Knowledge Base

Muscle strength

Muscle strength 

Muscle strength is created through muscle contraction. It is influenced by muscle volume, coordination, and energy reserves in the muscle and other factors. Muscle strength is needed to varying degrees in different sports. It is mainly developed when overcoming resistance, i.e. resistance or strength training (e.g. weightlifting). 

The impact of physical inactivity on muscle strength

For a young person, just two weeks without using their muscles (e.g. as a result of an accident or illness) means 25-30% of muscle strength is lost, and he or she ends up on the same level as a much older person. By contrast, regaining this lost strength takes around three times as long to recover as the time the muscle has been inactive.

The mechanism of muscle contractions

During muscle contraction, there is an active shift of the myosin along the actin, which serve as a track. The myosin filament is inserted between the adjacent actin filament, whereupon each sarcomere (see below) is shortened making the entire muscle shorten. 

In the relaxed muscle, the sarcomere is around 3 µm, while, during peak working, the muscle shortens to 1.5 µm (i.e. around 50%). This is the maximum possible shortening of the entire muscle at work. The actual values for shortening, however, are smaller thanks to the part of the muscle that does not contract when working (e.g. tendons).  

The sarcomere

The basic functional unit of a muscle fibre which is responsible for strength development is the sarcomere. It is made up of two basic proteins – actin and myosin – which implement the muscle contraction at the molecular level, and certain other proteins that either help or control the actin and myosin. Hundreds of actin and myosin molecules create long opposing fibres which partially overlap. Sarcomeres are arranged as chain links along the entire length of the muscle fibres.


Titin is one of the auxiliary proteins in muscle tissue – it is a huge elastic molecule and the largest known protein. We all have it in our muscles, where it functions as a spring that holds the myosin and actin fibres together. Thanks to its elasticity, titin helps return the muscle to the initial position after it has contracted, and also prevents it from slipping out of its position. This is why this protein needs to be so large. For comparison, if a titin molecule were a 2.5m rope, one molecule of actin, representing normal protein, would be just 1cm long and be the thickness of a single hair. 

The motor unit

A motor unit is a functional muscle unit – it is the set of muscle fibres controlled by one neuron, and they always work together. One motor unit has muscle fibres of just one type (either only slow-twitch  or only fast-twitch fibres). The number of muscle fibres in a motor unit can differ substantially depending on the muscle. The largest (and strongest) muscles are made up of motor units with thousands of fibres, whereas the small, light muscles can contain motor units with just 10 fibres. The fewer the muscle fibres in a motor unit, the more precisely the muscle can be controlled.

For proper exercising and adaptation of the muscles to a load, the work should involve all motor units. Henneman’s size principle applies, according to which motor units are recruited gradually according to the amount of strength required from the muscle. First, the small motor units are activated, made up of smaller, slower and persistent type I fibres. If more strength is required (for an intense load or in the case of significant muscle fatigue), the larger motor units are gradually recruited, made up of larger, stronger, faster, but also more easily fatigued, type II fibres. 

Types of strength training

There are three general approaches in weight training for exercising muscles, with each having its pros and cons:

  • Progressive resistance training – This is typical of most standard weight-training programmes.
  • Isometric training – When exercising, the joints do not move and the muscles do not change length. The exercises are static. One example of this is planking. Isometric training is a great rehabilitation technique.
  • Isokinetic training – Exercise is carried out at a constant speed regardless of how much effort is put into it (machines can automatically increase resistance, the more effort you exert). In theory, with isokinetic training, the highest amount of muscle fibres are connected during exercise.

Resistance training and intensity 

Weight or resistance training is an exercise where the muscle exceeds a certain resistance. It can be either weight training with weights or your own body or even rebound exercises or exercise using braking. Weight training is used to improve strength, speed and endurance in the majority of sports. 

High-intensity weight training generally involves training with weights around 70% of the maximum weight a person is able to lift. However, the number of repetitions is lower (e.g. 1–10). Low-intensity weight training is carried out with less than 30% of the maximum weight, and the number of repetitions is higher (usually over 15). When weight training (just as with any other exercise), it is necessary to take health, history of training and other elements into account.  

One-repetition maximum (1RM)

When setting the load in weight training, the term one-repetition maximum (one-rep max) is used (the maximum weight you are able to lift in one repetition, 1RM).  When measuring someone’s personal one-rep (repetition) maximum for a given muscle area, a close but lower value is estimated, and this is taken as the initial value. In subsequent steps, more weight is gradually added (from 1–5 kg, depending on the muscle area). Rest of 1–5 minutes is taken between individual repetitions, as the time that should be enough for the muscle to recover from the previous load. Just like the one rep maximum, it is possible to set a maximum for any number of repetitions (e.g. 10RM is the weight you are able to lift in 10 repetitions).

Estimating the one-repetition maximum

It is more practical and safer to estimate the one-repetition maximum based on your training rather than measure it. Just like 1RM, it is also possible to set 10RM, for instance, which is the weight someone can lift 10 times in one series. In general, for untrained people, the weight you are able to lift 7 to 10 times is the equivalent of around 68% of their maximum. For people who train, the weight they are able to lift 7 to 10 times is around 79% of their 1RM.