Different muscle fibre types exist in human muscle, but a single motor unit only contains one muscle fibre type; type 1 (slow twitch) or a subdivision of type 2 – 2a or 2b (fast twitch).
The smaller motor units contain slow-twitch fibres – these are named tonic units. These consist of type 1 fibres which are highly capillarised and rich in mitochondria which means they have a high capacity for aerobic metabolism, but are only capable of producing twitches with low peak tension and are slow to peak at about 60-120ms.
Tonic units are found in abundance in postural muscles such as the erector muscles of the back.
The larger motor units consist of fast-twitch (type 2a and type 2b) fibres and have been coined phasic units. These units rely on anaerobic metabolism due to their poor capillarisation and low amounts of mitochondria. However, they are capable of higher peak tensions than tonic units, and are much quicker at reaching that peak tension (10-50ms).
Phasic units are found in large quantities in the gastrocnemeus muscle, for example.
Burke and Colleges grouped motor units into 3 different types based on their fatigue index;
Type FF – fast twitch | fatigable
Type FR – fast twitch | fatigue resistant
Type S – slow twitch | fatigue resistant
These motor unit types correspond to the fibre types they contain – type 1, type 2a and type 2b, respectively.
Scanning EMG recordings (1) have shown that a single motor unit covers a cross-sectional area of 2-10mm in the biceps and anterior tibialis muscles, and that several motor unit territories overlap one another.
In pinnate muscles where fibres are arranged in a feather-like fashion, the distribution of fibres contained in a single motor unit have been shown to be fairly evenly arranged over a large area, smaller than the total cross-section of the muscle. Whereas motor units in multicompartment muscles are normally contained within a single compartment.Follow MuayThaiScholar