Reverse fiber type disproportion: A distinct metabolic myopathy

Area of Science:

• Exercise Physiology
• Muscle Biology
• Biochemistry

Background:

• Muscle fiber type composition influences physiological responses to exercise.
• Type II muscle fibers are associated with power and anaerobic metabolism.
• Understanding fiber type-specific responses is crucial for interpreting exercise performance and metabolic stress.

Purpose of the Study:

• To characterize the physiological and metabolic responses to incremental exercise in individuals with a predominance of type II muscle fibers.
• To compare these responses to those of individuals with a normal fiber type distribution.
• To propose a new classification for this specific muscle fiber phenotype.

Main Methods:

• 13 subjects with type II fiber predominance underwent incremental exercise testing.
• Measurements included maximal oxygen uptake (V˙O2 max), heart rate, ventilation, blood lactate, ammonia, and creatine kinase (CK).
• Muscle fiber type was assessed using myosin ATPase histochemistry.

Main Results:

• Muscle biopsies revealed a significant predominance of type II fibers (75%) compared to normal individuals.
• Subjects demonstrated normal maximal oxygen uptake and end-exercise lactate levels.
• Significantly higher levels of ammonia and creatine kinase (CK) were observed at maximal exercise.

Conclusions:

• Individuals with type II muscle fiber predominance exhibit exaggerated increases in ammonia and elevated CK levels during exercise.
• This condition is the opposite of congenital fiber type disproportion.
• The term "reverse fiber type disproportion" is proposed for this phenotype.