What is strength?

Strength is something that means different things, depending on the context.

In strongman competitions, competitors take part in several events, which are each designed to test their “strength” in different ways.

And while some strongman athletes are very dominant in their sport, and nearly always finish highly, the placing of the competitors is rarely identical for each event, indicating that each athlete expresses “strength” most effectively in different ways.

Also in sports science, “strength” frequently gets defined in very different ways, depending on the research group who are writing up the experiment, the previous literature on that topic, and the goals of the study.

Overall, this makes it very hard to have a meaningful conversation about “strength” and how to make people stronger, because everyone starts with a different picture in their minds of what “strength” means.

So what does “strength” actually mean?


What does “strength” mean?

From a scientific perspective, “strength” is a measure of force production.

If you can display “strength,” that means you can produce force. If you increase your “strength” by getting stronger, then that means you can produce more force than when we last tested you.

We can measure the ability to produce force in several ways, ranging from the very simple (lifting a the heaviest weight possible) to the complex (exerting force in a dynamometer).

And yet, even if we use exactly the same exercise (such as a biceps curl), the method that we use to we measure force production affects the value we subsequently record to such a degree that we might as well be measuring completely different things.


What affects our ability to produce force?

Even when we consider tests of the *exact* same muscle group, there are dozens of things that can affect our ability to produce force, and many of them are nothing to do with our muscles.

Some relate to the environment that we are in, or our psychological state.

Others these relate to the way in which our brain and central nervous system coordinate force production, at different joint angles, speeds, loads, and conditions of stability.

But there are also three very basic biological factors inside muscle that determine how much force we can produce. These are produced by the internal workings of the muscle fibers themselves.

These factors are:

  1. the length-tension relationship
  2. the force-velocity relationship
  3. force enhancement during lengthening

These basic biological factors inside muscle mean that we can *immediately* alter the amount of force we are able to produce simply by (1) finding the joint angle at which our muscles are capable of working hardest, (2) moving more slowly, or (3) allowing a muscle to lengthen while it is working.

Let’s take a closer look at each of these in turn.


#1. The Length-Tension relationship

The length-tension relationship is the observation that muscle fibers have an optimal length for producing force. This means that whole muscles also have a length at which they are strongest, and consequently, there is a joint angle at which our biceps curl (for example) force is greatest.

The main factor underpinning this observation is the amount of overlap between the strands or “myofilaments” inside the muscle fiber that move against one another to produce force.

We can segment these strands inside muscle fibers into chains of contractile units, called sarcomeres. These sarcomeres shorten, which makes the myofilaments reduce in length, and so the whole muscle fiber attempts to shorten, which produces a tensile force from one end to the other.

The sarcomeres shorten when thin strands (actin myofilaments) slide past thick strands (myosin myofilaments). This sliding action is produced by “crossbridges” on the thick strands, which repeatedly work their way up the thin filament in steps. When there are no further steps for the thick filament to take along the thin filament, the crossbridges are no longer able to contribute to force production.

It is the degree of overlap between the thick and thin strands in a sarcomere that determines how much active force can be produced by the muscle fiber. If there is full overlap, this means that all of the crossbridges can do their jobs, but if there are parts of the strands that are not in contact with each other (either because the muscle fiber is too short, or because it is too long), then the force that can be produced by the muscle fiber is reduced.

Additionally, if the muscle fiber is stretched too far, it begins producing force passively, because its internal structures start resisting, and this increases force production very quickly at long muscle lengths.

Overall, this means that muscle fibers (and muscles) tend to produce a low level of force at very short lengths, and a peak level of force at moderate lengths, before reducing slightly until they reach *very* long lengths, when force can increase again.


#2. The Force-Velocity relationship

The force-velocity relationship is the observation that muscle fibers produce less force whenever they contract more quickly. This means that whole muscles also produce less force when we move quickly, and consequently, we cannot exert much force when we perform high-speed sporting movements, like throwing a ball.

The main factor underpinning this observation is *again* the amount of overlap between the strands inside the muscle fiber that move against one another to produce force.

We know this because researchers have found that if they experimentally increase the force produced by a single muscle fiber, the number of attached crossbridges increases. Conversely, when they experimentally increase the contraction velocity of the muscle fiber, the number of attached crossbridges decreases.

Why does this happen?

The number of attached crossbridges is dependent upon the contraction speed of the fiber because the detachment rate of the crossbridges at the end of their working stroke increases, with increasing contraction velocity.

In other words, as a muscle fiber contracts faster, the crossbridges have to detach more quickly, and this reduces force.


#3. Force enhancement during lengthening

Until recently, we did not understand why muscle fibers were able to produce much greater force while they are lengthening, compared to when they are shortening.

In fact, while lengthening, single muscle fibers can produce up to 150% of the force we measure during similar shortening contractions. This in turn means that we are approximately 125–130% stronger when we lower a weight under control (over 3 seconds), compared to when we lift a weight in exactly the same exercise.

Fortunately, researchers have been able to discover that when our muscle fibers lengthen, a third strand inside the muscle fiber comes into play.

This strand, a giant molecule called “titin,” gradually unravels when we lengthen the fiber while it is actively producing force. It does not behave in the same way when the muscle fiber is not active, so it does not impede passive movements. Yet, as it unravels, it resists being elongated, and this contributes substantially to the force produced by the muscle fiber.

Later, as the muscle fiber reaches the end of its normal working length, its passive elements come into play, and these also contribute to resistance against the fiber being lengthened.

Overall, this makes force much greater when muscles are lengthening, compared to when they are shortening.


What is the takeaway?

Strength is the ability to produce force, but this ability changes radically depending on the muscle length, speed, and contraction type (shortening or lengthening) that we use.

And there three basic biological mechanisms that happen inside the muscle (the length-tension relationship, the force-velocity relationship, and force enhancement during lengthening) which explain why muscle force differs because of changes in these conditions, not to mention all the other factors that can affect strength.

Ultimately, no matter how hard we try, there will never be one single definition of “strength” because it differs depending on *how* and *when* we want to produce force.