Runners Guide to Muscles

Having a basic understanding of muscles helps us as runners understand what is going on! By which I mean how we move when we run, avoiding injuries and how to focus our training; both strength & conditioning as well as run training. Most of the bodies muscles are skeletal muscles. They attach to the skeleton…

Having a basic understanding of muscles helps us as runners understand what is going on! By which I mean how we move when we run, avoiding injuries and how to focus our training; both strength & conditioning as well as run training.

Most of the bodies muscles are skeletal muscles. They attach to the skeleton and are under voluntary control. Their fibres respond to the electrical impulses of the motor neurone network, which in turn is controlled by the central nervous system. Skeletal muscles mostly work in pairs on either side of a joint and which is how the joint is operated and partly how its range of motion is controlled. Muscles are connected to bones via tendons, which are a separate form of tissue. At its most simplistic level, electrical impulses cause the fibres in the muscle to pull on the bones and this is what causes movement.

Whilst the above describes the operation of a muscle contracting, there are different forms of contraction for us to consider:

  • Concentric: the muscle shortens during contraction
  • Eccentric: the. muscle lengthens during contraction
  • Isometric: The length of the muscle remains unchanged during contraction

As a runner this applies to you in two specific ways: eccentric contractions relate to the absorption (and storage) of ground reaction force. Whilst concentric contractions relate to how we drive the body forward.

Eccentric Muscle Contractions

The calf and the quadriceps engage eccentrically during the early loading phase, lengthening as they absorb the impact force of landing. The Achilles tendon also lengthens as it absorbs ground reaction force.

Concentric Muscle Contractions

The calves, quadriceps, proximal hamstrings and glutes engage concentrically during the terminal stance phase to propel the body forward into the next stride.

The Achilles Tendon

The Achilles tendon plays an important role in running. During the early stance phase it lengthens under tension, like stretching an elastic band. Thereby storing large amounts of energy from the ground reaction force. This is then released during the push off and drives us forward. For a relatively small tissue, the Achilles takes a significant load during running and is therefore often sighted as an injury. With a little care there is no need for the Achilles to be injured.

Muscle Repair

Muscles change and adapt as they repair themselves. This is a vital and fundamental part of the strengthening and “fitness” process. But what do we mean by repair and how did they become damaged?

Muscles are made of cylindrical cells bundled together and covered in connective tissue. Muscle damage triggers the repair process. This damage (often referenced as micro tears) occur when we load muscles so they contract beyond what they are used too (overload) and small tears in the fibre occur. White blood cells clear the dead tissue, then new fibres and connective tissue grows along with blood vessels and nerves. To be clear this is a normal regenerative process and is fundamentally the process that results from training, leading to adaption. This is not damage that is considered injury.

Muscle Types

Having considered how muscles operate, the forms of contraction and the ways muscles adapt, the final point to understand is concerning the different types of fibres found in muscles and how these relate to running.

Skeletal muscles comprise of two different sorts of fibre: slow-twitch and fast-twitch. You will almost certainly have heard of them.

Slow-twitch fibres are relatively resistant to fatigue and are used during steady state aerobic exercise. Fast-twitch fibres able to generate huge explosive forces but can only sustain doing this for very sort period of time. Therefore it follows that an Endurance Athlete will want a large arsenal of slow-twitch fibres to sustain long distance activity. A middle or long distance Triathlete will be racing for many hours and therefore the demands are very different to a 100m sprinter.

Although we cannot change a fibre from one type to another, the kind of training we engage in will determine the size and quantity of each type within different muscles. This change occurs during the muscle repair cycle described earlier and the particular change is a consequence of the stimulation applied to the muscle. The muscle (body) adapts to consistent demands placed upon it.

A final note here though is that even a long distance endurance athlete does need some fast-twitch fibres. For instance for a sprint finish or to execute an overtake. Or even a short sharp hill effort. Therefore there is a need to balance our training.


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