For movement to happen, muscles always work as a team: one carries out the movement, another brakes, and others stabilise. The muscle that produces the movement is called the agonist, its counterpart the antagonist and the supporting muscles the synergists. These roles change constantly – depending on the task and the direction of movement.
An example: when you lift a bag off the floor, your arm flexors work as agonists, while the arm extensors brake and control. At the same time, the shoulders and trunk stabilise the position. Muscles therefore do not act in isolation, but as a functional system.
A look inside – how muscles are structured
Regardless of whether a muscle is large or small, its internal structure is always organised in the same way. You can picture a muscle as a bundle of many thin threads. These “threads” are muscle fibres, that is, long muscle cells that can shorten.
Each muscle fibre contains myofibrils, and each myofibril in turn consists of the smallest units, the sarcomeres. A sarcomere is the functional basic unit of the muscle – this is where force is generated. Many sarcomeres in a row form a myofibril, many myofibrils a muscle fibre, and many muscle fibres a muscle. This structure explains why muscles can work so precisely and evenly: they consist of millions of small force modules.
How muscles generate force
For a muscle to work, proteins in the sarcomere must interact with one another. The most important are actin (thinner filaments) and myosin (thicker filaments). Myosin has small “grappling hooks” that attach to the actin and pull on it. This shortens the sarcomere – and with it the muscle.
What is fascinating is that the proteins themselves do not shorten, but slide into one another. This principle is called the sliding filament mechanism. To release the hooks again, the muscle needs ATP, the energy unit of the cell. Without ATP the muscle cannot relax – a mechanism that becomes noticeable during cramps.
In everyday life this process takes place thousands of times – whether you lift a coffee cup, climb stairs or sprint. The only difference lies in the number of activated muscle fibres and the signal strength from the nervous system.
How muscles are controlled
Muscles do not decide themselves when they work. They react to electrical signals that reach them via motor nerves. A single impulse produces a brief, barely visible contraction. Only a series of impulses leads to a smooth, powerful movement. This allows the nervous system to finely dose whether you lift a sheet of paper or a heavy bag – without consciously thinking about it.
Different types of muscle work
Muscles can develop force in three ways:
Concentric work: the muscle shortens while it produces force – for example, when you push yourself up from a chair.
Eccentric work: the muscle lengthens under tension, that is, while it brakes – for example, when lowering a dumbbell in a controlled way or walking downhill. This form is particularly efficient and highly stimulating.
Isometric work: the muscle does not change its length but builds up tension – for example, when you hold something or maintain a stable position.
In everyday life these forms rarely occur alone. Walking down stairs, for example, involves eccentric braking, isometric stabilisation and short concentric movements in alternation.
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Why there are different muscle fibre types
Not all muscles work the same way. The body has three fibre types that differ in speed, fatigue and energy supply:
Type 1 fibres are enduring and work with oxygen. They are used during long-lasting activities, such as prolonged standing, hiking or holding work.
Type 2A fibres are faster and more versatile. They can work both anaerobically (without oxygen) and aerobically (with oxygen) and are suited to repeated strength and speed tasks such as in team sports.
Type 2X fibres are the sprinter-like fibres. They generate a lot of force in a short time but fatigue quickly. They are active during sprints, jumps or explosive movements.
Every person has all three types. Differences in the mixing ratio partly explain why some people are strong sprinters, while others appear naturally enduring.
How muscles respond to training
Muscles are masters of adaptation. They change when they are loaded – and indeed in different ways depending on the type of training.
During strength training, muscle fibres store additional proteins, which increases the muscle's cross-section. This is called hypertrophy. In the absence of loading, the opposite occurs: atrophy, that is, muscle loss.
During endurance training, it is less about size and more about efficiency. The muscles improve their energy supply, increase the number of mitochondria (“cellular power plants”) and form more capillaries (small blood vessels) in order to transport oxygen and nutrients more quickly.
A third adaptation mechanism concerns the length of the muscle. Through regular training in stretched positions, muscles can add new sarcomeres in series. In this way the muscle adapts to new ranges of movement.
Open questions – and why we don't yet know everything
Despite extensive research, not all of the muscle's mechanisms are understood. People respond differently to training: some build muscle very quickly, others considerably more slowly despite an identical stimulus. Age, genes, hormones, nutrition and recovery interact here – but not all the connections are clarified.
Age-related muscle loss (sarcopenia) is also being intensively studied. It affects not only the muscle itself, but also the nervous system that controls it. This explains why regular training is a central factor for healthy ageing.
Conclusion – What we should understand about muscle tissue
Muscle tissue is far more than a supplier of force. It is an adaptable, metabolically active and health-relevant organ that responds to loading and safeguards everyday functions. Whoever uses their muscles keeps them – whoever does not use them loses them.
