How EMS recruits muscle fibres — the science in plain English

When you contract a muscle voluntarily — say, doing a squat — your brain sends an electrical signal down a nerve to recruit muscle fibres. But not all of them. Even at maximum effort, untrained adults only recruit about 30–60% of available muscle fibres. Highly trained athletes get up to 90%, and even they can't hit 100% under voluntary contraction.

Why? Because evolution. Recruiting all your muscle fibres simultaneously would tear your tendons off the bone. Your nervous system has built-in safety governors that limit voluntary recruitment to protect the joints.

EMS works around those governors. It's the most interesting thing about the technology — and the simplest reason it produces results.

How a muscle contracts (the basics)

A motor unit is one motor neuron plus the muscle fibres it controls — typically 100 to 1,000 fibres per unit. When the brain wants to contract a muscle, it doesn't fire all motor units at once. It recruits them in order: smallest first, largest last. This is called the "size principle" and it's a foundational concept in exercise physiology.

For light tasks (lifting a coffee cup), only the smallest motor units fire. For heavy tasks (deadlift), progressively larger motor units join in. The largest, most powerful units only get recruited at near-maximum effort.

For most adults, the very largest motor units rarely get recruited at all in daily life — or even in normal gym training. They're held in reserve.

What EMS does differently

When an electrical impulse passes through the muscle from the suit's electrodes, it bypasses the brain's recruitment ordering. The impulse depolarises whichever motor units are within range of the electrode — small and large units fire essentially together.

This means EMS recruits motor units that traditional training has trouble reaching, especially the largest, fastest-twitch fibres responsible for explosive strength and power.

Studies measuring muscle activation during EMS-augmented exercise consistently find higher recruitment percentages than the same exercise done without electrical stimulation. The "20 minutes equals 60–90 minutes" claim isn't marketing — it's the math of recruiting more muscle fibre per unit time.

Why this matters for results

Strength gains happen when muscle fibres are progressively challenged. Fibres that never get recruited never adapt. EMS reaches the under-trained fibres in your body and gives them a stimulus they don't normally get.

For an untrained adult, this is dramatic — the unused capacity is large. For a highly trained athlete, the gain is smaller but still meaningful, which is why elite teams use EMS for in-season conditioning.

It's also why EMS is so effective for rehab: when an injury or surgery makes voluntary contraction painful or impossible, EMS can keep the muscle activated and prevent atrophy.

What about the safety the brain was protecting you from?

Modern EMS systems control intensity and impulse parameters within a safety envelope established by clinical research. The contractions are strong, but they're not so violent that they tear tendons. The 20-minute session length and the periodised impulse pattern are designed to give meaningful stimulus without exceeding what tissues can handle.

This is also why we recommend a maximum of 2 sessions per week and at least 48 hours between sessions — the recovery requirement is higher than for conventional training because more muscle fibre is being challenged.

The honest limit

EMS is excellent at recruiting muscle fibres for strength and conditioning. It is not a substitute for cardiovascular training (the heart and lung adaptations need sustained aerobic work), and it doesn't directly train movement skill (technique still requires practice).

For strength, body composition, rehab, and time-efficient conditioning — the science holds up. For aerobic fitness or specific sport skill — you still need traditional training.

The smartest training programs combine both, which is what most of our long-term clients end up doing.

Read the underlying research on our Science page, or see exactly how a 20-minute Onyx session is structured.