Contraction, Rigor and Conditioning


In this section we will be looking at how thick and thin myofilaments slide past each other within the sarcomere to cause contraction of the myofibrils and myofibres. Remember, the sarcomere is the muscle unit along a myofibril from one Z line to the next? Remember, the prefix myo means muscle?  Thus, for example, myofilament = muscle filament. Here is one sarcomere.



So this is the unpacked arrangement shown above.  Close them together laterally, and they would form a long bundle - the thick filament.

Calcium ions - the trigger for contraction

The cytoplasm within a muscle fibre has a special name, sarcoplasm, but still we use the normal name for the aqueous phase of the cytoplasm which surrounds cellular organelles, cytosol. The calcium ion concentration of the muscle fibre cytosol is regulated by the sarcoplasmic reticulum.

A key point to get straight is the relationship between the sarcoplasmic reticulum and the transverse tubular system.

 Only the transvere tubules open onto the surface of the muscle fiber.

The sarcoplasmic reticulum surrounds the fibrils within a fibre and occupies about 4% of the volume of the muscle fibre. In a resting muscle fibre in a live animal, the cytosol is kept almost free of calcium ions by the rapid capturing or sequestering of calcium ions by the sarcoplasmic reticulum. The concentration of calcium ions in the cytosol of a resting muscle is about 5 x 10-8 M, which increases to about 5 x 10-6 M when the muscle contracts. The protein calsequestrin binds and stores calcium ions within the sarcoplasmic reticulum, and the calsequestrin concentration is higher in fast-contracting muscle fibres than in slow fibres.

Transverse tubules and sarcoplasmic reticulum

The sarcoplasmic reticulum initiates muscle contraction by releasing calcium ions when prompted to do so by the transverse tubular system. Each transverse tubule is a finger-like inpushing from the surface membrane of the muscle fibre, coming into very close contact with elements of the sarcoplasmic reticulum. If this is hard to visualise, imagine you are a kid again with your nose pressed up against an inflated balloon. Look inside the balloon, then push your fingers into the balloon, and you will see your fingers carrying in the surface membrane of the balloon into its interior - just like transverse tubules. Thus, transverse tubules may conduct electrical action potentials from the surface of the muscle fibre deep into the interior of the fibre. Transverse tubules occur at the level of each A-I junction. Communication between a transverse tubule carrying an action potential and the sarcoplasmic reticulum is mediated by protein bridges between the adjacent membranes of the sarcoplasmic reticulum and the transverse tubule.

Muscle relaxation

After muscle contraction no longer required is required, it is turned off by the sarcoplasmic reticulum resequestering all the calcium ions it just released. Sustained muscle contraction or tetanus is the result of the fusion of individual muscle twitches. The peak tension generated by a single twitch occurs a few milliseconds after the action potential on the muscle fibre membrane, when about 60% of the maximum calcium ion release has occurred.

Initiation of contraction by the nervous system

The main steps involved in a single contraction are as follows.

(1) Voluntary activity from the brain or reflex activity from the spinal cord computes that a contraction is needed.

(2) The impulse is passed down the spinal cord to a motor neuron, and an action potential passes outwards in a spinal nerve, carried by an axon linking the motor neuron to all its muscle fibers.

(3) The axon branches to supply all its muscle fibres (motor unit), and the action potential is conveyed to a neuromuscular junction on each muscle fibre.

(4) At the neuromuscular junction, the action potential causes the release of packets or quanta of acetylcholine into the small space (synapse) between the axon and the muscle fibre.

(5) Acetylcholine causes the electrical resting potential of the muscle fibre membrane to change, and this then initiates a new action potential that passes in both directions along the surface of the muscle fibre.

(6) The action potential spreads deep inside the muscle fibre, carried by transverse tubules.

(7) Where transverse tubules touch parts of the sarcoplasmic reticulum, the sarcoplasmic reticulum releases calcium ions.

(8) The calcium ions cause the movement of troponin and tropomyosin on their thin filaments, which then enables the myosin molecule heads to "grab and swivel" their way along the thin filament.


Automatic relaxation by the muscle

Contraction is turned off by the following sequence of events.

(9) The acetylcholine at the neuromuscular junction is destroyed by an enzyme (acetylcholinesterase), and this terminates the stream of action potentials along the muscle fibre surface.

(10) The sarcoplasmic reticulum ceases to release calcium ions, and immediately starts to resequester all the calcium ions that were just released.

(11) Without calcium ions, a change in the configuration of troponin and tropomyosin blocks the action of the myosin molecule heads so that they cannot reach the thin filaments any more, and contraction ceases.

 (12) In the living animal, an external stretching force, such as gravity or an antagonistic muscle, is required to pull the muscle back to its original length.

Different forms of contraction

Muscle contraction may take either of two different forms, or a combination of both. In isotonic contraction, the muscle shortens against a constant load and the load is moved (eg., lifting a barbell). In isometric contraction, the load is too great to be moved, and the muscle generates increasing tension as it attempts to shorten (eg., pushing against a brick wall).


Explaining the development of rigor is a serious undertaking, because first you must understand the biochemistry of muscle contraction. Fortunately, for our purposes, we can make some grand simplifications. When the myosin molecules of the thick filaments are "grabbing and swivelling" their way along the thin filaments, causing the filaments to slide past each other for muscle contraction, they require a constant stream of energy from ATP. Before a myosin molecule of a thick filaments can release itself from an actin molecule of the thin filament, it requires some new ATP. Without ATP, myosin stays locked onto actin, even if the muscle is trying to relax. Thus, when living muscle finally runs out of ATP after slaughter, then rigor mortis develops.

Sarcomere length

When a fibril contracts, the thick filaments slide between the thin filaments so that the I band gets shorter, while the length of the A band remains constant. If a muscle is at its resting length, the gap between opposing thin filaments at the mid-length of the sarcomere causes a pale H zone in the A band. This sliding filament theory of muscle contraction was proposed in the early 1950s by two unrelated scientists working independently but, confusingly, both called Huxley (Andrew Huxley in Cambridge, England, and Hugh Huxley in Cambridge , Massachusetts). Back in the 1950s, as news of the sliding filament theory of muscle contraction spread around the world, the late Dr. Ron Locker at the New Zealand Meat Research Institute immediately could see how sarcomere length might cause meat toughness. The relationship between sarcomere length and meat tenderness now is widely known in the meat industry, and the volume of scientific research on the topic is somewhat overwhelming, but Locker made four key discoveries.

(1) As rigor develops after slaughter, carcass muscles may be stretched or contracted, depending largely on their position in the hanging carcass.

(2) Relaxed muscles produce meat that is more tender than that from contracted muscles.

(3) Rapid cooling before the start of rigor causes muscles to shorten (cold shortening). Sequestering calcium ions takes a lot of energy, so when the sarcoplasmic reticulum is cooled down, its efficiency drops, and it cannot then mop up all the calcium ions released by reflex muscle activity during slaughter and by leakage through the sarcoplasmic reticulum membrane.

(4) Freezing of meat before the completion of rigor causes extreme shortening when meat is thawed (thaw shortening), because ice crystals have slashed open the sarcoplasmic reticulum allowing massive contraction once the system is warm enough to respond.

To avoid cold shortening, meat must not reach 10oC within 10 hours post mortem in lamb carcasses. Beef sides should not be exposed to air below 5oC or faster than 1 metre / second within 24 hours after slaughter. Although thaw shortening causes extreme toughness in beef, the situation is different in poultry. Poultry meat toughness reaches its maximum level as recently slaughtered carcasses are washed, but the meat becomes more tender as carcasses are chilled. Maximum tenderness is reached after blast-freezing and thawing.

Electrical stimulation

Stimulating beef carcasses after slaughter causes an acceleration of all the biochemical changes that normally occur. Thus, rigor develops earlier, and the risk of cold shortening is reduced. Once rigor has fully developed, filament sliding is impossible, so it does not matter how cold the meat gets. Electrical stimulation also may create quite a lot of microscopic damage, somewhat equivalent to whacking a steak on the block with a heavy beater. Whacking steaks makes them tender but spoils their shape and appearance, but the same tenderizing effect may be achieved in a more sneaky fashion microscopically without upsetting anyone (unless they measure the cooking losses, which may be increased by electrical stimulation).

Aging or conditioning

All traditional butchers know that beef taste and tenderness are improved by aging or conditioning, although this vital fact is cheerfully ignored by many cut-price supermarkets intent on off-loading mountains of bright red meat as fast as they can.

Meat tenderness and taste definitely are improved if carcasses or vacuum packed cuts are conditioned after slaughter but, paradoxically, beef is quite tender just two hours after slaughter, and several days of conditioning are required to recover this degree of tenderness. After this point, the beef becomes progressively better, with a higher temperature allowing a faster rate of conditioning. Gains in tenderness from conditioning are particularly important in grass-fed beef. When beef is from youthful animals without risk of being cold shortened, then lack of conditioning is a serious cause of beef toughness.

In early research, it was though that increased tenderness might originate from the breakdown of rigor bonds between thick and thin filaments, but now this appears unlikely, because increased tenderness during conditioning still occurs when sarcomeres are at a stretched length that virtually eliminates any overlap of thick and thin filaments. Another factor that contributes to the increased tenderness of conditioned meat may be an increase in ionic strength that solubilizes myofibrillar proteins, particularly those of the thick filament.

Calcium-activated protease (Calpain) is an enzyme located in the cytosol. It slowly disrupts Z lines by releasing alpha actinin, a protein that holds the thin filaments into the Z line. This makes an important contribution to meat conditioning, but many aspects of the calpain system still are unknown. Calpains occur in all vertebrate cells, where they are involved in general purpose enzymatic activity related to maintenance of cell shape and the response of cells to hormones. Also there are other enzymes inside the muscle fibre which might be involved in the conditioning effect. Cathepsin B and D occur in lysosomes (suicide bags to destroy unhappy cells) and parts of the sarcoplasmic reticulum. Acid phosphatase is another enzyme that has been found inside muscle fibres.

The enhancement of meat taste and aroma during conditioning still is not fully understood. Conditioning causes changes in a number of water-soluble compounds that affect meat taste, including free amino acids, metabolites of ATP, organic acids and sugars. Free amino acids released by aminopeptidase activity are increased during the conditioning of pork, chicken and beef, although the changes in beef are less than anticipated.

Some of us are worried that conditioning might be reduced by factors in modern animal production, such as the genetic selection for rapid growth rate. Even animals that are growing very rapidly have a balance between the synthesis of new tissue (anabolism) and the break-down of worn or damaged tissue components (catabolism). Thus, a young animal with rapid growth has much more anabolism than catabolism. But when it reaches its full adult size, anabolism and catabolism are equal. If selection for rapid growth has been achieved by decreasing catabolism as well as increasing anabolism, then we may find that we have weakened the system responsible for the favourable effects of conditioning. Perhaps we are worrying about nothing and there is no problem, but that is what the cod fishermen said about over-fishing, right up until the time that the cod disappeared.


We have seen how muscle contraction is caused by thick and thin filaments sliding past each other to decrease the length of their sarcomeres. In turn, this decreases the length of fibrils, fibres and the whole muscle. If the muscle happens to be at a contracted length when rigor develops, we have a major problem, because the massive overlap of thick and thin filaments causes severe meat toughness. We avoid cooling meat too rapidly after slaughter, otherwise it cold-shortens and gets tough. On the positive side, provided we have not allowed cold-shortening, conditioning will increase meat tenderness. But with too much conditioning, we are going to worry about weight losses, surface spoilage, and the cost of refrigerated storage.