Force accelerates mass.  The heavier the mass, the smaller the acceleration.  The greater the force, the greater the acceleration.

When looking at motor skills, the forces involved include those generated by muscles (the muscular tension), gravity (both on the body and objects the body interacts with), internal body reaction forces and forces from interacting with external objects.

Although analysis of movements can become very complicated, general principals can be taken form the science literature and applied in a straight forward manner.

Muscles work by creating tension.  The result of the tension depends on the load of the muscle.  The load is the sum of internal body forces (e.g. the weight of the body parts involved) and external forces (e.g. holding weights) that act against the muscle.  The results are defined as follows:

Tension compared to load	Muscle length	Action
Greater	Contracts	Concentric
Equal	Stays same	Isometric
Less	Lengthens	Eccentric

Bones are rotated around joints because of the forces acting upon them.  Muscles connect over one or more joints and as a muscle contracts (concentric action), it pulls the bones that it connects to together.  This action results in the bones rotating around the joints that the muscle spans.  Alternatively, muscles can resist other forces and either keep their length constant (isometric) in which case no movement is caused, or lengthen in a controlled way (eccentric) where forces outside the muscle cause the movement but the muscle slows down that movement.

As an example of the above, imagine picking up an object.  At first, the muscles doing the work contract as the object is lifted.  This is concentric action.  Then the object is held in a fixed position.  Some muscles are still creating tension to counteract the weight of the object but because everything is static, their length does not change.  This is isometric action.  Finally, the object is lowered back to the ground.  The muscles are still resisting gravity so the object does not just fall to the floor, but the tension is less than gravity.  The object is lowered and the muscles lengthen.  This is eccentric action.

Muscles usually have complex effects on movement do to their geometry.  For example, it is common for muscles to cause rotations in more than one plane around a joint and some to affect rotations around more than one joint.  Presumably this is due to evolution packing in as much muscle capability into the given space.  It requires the control system (nervous system) to often activate several muscles to achieve a seemingly simple movement.

When looking at a movement, muscles are divided into three groups based on their contribution to that movement:

• Agonist prime mover: these muscles generate the bulk of the force in the direction of the desired movement
• Agonist synergist: these muscles assist the prime movers, providing less force but adding control
• Antagonist: these oppose the desired movements for control and for protection

In general we shall group prime movers and synergists together and refer to them simply as agonists.

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