What is defined as the amount of force that can be created by a muscle or group of muscles?

If you're training your clients for optimal strength or power gains you must understand how these components differ and how force is regulated. The answers are all right here for you.

How is the force generated by skeletal muscle graded?

There is potential for a huge amount of force to be created when muscles contract, however we don’t always want all of that force.  Imagine typing on your computer if the muscles of your hands and arms contracted with their full force?  Microsoft would like it as you'd go through keyboards with alarming regularity.  So in order to develop simple and complex skills like writing, talking, running and jumping, the force produced by muscles must be graded specifically.

Force in muscles is ‘graded’ through the control of the frequency (how often) of the firing of impulses at the neuromuscular junction (point where the motor neuron connects to the muscle).  

Looking at the diagram below you can see the nerves feeding into the muscle.  The more muscle fibres one nerve controls the bigger the motor unit. 

In areas of your body where lots of force is produced and finesse is not a requirement, (think legs for instance) there will be few motor units and those motor units will be large.  In areas of your body where finesse is required, fine movements with gentle force, lots of small motor units will exist.  This simply gives the brain more control over that part of the body (think fingers in this case).

If we want a tiny amount of force the motor unit might just receive one little brief signal from the nerve, calcium would flood the muscle cell and a contraction would occur, and the calcium would quickly be pumped back out and the contraction will finish.  When a motor unit does this it’s called a single or simple twitch, as seen on the image below.  Think of it as a ‘blip’ on the radar.  With a single or simple twitch not much force is produced as only a limited number of actin and myosin cross bridges have time to bind.

If more force is required the message at the neuromuscular junction of the motor unit is sustained.  This means that although the calcium pump in the muscle cell is removing calcium, new calcium is being cycled back into the muscle cell to enable the contraction to continue.

In this situation more and more myosin/actin cross bridges can form and the force produced in the motor unit climbs.  This is called ‘summation’ (or in lay terms - adding up) where a motor unit begins to develop more force due to a higher frequency of nerve signal, as seen in the image above.

When a motor unit receives continuous stimulation from the nerve feeding it, which is at a frequency that is high enough that calcium is always present in the muscle tissue, it will eventually reach its full force producing capacity.  This is called a ‘tetanic’ contraction, as seen in the image above.

If the nervous signal is continuous tetanus can be sustained until either ATP (energy) can no longer be provided in the muscle, or the nervous system fatigues and can no longer create impulses quickly enough to keep the signal going and cause the release of calcium into the muscle cell which is vital for muscular contraction.

The grading of muscular force and exercise performance

We know that muscular contractions can be graded from a single twitch through to a tetanic contraction depending on the frequency of the impulses being received at the muscle, but what does that mean when it comes to exercise performance?

So if you get enough single twitches in quick succession it will lead to summation and eventually reach the threshold of tetanus and the full contraction and subsequent movement (i.e. walking, typing, jumping) will occur.  From this point the longer the muscle fibres are in a state of tetanus or if more motor units are also stimulated to the point of tetanus the greater the force produced.

For example, in order to do a body weight squat sufficient frequency in impulses will be received by the working muscles to take them from a state of rest through to single twitch, summation and eventually tetanus (squatting).  The way in which the impulses are graded means the movement ends up smooth and flowing.

If you decided to put a big weight on your back and do some more squats you would require more force to be produced in order to complete the movement.  At this point the same sequence of events occurs, only more motor units reach tetanus at the same time.  This results in more muscle being contracted and therefore enough force is produced for you to complete the movement.

How do force, velocity and power relate?

Force (strength), velocity (speed) and power (combination of strength and speed) all relate to our muscular contractions and how they are graded i.e. fast, slow, strong, weak etc.  They also relate directly to the way you train your personal training clients.

For example a client might want to improve their maximal strength in the bench press or their squat jump height for volleyball.  Understanding how these three elements relate will help ensure you train your clients in the right way to get them their desired results.

The graph below shows the relationship between the velocity (speed) of movement and the amount of force generated.  Remember that at any velocity fast twitch fibres can produce more force.  This is most dramatic at higher velocities.

At zero velocity you can see the muscles can produce maximal force.  This is because the load isn’t moving (think of pushing as hard as you can against a wall or a weight you couldn’t shift) so the force you can generate will be maximal because your muscles will have all the time they need to contract. 

As the velocity of the movement increases the force you can produce starts to drop.  This is because less of the actin and myosin filaments have a chance to be bind and contract.

At very fast velocities the force you can produce is quite low as very few of the actin and myosin cross bridges have time to bind, and very few are bound at any moment.  The faster you go the less force you have time to produce within the muscle.

When we look at the relationship between power and velocity on the graph below, we see a different shaped curve. 

Power is calculated as follows:

Power = work / time.  Work is the force x the distance.  So if you moved a 60kg weight one metre in just one second you could calculate power as follows:  60 x 1 / 1 = 60 watts

So, if we move fast we have increased power because we are doing things in less time.  However, there is a trade off in that as we speed up more and more we produce less force, which means less power again.  So the power curve peaks at an ‘optimal’ velocity in the middle where the time taken and the force being produced is balanced.

The implications of this are that if you want to be very powerful with an action (for instance a punch) you need to move quickly and forcefully, but not try to go so fast that you don’t allow your muscles the time they need to generate all the force they could. 

An example is that a jab in boxing is fast, but not as powerful as a right cross.  In the jab you move very quickly but the force you can muster in your muscles is limited due to the velocity of the movement.  There is also less time for calcium to enter the muscle and therefore fewer actin and myosin cross-bridges form and thus less power can be created.  Conversely, the hook is slower, but more powerful, as you have more time to use more of the muscle available to you.

Luckily, with training, you can improve power at each of the velocities on the curve.  So maybe one day your hook can be as quick as your jab, but very forceful still.

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