Muscle Contraction: Sliding Filament Theory
The relationship between stretching, strengthening, speed and force of contraction and muscle growth is complex, but they are all complementary. When muscle is viewed under extremely high magnification many features can be deduced about its structure. In the 1960's the sliding filament theory was developed by Huxley and accounted for many of these features and how muscle works.
When dealing with very small structures, the micron is used as the unit of measurement and is equal to 1/1000000 of a metre. At the molecular level, Angstrom units (Å = 1/10000 micron or 1/10000000000 metre) are used. For the purposes of the discussion that follows, the absolute size is not as important as the relative dimensions and the effects of change within muscle.
The smallest working unit of muscle is known as the sarcomere and is approximately 2.5 micron (25000Å) in length. A study of the structure of a single sarcomere reveals a great deal about muscle, the derivation of contractile power and the effects of lengthening (stretching). The manner in which sarcomeres are arranged into myofibrils and these layered into larger fibres builds up the general picture of muscle. The chemical origin of energy and its conversion into mechanical work is controlled electrically by nerve impulses carried down pathways from the brain and act upon the whole fibre: sarcomeres do not function in isolation.
Molecules of the protein actin aggregate into long thin filaments about 1 micron in length and are arranged linearly in a regular fashion. The filament appears as a helical structure much like two parallel lengths of string twisted around each other. This provides a very stable structure by internal binding and reinforcement. Along its length there are key locations regularly positioned 360º apart. The structure of the protein and the helical arrangement ensures that other binding sites are positioned at 120º, but 120º further around and interact with other protein filaments called myosin.
Each individual molecule making up the myosin aggregate has a globular shaped head (110Å by 40Å) and long tail (1400Å). These long molecules are coiled around each other in pairs and aligned such that all the heads in the aggregate point towards one end and tails the other. Not only this, but tails come together from another aggregate with all the heads pointing in the opposite direction forming a filament twice as long. The myosin filaments are in total about 1.5 micron in length and in the centre there is a short distance (0.2 micron) where only the tails overlap. It is a region devoid of the globular heads.
The aggregate arrangement of myosin filaments has a coiled-coil structure and pairs of molecules are themselves coiled around other pairs. One end of the actin filament is anchored to one end of the sarcomere. Facing this across a short distance is another filament of actin and is tethered to the other end. A myosin filament is positioned between these two actin filaments partially overlapping with both towards its ends. In three dimensions, other actin and myosin filaments are arranged into a regular hexagonal array where each thin filament has three neighbouring thick filaments and each thick filament is encircled by six thin filaments. In the muscle growth process these aggregate filaments, more myosin than actin, increase in thickness as more molecules are laid down lengthways. The number of myofibrils also increases making each individual muscle fibre larger.
In stretched muscle, the number of sarcomeres in the myofibril can increase so lengthening the myofibril. The action of muscle to generate contractile power is the interaction of the myosin heads with the regular key locations of the actin filaments to form cross-bridges. In resting muscle, the sarcomere length is roughly 2.5 micron but this distance shortens as the muscle contracts. The overlap between actin and myosin in each half of the sarcomere can close by 0.25 micron until the ends of two actin filaments come together (20% contraction) and corresponds to the maximum tension possible (the actin/myosin overlap). A further 0.125 microns closure is then possible and this amounts to a total contraction of 30%. The actin filaments cross over and close over that part of the myosin filament that has no globular heads and cannot form cross bridges, so this generates no further tension in the muscle only shortening.
If the distance between binding sites on actin is 360º then the myosin filament must move over about 10 of these sites for full contraction, or apart by the same amount for full stretch, in each half. The mechanism by which this is thought to occur is quite complex, but essentially the globular heads in the myosin attach to an actin binding site to start the process, detach and re-attach to the next one along, continuing to detach and re-attach along the actin filament and so on causing the shortening of the sarcomere and the muscle as a whole. In effect the myosin "walks along" the actin, pulling as it goes much like pulling on a rope with one hand jumping over the other hand. Contraction to even shorter lengths allows the myosin to continue to "walk along" the actin but tension begins to decrease. The globular heads of myosin have directionality and for sarcomeres at lengths of less about 1.75 micron, the actin filaments not only overlap with each other, but place some of the myosin heads into the wrong half of the sarcomere with the heads pointing the wrong way. Little or no contractile force can be developed from this arrangement and binding sites are lost.
In stretching, the total sarcomere length can increase by about 30% (up to 3.25 micron) and still form some actin/myosin cross-bridges. As the length of the muscle gets greater the tension within it becomes less as the two contractile proteins move further apart so that binding sites become less in number until beyond about 3.5 micron (40% length increase) muscle tension drops to zero (a protein called titin holds the sarcomere together when stretched beyond the limit of cross-bridge formation). This is supported by evidence that stretching a muscle weakens it. The movement between actin binding sites is the power stroke and is that part of the contraction process that requires energy - where work is done.
Neuromuscular efficiency is a term applied to the recruitment of muscle fibres to do work. Normally the number of muscle fibres involved in muscular work is sufficient only to perform usual activities. Although the brain is very capable of stimulating more fibres, it is somewhat lazy in only doing the minimum. As the requirement for work increases by using progressively higher loads (weight resistance training) the number of fibres used will increase. As the demand continues to increase the recruitment of fibres also increases. The brain learns to use more and more fibres to do the work and thus prevent muscle overload and damage. It is essentially a protection mechanism.
Four types of human muscle fibre have been identified. Type I fibres are referred to as the slow oxidative fibres and used to be called slow twitch fibres. The term slow is relative since this type of fibre can move a limb over 1000º/sec. The fibre responds to low intensity work and uses oxygen for long periods in the energy forming processes. A second type of fibres is classified into IIa, IIb and IIab. These used to be called fast twitch fibres. The differences are that Type IIa fibres are fast, but use oxygen and are fatigue resistant. Type IIb fibres are fast, glycolitic and do not use oxygen but quickly tire. They perform high intensity work for short periods. Type IIab fibres fall between Type IIa and Type IIb fibres being able to perform oxidatively and glycolitically with intermediate fatigueability. The term glycolitic means without oxygen (anaerobic) Depending on the work to be performed, the recruitment pattern follows Type I first then IIa, IIab and finally IIb as the demand rises. A principle of size recruitment. So, not only does an increase in work result in the recruitment of more muscle fibres but the type, depending on the intensity of work. There is no evidence to suggest a direct recruitment jump from Type I to Type IIb without any new involvement of Types IIa or IIab. This is the reason why no end of light load training will develop muscle strength, only endurance. High intensity loading must be used for strength improvements.
When dealing with very small structures, the micron is used as the unit of measurement and is equal to 1/1000000 of a metre. At the molecular level, Angstrom units (Å = 1/10000 micron or 1/10000000000 metre) are used. For the purposes of the discussion that follows, the absolute size is not as important as the relative dimensions and the effects of change within muscle.
The smallest working unit of muscle is known as the sarcomere and is approximately 2.5 micron (25000Å) in length. A study of the structure of a single sarcomere reveals a great deal about muscle, the derivation of contractile power and the effects of lengthening (stretching). The manner in which sarcomeres are arranged into myofibrils and these layered into larger fibres builds up the general picture of muscle. The chemical origin of energy and its conversion into mechanical work is controlled electrically by nerve impulses carried down pathways from the brain and act upon the whole fibre: sarcomeres do not function in isolation.
Molecules of the protein actin aggregate into long thin filaments about 1 micron in length and are arranged linearly in a regular fashion. The filament appears as a helical structure much like two parallel lengths of string twisted around each other. This provides a very stable structure by internal binding and reinforcement. Along its length there are key locations regularly positioned 360º apart. The structure of the protein and the helical arrangement ensures that other binding sites are positioned at 120º, but 120º further around and interact with other protein filaments called myosin.
Each individual molecule making up the myosin aggregate has a globular shaped head (110Å by 40Å) and long tail (1400Å). These long molecules are coiled around each other in pairs and aligned such that all the heads in the aggregate point towards one end and tails the other. Not only this, but tails come together from another aggregate with all the heads pointing in the opposite direction forming a filament twice as long. The myosin filaments are in total about 1.5 micron in length and in the centre there is a short distance (0.2 micron) where only the tails overlap. It is a region devoid of the globular heads.
The aggregate arrangement of myosin filaments has a coiled-coil structure and pairs of molecules are themselves coiled around other pairs. One end of the actin filament is anchored to one end of the sarcomere. Facing this across a short distance is another filament of actin and is tethered to the other end. A myosin filament is positioned between these two actin filaments partially overlapping with both towards its ends. In three dimensions, other actin and myosin filaments are arranged into a regular hexagonal array where each thin filament has three neighbouring thick filaments and each thick filament is encircled by six thin filaments. In the muscle growth process these aggregate filaments, more myosin than actin, increase in thickness as more molecules are laid down lengthways. The number of myofibrils also increases making each individual muscle fibre larger.
In stretched muscle, the number of sarcomeres in the myofibril can increase so lengthening the myofibril. The action of muscle to generate contractile power is the interaction of the myosin heads with the regular key locations of the actin filaments to form cross-bridges. In resting muscle, the sarcomere length is roughly 2.5 micron but this distance shortens as the muscle contracts. The overlap between actin and myosin in each half of the sarcomere can close by 0.25 micron until the ends of two actin filaments come together (20% contraction) and corresponds to the maximum tension possible (the actin/myosin overlap). A further 0.125 microns closure is then possible and this amounts to a total contraction of 30%. The actin filaments cross over and close over that part of the myosin filament that has no globular heads and cannot form cross bridges, so this generates no further tension in the muscle only shortening.
If the distance between binding sites on actin is 360º then the myosin filament must move over about 10 of these sites for full contraction, or apart by the same amount for full stretch, in each half. The mechanism by which this is thought to occur is quite complex, but essentially the globular heads in the myosin attach to an actin binding site to start the process, detach and re-attach to the next one along, continuing to detach and re-attach along the actin filament and so on causing the shortening of the sarcomere and the muscle as a whole. In effect the myosin "walks along" the actin, pulling as it goes much like pulling on a rope with one hand jumping over the other hand. Contraction to even shorter lengths allows the myosin to continue to "walk along" the actin but tension begins to decrease. The globular heads of myosin have directionality and for sarcomeres at lengths of less about 1.75 micron, the actin filaments not only overlap with each other, but place some of the myosin heads into the wrong half of the sarcomere with the heads pointing the wrong way. Little or no contractile force can be developed from this arrangement and binding sites are lost.
In stretching, the total sarcomere length can increase by about 30% (up to 3.25 micron) and still form some actin/myosin cross-bridges. As the length of the muscle gets greater the tension within it becomes less as the two contractile proteins move further apart so that binding sites become less in number until beyond about 3.5 micron (40% length increase) muscle tension drops to zero (a protein called titin holds the sarcomere together when stretched beyond the limit of cross-bridge formation). This is supported by evidence that stretching a muscle weakens it. The movement between actin binding sites is the power stroke and is that part of the contraction process that requires energy - where work is done.
Neuromuscular efficiency is a term applied to the recruitment of muscle fibres to do work. Normally the number of muscle fibres involved in muscular work is sufficient only to perform usual activities. Although the brain is very capable of stimulating more fibres, it is somewhat lazy in only doing the minimum. As the requirement for work increases by using progressively higher loads (weight resistance training) the number of fibres used will increase. As the demand continues to increase the recruitment of fibres also increases. The brain learns to use more and more fibres to do the work and thus prevent muscle overload and damage. It is essentially a protection mechanism.
Four types of human muscle fibre have been identified. Type I fibres are referred to as the slow oxidative fibres and used to be called slow twitch fibres. The term slow is relative since this type of fibre can move a limb over 1000º/sec. The fibre responds to low intensity work and uses oxygen for long periods in the energy forming processes. A second type of fibres is classified into IIa, IIb and IIab. These used to be called fast twitch fibres. The differences are that Type IIa fibres are fast, but use oxygen and are fatigue resistant. Type IIb fibres are fast, glycolitic and do not use oxygen but quickly tire. They perform high intensity work for short periods. Type IIab fibres fall between Type IIa and Type IIb fibres being able to perform oxidatively and glycolitically with intermediate fatigueability. The term glycolitic means without oxygen (anaerobic) Depending on the work to be performed, the recruitment pattern follows Type I first then IIa, IIab and finally IIb as the demand rises. A principle of size recruitment. So, not only does an increase in work result in the recruitment of more muscle fibres but the type, depending on the intensity of work. There is no evidence to suggest a direct recruitment jump from Type I to Type IIb without any new involvement of Types IIa or IIab. This is the reason why no end of light load training will develop muscle strength, only endurance. High intensity loading must be used for strength improvements.
posted by Louis Brothnias @ 10:01 AM
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