25 Double Muscling

25.1 Introduction

This is a short lecture.  The idea is to look at one small topic, but to use and integrate ideas we have encountered in earlier lectures.

3. Embryology.
5. Somites.
6. Nervous system.
7. Myogenesis.
8. Fibre types.
12. Glycolysis.
14. PSE & DFD.
16. Fibrous connective tissues.
23. Radial growth of myofibres.
24. Longitudinal growth of myofibres.

For many years, cattle with greatly enlarged muscles and a scarcity of adipose tissue have caught the attention of beef producers. The large superficial muscles of the shoulder and proximal hindlimb are usually more enlarged than the distal limb muscles or deep muscles of the carcass.

Double muscling occurs in many countries, so there are many names for it:

The effect of double muscling on meat flavor and overall acceptability is very slight.

25.2 Problems associated with double muscling

Although the greatly increased yield of lean meat in double muscled animals is of considerable interest commercially, there are many disadvantages and physiological abnormalities associated with the condition. These abnormalities do not necessarily prohibit the exploitation of double muscling, but they explain many past failures.

25.3 Genetics

25.4  Development of double muscling

A double-muscled foetus.

 A apparent numbers of myofibres have been counted in sartorius and extraocular muscles (a muscle around the eyeball). In the sartorius, apparent numbers were doubled at each stage of development, relative to normal fetuses. Numbers were adjusted to a common basis, relative to the number present at a crown‑rump length of 26 cm, to determine if the extra myofibres in double muscled animals were formed before, or after foetuses had reached a length of 26 cm. Evidence of extra myofibre formation after 26 cm was minimal, and it was concluded the hyperplastic condition had an earlier origin. The major origin of extra myofibres was from an increase in the real number of primary myotubes and to cellular changes prior to 26 cm crown‑rump length. Myofibre diameters did not differ between normal and double muscled foetuses. In contrast to the situation in the sartorius muscle, myofibre numbers were not doubled in the extraocular muscle examined (the anterior rectus). The embryological origin of extraocular muscles is from the pro-otic somites of the head (the three segments anterior to the developing ear). These somites have an unusual development and innervation, but whether this has any connection with their lack of response to the gene for double muscling is unknown.

The extent to which increases in myofibre diameter may contribute to muscle enlargement in double muscled animals is unclear. Some researchers have found increased numbers of fast-contracting myofibres - and these tend to be larger in diameter than slow-contracting myofibres.  Double-muscled animals may produce meat with a rapid rate of post-mortem glycolysis, and it tends to be more pale than normal.

25.5 Conclusion

At present, therefore, it appears muscle enlargement observed in double‑muscled cattle is caused primarily by an increased real number of myofibres (myofibre hyperplasia) supplemented to varying degrees by increases in myofibre diameter (radial  hypertrophy).

Further information

Structure and Development of Meat Animals and Poultry.  Pages 379-384.