Logan Steele
Muscle protein synthesis (MPS) is the biological process that supports muscle growth, repair, and recovery. It is triggered by exercise and protein ingestion, with the latter providing the body with amino acids, which are essential for MPS. However, the body can only utilize a finite amount of amino acids, and excessive protein intake may lead to the breakdown and excretion of excess amino acids by the liver. Exercise, particularly resistance training, plays a crucial role in MPS, with acute exercise stimuli causing muscle contraction and promoting MPS responses. The sliding filament theory explains that muscle contraction occurs when actin slides past myosin, resulting in the shortening of the sarcomere and, consequently, the muscle.
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What You'll Learn
Sliding filament theory
The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement. The sliding filament theory was introduced in 1954 by two research teams, one consisting of Andrew Huxley and Rolf Niedergerke from the University of Cambridge, and the other consisting of Hugh Huxley and Jean Hanson from the Massachusetts Institute of Technology. It was originally conceived by Hugh Huxley in 1953.
The theory states that the sliding of actin past myosin generates muscle tension. Myosin (thick filaments) of muscle fibres slide past the actin (thin filaments) during muscle contraction, while the two groups of filaments remain at a relatively constant length. The A band contains thick filaments of myosin, which suggested that the myosin filaments remained central and constant in length while other regions of the sarcomere shortened. The I band, rich in thinner filaments made of actin, changed its length along with the sarcomere.
The theory directly introduced a new concept called cross-bridge theory, which explains the molecular mechanism of sliding filament. Cross-bridge theory states that actin and myosin form a protein complex by the attachment of the myosin head on the actin filament, thereby forming a sort of cross-bridge between the two filaments.
The sliding filament theory has been supported by various experiments and observations. For example, in 1994, William Lehman and his colleagues demonstrated how tropomyosin rotates by studying the shape of actin and myosin in either calcium-rich solutions or solutions containing low calcium. They found that the presence of calcium is essential for the contraction mechanism as it exposes the myosin-binding sites on actin. Once the myosin binds to actin, the sarcomere shortens and the muscle contracts.
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Myosin and actin filaments
Muscle proteins in humans and other mammals are more abundant than any other type of protein. Muscle fibres are composed of myofibrils, which include the proteins actin and myosin. These two proteins are the most abundant in muscle and are directly involved in a muscle's ability to contract and relax.
Myosin is a contractile protein that is insoluble in water. It consists of an elongated, probably double-stranded, peptide chain, which is coiled at both ends in such a way that a terminal globule is formed. The length of the molecule is approximately 160 nanometres, and its average diameter is 2.6 nanometres. Myosin contains many amino acids with positively and negatively charged side chains, which form 18 and 16 per cent, respectively, of the total number of amino acids.
Actin is a thinner filament that makes up the "I band" in the sarcomere arrangement. Actin is tethered to structures located at the lateral ends of each sarcomere called Z discs or "Z bands". The sliding filament theory states that the sliding of actin past myosin generates muscle tension. Therefore, any shortening of the actin filament length would result in a shortening of the sarcomere and thus the muscle.
The sliding filament theory also explains how myosin is able to pull upon actin to shorten the sarcomere. The globular end of each myosin protein nearest actin, called the S1 region, has multiple hinged segments that can bend and facilitate contraction. The bending of the myosin S1 region helps explain how myosin moves or "walks" along actin. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility and rotates in concert with the S1 contraction. The movements of myosin appear to be a kind of molecular dance. The myosin reaches forward, binds to actin, contracts, releases actin, and then reaches forward again to bind actin in a new cycle. This process is known as myosin-actin cycling.
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Muscle protein synthesis
MPS is the driving force behind adaptive responses to exercise and is a widely adopted proxy for gauging the efficacy of acute interventions such as exercise and nutrition. An acute exercise stimulus, especially resistance exercise, and protein ingestion both stimulate MPS and work synergistically when protein consumption occurs before or after resistance exercise. The anabolic effect of exercise is long-lasting (at least 24 hours) but likely diminishes with increasing time post-exercise. Nutrient-driven increases in MPS are of finite duration (approximately 1.5 hours), switching off despite sustained amino acid availability.
MPS rates vary depending on the type of exercise performed. Endurance-type exercises such as running or cycling are associated with increased synthesis of mixed muscle proteins. However, these acute responses are not associated with significant changes in muscle mass. In contrast, resistance training leads to a persistent positive MPS balance, resulting in the accumulation of contractile material (actin and myosin) and muscle hypertrophy. Studies have shown that MPS responses are similar regardless of the mode of exercise, with the duration of sensitization being the main difference.
The measurement of MPS is commonly expressed as the rate of amino acid incorporation into bound muscle protein over a given time, typically an hour or a day. The most common approach to measuring MPS is the precursor-product method, which allows for the determination of the muscle protein fractional synthesis rate (FSR). This method utilizes stable isotope-labeled amino acids, typically administered intravenously under controlled laboratory conditions, to directly trace the incorporation of free amino acids into newly synthesized bound muscle proteins.
For building and maintaining muscle mass, a daily protein intake of 1.4-2.0 g protein/kg body weight/day is sufficient for people exercising. Higher protein intakes (>3.0 g/kg/day) may have positive effects on body composition in resistance-trained individuals, promoting the loss of fat mass. The ideal protein intake per serving for athletes to maximize MPS is approximately 0.25 g of high-quality protein per kg of body weight or an absolute dose of 20-40 g, distributed evenly throughout the day.
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Contractile proteins
Muscle proteins are the most important component of striated skeletal muscle. Muscle fibres are composed of myofibrils, which include the contractile proteins myosin and actin. Myosin constitutes approximately one half of the total myofibrillar protein, while actin makes up about one-fifth. These two proteins are responsible for the ability of muscles to contract and relax.
Myosin is a contractile protein that occurs in muscle fibres and blood platelets. It is insoluble in water and consists of an elongated, probably double-stranded peptide chain, coiled at both ends to form a terminal globule. The length of the molecule is approximately 160 nanometres, with an average diameter of 2.6 nanometres. Myosin contains many amino acids with positively and negatively charged side chains, which form 18 and 16 per cent, respectively, of the total number of amino acids.
Actin is the principal component of thin myofilaments. It is tethered to structures called Z discs or Z bands, located at the lateral ends of each sarcomere. The sliding movement of the actin-myosin protein conjugate is responsible for contracting muscles. During contraction, the myosin filaments remain centred while the actin filaments slide past, generating muscle tension and resulting in the shortening of the sarcomere and thus the muscle.
The contractile proteins are soluble in salt solutions and susceptible to enzymatic digestion. The energy required for muscle contraction is provided by the oxidation of carbohydrates or lipids.
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Resistance exercise
There are many ways to perform resistance exercises, including:
- Free weights (e.g. dumbbells, barbells, kettlebells)
- Weight machines
- Resistance bands
- Body weight exercises (e.g. squats, push-ups, chin-ups)
A typical beginner's strength training program involves eight to ten exercises targeting the major muscle groups, performed two to three times per week. It is recommended to start with one set of each exercise, consisting of eight repetitions, and gradually increase to two to three sets of eight to twelve repetitions. It is important to warm up before starting strength training exercises, such as with light aerobic exercise and dynamic stretches.
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Frequently asked questions
Muscle proteins are the basic material of tissue structure and are the most important component of striated skeletal muscle.
The sliding filament theory states that the sliding of actin past myosin generates muscle tension and shortens the sarcomere and thus the muscle.
The sliding filament theory is a theory that describes the molecular basis of muscle contraction. It was proposed by scientists in 1954 and has remained impressively intact.
Myosin is a contractile protein that is involved in the ability of a muscle to contract and relax. It constitutes as much as 35% of the total protein in muscles.
Exercise training, especially resistance exercise, shortens the duration of the anabolic response and stimulates muscle protein synthesis (MPS).
