The secret life of muscles

By understanding the way muscles approach a unit of work, you can gain valuable insights into training methodologies you may not have previously considered, writes Tony Podpera.

As a personal trainer, a great deal of your work revolves around muscles. But do you really understand how they actually work – and how that knowledge can improve the way you train your clients, and yourself? Let’s take a closer look at two aspects of muscle function, and apply them to the world of resistance training.

Truth 1: A whole muscle never contracts

During a weights exercise, the whole muscle appears to be contracting, but it isn't. To conserve energy, the body only uses as much muscle as necessary at any given time. As we know, muscles are divided into segments called motor units – bunches of muscle fibres under the control of one nerve (so all the individual fibres within the same motor unit behave identically at all times). During any movement, our body will calculate exactly how many motor units, and which ones, will be needed to do the job. It performs this calculation several times per second, so as the movement gets harder, the body enlists more motor units to help. The body constantly adjsts the number, location and effort of motor units in the muscle for peak efficiency, so that the muscle’s entire system of motor units is never working at the same time. If the whole muscle contracted every time you did a biceps curl, you would use as much energy scratching your nose as you would doing a 20 kilo barbell curl. The body is too clever to waste energy like that.

The only reason we exercise with weights is to force our body to use more motor units during a movement. That’s to say, to get a greater amount of the target muscle working, which is a key element of successful weight training for size and strength. Of course, when only a portion of motor units are working, the rest have no choice but to go along for the ride, so it always looks like the whole muscle is doing the work.

Truth 2: Muscle fibres have to be reminded constantly to contract

A biceps curl might take two seconds to complete, but, despite appearances, the muscle fibres will not be contracting for two seconds. What is really happening is that all the muscle fibres within each working motor unit are continually contracting and relaxing in a matter of milliseconds.

Fast-twitch fibres (of which there are several sub-types) can contract in as little as 30 milliseconds. This is important for movements which need to harness maximum power quickly, or where additional short-term strength is required. They tend to fatigue quickly, so they are only used when necessary.  On the other hand, slow-twitch fibres can keep working for longer. The trade-off with them is that they take a bit longer to contract, around 120 milliseconds. So a biceps curl which takes two seconds involves lots of repeated muscle fibre contractions – not just one long contraction.

Here's what might happen: the body will begin using the energy-efficient slow-twitch fibres as we start our curl and will recruit more as the curl ascends, constantly adjusting the usage of motor units based on their position, size and readiness to work. As the curl becomes harder, the body may recruit some motor units containing fast-twitch fibres to provide the extra force to get through the curl's sticking point. The body will use the most efficient combination of motor units to complete the curl while using as little energy as possible.

So, a muscle fibre is designed to only contract once and then relax in milliseconds. However, in the gym, our efforts last longer than milliseconds. So, during any sustained effort (like the two-second biceps curl), the muscle fibres never get the chance to relax fully. If muscle fibres are in a motor unit that the body wants to keep using, they will have just started to relax after a contraction when another signal will come from the brain telling them to contract again because the load still needs to be lifted.  The contraction of muscle fibre when it hasn't fully recovered from its previous contraction is called summation. Summed contractions can lead to a higher force being generated by a muscle fibre because the fibre has not fully relaxed and is still partially contracted when the next signal to contract comes along. The contractions build on each other to increase tension in the fibre. Each contraction can only exert a certain force, but add them up and the whole muscle fibre can build a tension greater than that of a single contraction. If the signals to contract come quickly enough (say, 50 signals per second) this can lead to muscle fibres remaining effectively contracted permanently in tetanus. The moment those signals stop or the muscles run out of energy, the muscle fibre will stop contracting.

In basic terms, overloading muscle fibres by heaping contractions together like this is the base from which muscles develop the adaptations which make them bigger and stronger.

So what happens when we train?

This information tells us that, when training for size and strength, we should aim to recruit as many motor units in a muscle as possible and contract them for as long as possible. Most basic weight training programs manage to do this pretty well.

But it's not that simple. One of the first adaptations the body makes to training is to learn to signal motor units more efficiently. This is the reason that new weight trainers get stronger before they get bigger. The body knows that it can manage the increased workload by being smarter about motor unit recruitment.

However, as the loads increase over time and muscle fibres are forced to generate greater force through summation of contractions, the body realises that clever motor unit recruitment may not be enough and that the fibres themselves may need to be upgraded if the level of activity is going to continue in the long term. Upgraded muscle fibres are bigger and stronger – which is what those training for strength want.

Training applications

This snapshot of weight training from the muscle fibre level can provide insight into the training methods we use and help us to think about them in a different way.

For example, considering this information about muscle contraction, do you appreciate why the concept of ‘time under load’ is important for muscle development? Does it influence your perception of forced reps, negative reps, range of motion, training to failure, and training for fast-twitch vs slow-twitch muscle development in athletes?

Muscles are amazing things – it pays to develop a greater understanding of exactly how they work. But even if you are armed with only the basics of muscle function, you should be able to consider any weight training methodology and determine whether it is likely to work or not and be able to explain why.


Tony Podpera
Tony is a Canberra-based fi tness instructor. He began bodybuilding in the early 1980s and has advised clients on weight training and strength and conditioning issues for over 20 years. He has been teaching group fitness classes since 1994 and has taught throughout the ACT and internationally.