25 Double Muscling
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.
6. Nervous system.
8. Fibre types.
14. PSE & DFD.
16. Fibrous connective tissues.
23. Radial growth of myofibres.
24. Longitudinal growth of myofibres.
many years, cattle with greatly enlarged muscles and a
scarcity of adipose tissue have caught the attention of beef producers.
large superficial muscles of the shoulder and proximal hindlimb are
enlarged than the distal limb muscles or deep muscles of the carcass.
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.
- A groppa doppia
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
These abnormalities do not necessarily prohibit the exploitation of
muscling, but they explain many past failures.
- Underdevelopment of male reproductive tract.
- Underdevelopment of female reproductive tract.
(enlarged tongue) which makes it difficult for
the calf to nurse.
- Weak bones, often leading to bone damage of the hooks.
- Changes in pelvic dimensions associated with difficult
- Poor lactation.
- Reduced adipose reserves when subjected to nutritional
- Reduced weight of hide.
- Reduced weight of liver.
- Reduced collagen
content, measured by reduced hydroxyproline
content. This is a good feature of the meat, but makes the
- Longer gestation period.
- Low levels of thyroxin
(used in control of body heat generation).
- Susceptibility to heat stress.
- Small feet and cannon bones (nice on the carcass but not for
- Slow growth.
- The genetic basis of the condition is a single pair of autosomal genes.
animals may range from normal to
extremely double muscled phenotypes.
(The genotype is the animal's DNA. The phenotype is the result of
the DNA working in the environment to produce the animal).
penetrance and modifier genes
are the usual explanations proposed to explain
the phenotypic range of heterozyous animals, but it is difficult to
proof the range is due solely to genetic interactions at the DNA level.
- The gene for double muscling is pleiotropic - it affects the
physiological and anatomical systems listed above, not just the meat of
- There is evidence the expressivity of double muscling in
heifers is affected by
their plane of nutrition.
25.4 Development of double muscling
muscling is caused primarily by an increase in the apparent number of myofibres.
- As far as is known at present, the increase in apparent
is based on an increase in real
numbers as well.
pleiotropic nature of the single gene
involved, together with the considerable involvement of various types
connective tissues, suggests that the development of double‑muscling
a shift in the programmed sequence of muscle cell differentiation.
adipose cells and fibroblasts are both deficient in double‑muscled
increase in the number of myoblasts appears to have been made at the
the fibroblast stem‑cell population.
- Genes have been identified causing
- In addition to hyperplasia of extrafusal myofibres
(ordinary myofibres), double
muscled cattle also have an increase in the number of the intrafusal
myofibres. (These are miniature myofibres inside the sensory organs -
neuromuscular spindles - responsible for generating information on
muscle length for the cerebellum).
- With all the extra myofibres, how does the nervous system
respond? Are there extra motor neurons or not? The answer: there is a normal number of motor
neurons, but the amount of branching in terminal axons is increased.
Thus, motor unit side is larger in
double muscle cattle than in normal cattle.
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
of development, relative to normal fetuses. Numbers were
adjusted to a common basis, relative to the number present at a
length of 26 cm, to determine if the extra myofibres in double muscled
were formed before, or after foetuses had reached a length of 26 cm.
extra myofibre formation after 26 cm was minimal, and it was concluded
hyperplastic condition had an earlier origin. The major origin of extra
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,
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
any connection with their lack of response to the gene for double
extent to which
in myofibre diameter may contribute to muscle enlargement in double
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.
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).
Structure and Development of Meat
Animals and Poultry. Pages 379-384.