LEC24

24 Longitudinal Growth of Muscles

24.1 Introduction

Think of the length of any major muscle in a calf or piglet, then think of the length of the same muscle in a commercial beef or pork carcass. There has been a major increase in length. Thus, longitudinal growth of whole muscles makes a major contribution to meat production in all our meat animals.
But the longitudinal growth of individual muscles is achieved by the longitudinal growth of individual myofibres, which is complicated by the angular insertion of myofibres in most muscle. Only a few muscle like the Sartorius in the hindlimb have parallel myofibres. In most muscles, the myofibres are arranged at angles to modify the effects of individual myofibre contraction. If a myofibre contracts over a greater distance than the length of the whole muscle, then leverage is obtained and the force of contraction is magnified at the expense of distance. Often distance and force are unchanged but the angular arrangement of myofibres allows more myofibres to be connected to the tendon than would be possible if the myofibres were all parallel.

When myofibres have an angular insertion, the muscle is said to be pennate, as in the example above. This type of muscle is very common in less expensive cuts of meat such as the shank meat of the legs. The large amount of intramuscular collagen makes these muscles very tough unless they are given long cooking with moist heat.

24.2 Longitudinal growth of myofibres

Even when the longitudinal axes of myofibres and their fasciculi are parallel to the longitudinal axis of a whole muscle, it is extremely difficult to ascertain the length of individual myofibres.
Longitudinal growth originates from the formation of new sarcomeres at the ends of myofibres.
The addition of new sarcomeres is regulated. Sarcomere length and the degree of overlap of thick and thin filaments must be maintained at an optimum for efficient muscle contraction.
Regulation may be mediated by the protein dystrophin as a mechanochemical transducer.
However, the system must be very complex, because sarcomere length shortening during contraction differs between muscles and is related to the angular changes of nearby skeletal joints.
Myofibrils insert obliquely at their ends. This enables new myofilaments to be added without interfering with the mechanical continuity of the myofibrils at their attachment to the cell membrane.
The ratio of muscle belly length to free tendon length remains approximately constant during growth.

Rabbit muscles have been marked with ink and implanted wires to find where new muscle tissue is added during longitudinal growth. New tissue is added evenly along the muscle length - rather than being restricted to the ends of the muscle. But remember - many myofibres terminate intrafascicularly with tapered endings. The problem with this type of experiment is the possibility of ink marks or wire implants following the growth of intramuscular connective tissue rather than the growth of myofibres.

An increase in the girth of a deep muscle (usually from the radial hypertrophy of myofibres) causes overlying superficial muscles to bulge outwards.
Thus, the dominant pattern of cellular growth in the superficial muscle may be longitudinal growth, since its length from origin to insertion is increased in a curvilinear manner as the deep muscle bulges outwards.
This is why a subjective assessment of muscle bulging in a carcass can be used in carcass evaluation as a guide to muscularity (it is used in many types of beef carcass grading).
The longitudinal growth of muscles follows that of the skeleton. Thus, bone length is related to meat yield.

24.3 Fascicular length

In complex carcass muscles it may be possible to measure fascicular length, although fascicular length conveys no information about nyofibre length unless the distribution of intrafascicularly terminating muscle fibers is known. Fascicular length is one of the factors contributing to the cross sectional area of the longissimus dorsi muscle. The width and the depth of longissimus dorsi have long been recognized as partially independent variables, with depth being a more sensitive indicator than width for the prediction of muscularity. For example, ewes fed at a supermaintenance level show proportionately greater gains in depth relative to width. Similarly, submaintenance feeding causes a greater reduction in depth than in width. In pork carcasses, the width of the longissimus dorsi is more strongly correlated with total carcass muscle than is depth. Width is weakly correlated with myofibre diameter whereas depth is not correlated with myofibre diameter.

The partial independence of depth and width in the longissimus dorsi is caused by muscle fasciculi passing through the muscle cross sectional area in a plane parallel to depth.
Thus, when fasciculi grow in length, they contribute to depth but not to width.
When the longitudinal growth of fasciculi is proportionately faster than growth in muscle length, the increased dorsal overlapping of fasciculi causes an increase in the apparent numbers of myofibres (explained below).
Every millimetre of gain in fascicular length adds 0.77 mm to the depth of the Longissimus thoracis (the thoracic part of Longissimus dorsi) in a pork carcass.

24.4 Rate of new sarcomere formation

About every 20 minutes, a new sarcomere is added to the length of myofibres in pigs growing to market weight. In muscles with relays of intrafascicularly terminating fibers along their length, or relays of fasciculi along their length, the number of points at which longitudinal growth occurs is increased. If two growth points, one at each end of a myofibre, can add a sarcomere every 20 minutes, then the formation of each sarcomere must take about 40 minutes. Thus, a muscle with four relays of intrafascicularly terminating myofibres (or four relays of fasciculi) might add a new sarcomere to its length in about 5 minutes because it has eight points for longitudinal growth. Hence, in pigs growing to market weight, muscles deep in the ham may add a new sarcomere to the length of the muscle every five minutes but, in superficial muscles which are increasing in length at a faster rate, the time for sarcomere addition may be as short as two minutes.

Using the number of sarcomeres along the length of a fasciculus as a measure ot its length, some interesting growth patterns have been detected in turkey muscle. The pattern of growth in the parallel-fibred sartorius is a simple decelerating curve to reach a plateau at about 15 weeks with males having longer muscles than females. In the pennate supracoracoideus, trapped in an osteofascial compartment limiting growth against the keel of the sternum, fascicular length reaches a maximum at 13 to 14 weeks and then may decline, with only slightly longer fasciculi in males than females. (The osteofascial compartment is formed by a sheet of connective tissue trapping the muscle in a depression on the sternum).

24.5 Apparent number of myofibres

From the diagram below, we can see the number of myofibres appearing at the midlength of a muscle may be less than the real number of myofibres in the muscle. Myofibres can terminate intrafascicularly along the length of a muscle. When they grow in length, they cause an increase in the apparent number of myofibres (but there has been no change in the real number!).

Likewise, if myofibre longitudinal growth is LESS than in the whole muscle, the apparent number may decrease. Increases in apparent number occur in young animals. Decreases in apparent number occur in older animals.

Further information

Structure and Development of Meat Animals and Poultry. Pages 396-401.