Muscular Adaptations

Muscular adaptations to physiological training stimuli generally occur through an increase in muscle cell volume (growth in thickness, growth in length) and through metabolic-contractile reprogramming (Goldspink 1985). These adaptations are movement-unspecific and transferable, provided the adapted muscle fibres are also used in the sport in question. The aim of strength training is therefore to develop the muscle as broadly as possible, so that it can then meet the specific demands of sport and everyday life as effectively as possible.

Radial growth (growth in thickness)
+ Hypertrophy/(hyperplasia) (more sarcomeres arranged in parallel)
– Atrophy (fewer sarcomeres arranged in parallel)
Adaptations: longitudinal growth (growth in length)
+ Hypertrophy/elongation (more sarcomeres in series)
– Atrophy/shortening (fewer sarcomeres in series)

Fibre type distribution
Metabolic-contractile reprogramming

Definitions and explanations
Hypertrophy
Muscle fibre hypertrophy is defined as an increase in the cytoplasmic (i.e. sarcoplasmic) volume of the muscle fibre, regardless of whether the number of nuclei changes as a result. Muscle fibre hypertrophy does not mean an increase in the number of muscle fibres. Since muscle fibres can no longer divide after birth, the primary mechanism behind an increase in muscle mass in humans is assumed to be muscle fibre hypertrophy.
1. Radial hypertrophy
An increase in the physiological cross-section of the muscle as a result of an increase in the cross-section of individual muscle fibres.
2. Longitudinal hypertrophy
An increase in the length of individual muscle fibres while the cross-section stays the same. Muscles can adapt to a new functional length by adding new sarcomeres in series at the ends of the myofibrils.

Atrophy
A decrease in muscle mass as a result of inactivity or immobilisation.
1. Radial atrophy
A decrease in the physiological cross-section of the muscle as a result of a decrease in the cross-section of individual muscle fibres or the loss of muscle fibres.
2. Growth in length
A decrease in the length of individual muscle fibres. Muscles can remove sarcomeres in series at the ends of the myofibrils.

Fibre type distribution
1. Metabolic-contractile reprogramming
Changes in muscle fibre distribution are only possible to a limited extent. This is accompanied by adaptations in vascularisation, mitochondrial content and so on. The adaptation therefore takes place primarily in the oxidative capacity of the muscle. It is assumed that, in humans, crossing the boundary between type 2 and type 1 is unlikely to happen under normal circumstances. Contrary to popular belief, endurance training therefore does not, on average, lead to an increase in type 1 fibres.
Contrary to popular belief, "explosive" training makes the muscle fibres slower at the level of their molecular motors (a switch from 2X to 2A). Inactivity (voluntary or enforced) makes the muscle fibres "faster" (a switch from 2A to 2X).
Hyperplasia
An increase in the physiological cross-section of the muscle as a result of an increase in the number of muscle fibres. (In humans, there is no conclusive scientific evidence, and it is assumed that no training-induced increase in the number of muscle fibres occurs after birth.)

Sources: Goldspink G. (1985): Malleability of the motor system: a comparative approach. J Exp Biol 115: 375–391. Theorie Kraft, Scientifics AG