The All or Nothing law is a well-established principle of the recruitment of muscle fibre; how much of a muscle is working during a given exercise. Before we talk about it, we briefly need to explain what a motor unit is.

What Are Motor Units?

A motor unit is made up of two main components: motor neurons and muscle fibres. Motor neurons are nerve cells that travel from the brain all the way to a group of muscle fibres, within a muscle (remember, every muscle is full of many motor units and their muscle fibres).

Each motor neuron is attached to one type of muscle fibre: slow twitch, fast twitch I or fast twitch II fibres.

The breakdown below shows the differences between each fibre type…

Slow Twitch

Size - Small

Force - Low

Speed - Slow

Fatiguability - Low

Example Exercises - Endurance exercises such as running, cycling and walking

Fast Twitch 1

Size - Large

Force - High

Speed - Fast

Fatiguability - High

Example Exercises - Strength and power exercises such as resistance training, jumping and sprinting

Fast Twitch 2

Size - Large

Force - Very high

Speed - Very fast

Fatiguability - Very high

Example Exercises - Strength and power exercises such as resistance training, jumping and sprinting

What is the All or Nothing Law?

The all or nothing law states that once a stimulus exceeds the contractile threshold of a motor unit, it contracts FULLY. If a stimulus does not exceed the contractile of a motor unit, it does not contract at all; in simpler terms, this means an individual muscle fibre is either fully contracted or at complete rest.

How Does the All or Nothing Law Impact Training?

The importance of this can be explained by Henneman’s Size Principle. This principle states that motor units’ thresholds are not simply at random order, but instead a sequential order, based on the type of muscle fibre and the amount of stimulus/force required to activate them.

You cannot partially contract each fibre; to train more fast twitch fibres, you have to train muscles at high intensities, they won’t get trained partially when you train at low relative intensities.

Should You Always Train At Maximal Force?

No. While maximal force training, hitting lots of fast twitch type II fibres is great, it develops a lot of fatigue and muscle soreness and thus cannot be maintained for longer durations, not in too much volume throughout the week. Exercises associated with this include the back squat or clean and jerk, which are great for strength and power development, but these ‘fast-twitch exercises’ may miss out other important health related components of fitness.

Slow twitch fibres are associated with oxidative energy systems; they’re used in the gym during cardiovascular exercise, for example, running or cycling.

Why is This Important?

Training at low intensities for longer periods is great and should be encouraged for the associated respiratory benefits. Benefits of cardio include -

• Improved aerobic conditioning

• Decreased risk of heart disease

• Decreased resting heart rate

• Decreased blood pressure

But we also need to train at high intensities to benefit from the associated strength and power adaptations, especially given that they play key roles in functional capacity as we age, not to mention the importance of all of these training qualities in excelling in your sport. Benefits of strength and power training include -

• Decreased risk of musculoskeletal injuries

• Increased functional capacity in day-to-day tasks

• Increased force production capabilities

• Increased bone density

While the all-or-nothing law is always present, remember your training does not have to be -all-or-nothing, every little helps and staying consistent with your cardio and heavy strength and power training is the key to long-term success in the gym, balancing your training across slow-twitch and fast-twitch dominant exercises to stay fit and healthy.

Written by Ewan Hammond

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Tags: Misc > Lifestyle