocity relationship'. This is misleading.

Peak vs. Instantaneous. This graph plots peak values from an entire movement, not the instantaneous force at a specific velocity during the movement. Peak force and peak velocity often occur at different times in a movement.

Load-Velocity vs. Force-Velocity. The graph actually shows a load-velocity relationship (heavier loads move slower) and, due to the impulse-momentum relationship, how peak force correlates with the net impulse (and thus velocity change) across different loads. It does not prove an instantaneous force-velocity relationship for the movement itself.

12. Stiffness is task-dependent and distinct from flexibility; both can be trained.

Equally, it should be emphasised that being flexible does not preclude us from being stiff.

Stiffness defined. Stiffness refers to the resistance to stretch or deformation (Force / Length Change). It describes the spring-like behavior of tissues, joints, or limbs when subjected to external force. It's highly regulatable via muscle activation.

Flexibility vs. Stiffness. Flexibility is typically the passive range of motion at a joint. Stiffness is the dynamic resistance to stretch during movement. An athlete can have good flexibility (large range of motion) but also high stiffness (resist stretch effectively during dynamic tasks).

Training implications. Tendon stiffness is crucial for efficiently storing and returning elastic energy (bounce strategy). Muscle stiffness can be regulated by the nervous system. Training needs to consider the desired stiffness behavior for the sport skill – sometimes requiring high stiffness (sprinting, jumping), other times requiring lower stiffness (landing). Static stretching might reduce stiffness, but dynamic training can improve the ability to regulate it.

Last updated: June 8, 2025