Athena Vale
Human skeletal muscle is composed of a heterogeneous collection of muscle fiber types, including slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) fibers. The ability of muscles to grow in length is referred to as the Flexibility Growth Factor (FGF). This growth occurs through the addition of new sarcomeres, which are added when muscles are stretched properly or through exercises such as eccentric tension exercises and contract-relax stretches. Studies have shown that a muscle can be contracted up to 70% of its normal resting length or stretched up to 130% of its normal resting length. While the biomechanical effects of long-term stretching programs are not yet fully understood, it is known that muscle length does increase during stretching due to the viscoelastic properties of muscle.
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What You'll Learn
Muscle extensibility increases with stretching
Muscle extensibility, or flexibility, is an important aspect of physical health and fitness. It refers to the ability of a muscle to extend or stretch beyond its resting length. While most people are aware that muscles can get bigger and thicker with strength training, they are often unaware that muscles can also grow longer and become more supple through stretching.
Stretching is a common activity for athletes, older adults, rehabilitation patients, and those engaged in fitness programs. It is well-established that stretching increases muscle extensibility or flexibility. This can be achieved through various types of stretching exercises, including static, dynamic, and pre-contraction stretching. The effectiveness of stretching is usually observed as an increase in joint range of motion (ROM), often measured by placing a standardized load on the limb and quantifying the resulting joint motion.
Several studies have shown that active stretch is necessary for regulating muscle fiber length. This is achieved by increasing the number of sarcomeres in series, which are the basic units of muscle fibers. Elevated cytoplasmic calcium is believed to be a critical component in activating the signaling pathways required for sarcomere number regulation. Additionally, eccentric exercises, when applied to muscles at long lengths, have been shown to improve the length force-characteristics by increasing both the maximum force and the length range of active force exertion.
The impact of stretching on muscle extensibility can be observed after a single stretching session and short-term stretching programs (3-8 weeks). However, it is important to note that the biomechanical effects of long-term (>8 weeks) and chronic stretching programs have not yet been extensively evaluated. Furthermore, the effectiveness of stretching may vary based on age and sex, with men and older adults under 65 years tending to respond better to contract-relax stretching, while women and older adults over 65 benefit more from static stretching.
In conclusion, muscle extensibility does increase with stretching, and this has been demonstrated in various studies. By engaging in stretching exercises, individuals can improve their muscle flexibility, range of motion, and overall physical health.
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The addition of sarcomeres lengthens muscle fibres
Muscles do not stretch like rubber bands; instead, they grow longer. This growth in length is referred to as the Flexibility Growth Factor (FGF). Muscles are made up of muscle fibres that are arranged in bundles parallel to each other. Each muscle fibre contains bundles of myofibrils, the subcellular contractile apparatus that consist of a highly ordered arrangement of longitudinal myofilaments. The basic unit of this arrangement is the sarcomere, which is defined as the periodic segment between two neighbouring Z-lines. The thick filaments are predominantly made of myosin, whereas the thin filaments are mainly composed of actin, tropomyosin, and troponin.
Eccentric exercises also stimulate the addition of parallel-arranged sarcomeres within the muscle fibres, resulting in increases in both fCSA and PCSA. Immobilization in a lengthened position, such as full dorsi flexion, results in a considerable addition of serial sarcomeres, ensuring that the optimum muscle length is attained. Studies have shown that lengthening immobilization of certain muscles in rats leads to hypertrophy.
Active stretch is necessary for regulating muscle fibre length, which is influenced by the number of series sarcomeres. Elevated cytoplasmic calcium is believed to be the component of contractile activity required to activate signalling pathways for sarcomere number regulation. Passive stretch, on the other hand, reduces muscle tissue stiffness.
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Muscle fibres can be lengthened with eccentric exercises
During an eccentric contraction, the muscle is stretched by an external force while contracting, and the muscle force developed is higher than that developed with a concentric contraction. This higher force production is due to the rate at which actin and myosin detach from one another being inhibited during the eccentric phase, allowing for more contractions. Eccentric exercises also stimulate the addition of parallel-arranged sarcomeres within the muscle fibres, increasing the length of the muscle.
Eccentric exercises can be used to increase muscle strength and mass, as they stimulate muscle hypertrophy and increase the fascicle pennation angle, fascicle length, and neural activation. They can also be beneficial for rehabilitation, particularly for microtrauma and overuse injuries, and for conditions such as tendinopathy, arthritis, and Parkinson's disease.
Some examples of eccentric exercises include modified push-ups, squats, and heel drops, where the person moves slowly while lowering the body and more quickly when raising it. It is important to note that eccentric exercises can cause muscle soreness or injury, so it is recommended to consult a doctor or physical therapist before starting a new exercise routine.
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Muscle length is increased with passive stretch
Muscle length can be increased with passive stretching. Passive stretching is a common intervention performed during rehabilitation to increase muscle length and range of motion (ROM) or to align collagen fibers during the healing process of a muscle. It is also used to treat muscle tightness, which can result from postural adaptation, scarring, spasms, or contraction.
Passive stretching can be performed by a clinician or physical therapist, or by the individual themselves. During passive stretching, the muscle is transmitted via the connective tissue (perimysium and endomysium) to the muscle fiber. The contractile apparatus must be linked with the noncontractile apparatus to lead to an immediate increase in sarcomere length. This process involves signal sensing, signal transduction, and subsequent gene transcription, resulting in sarcomere assembly.
Several studies have demonstrated the effectiveness of passive stretching in increasing muscle length. For example, a study on children with cerebral palsy found that passive dorsiflexion stretches performed for 5 sets of 20 seconds on each leg resulted in a significant increase in maximum dorsiflexion, medial gastrocnemius muscle length, and fascicle length. Another study on rat muscles found that immobilization in a lengthened position resulted in a considerable addition of serial sarcomeres, leading to an increase in optimum muscle length.
It is important to note that the biomechanical rationale alone cannot fully explain the long-term, permanent changes in muscle length observed after a stretching program. The muscle appears to actually become longer by adding sarcomeres in series, allowing for further excursion. This suggests that passive stretching can lead to a permanent adaptive response, resulting in a longer and functionally intact muscle.
Additionally, it is worth mentioning that active stretch is also necessary for regulating muscle fiber length. Active stretching involves moving a limb through its full range of motion and can be performed by the individual themselves. By combining passive and active stretching techniques, individuals can effectively increase muscle length and improve their overall flexibility and range of motion.
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Muscle growth and length are linked to myofibrillogenesis
The radial growth of muscle fibres, or an increase in their cross-sectional area (CSA), occurs in response to increased mechanical loading, such as resistance exercises. This growth can be mediated by either myofibril hypertrophy (an increase in the size of individual myofibrils) or myofibrillogenesis (an increase in the number of myofibrils). While the exact mechanisms are still being studied, research has shown that myofibrillogenesis plays a significant role in muscle growth.
Studies using novel imaging techniques, such as FIM-ID, have provided valuable insights into the role of myofibrillogenesis. By visualizing myofibrils in mouse and human models, researchers found that mechanical loading led to a significant increase in the CSA of muscle fibres, particularly Ox and Gly fibres. This increase was attributed to a rise in the area occupied by myofibrils, indicating that myofibrillogenesis contributes significantly to muscle growth.
Additionally, muscle length is not solely determined by stretching but also by the addition of new sarcomeres, the basic units of muscle fibres. Eccentric exercises and contract-relax stretches performed effectively can increase muscle length by adding new sarcomeres in parallel, improving both the maximum force and the length range of active force exertion. Therefore, muscle growth and length are intricately linked to myofibrillogenesis and the addition of sarcomeres, respectively.
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Frequently asked questions
Muscle fibers are the bundles of parallel fibers that make up muscles. There are three types of muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG).
Lengthening muscle fibers can be achieved through various stretching exercises, such as eccentric tension exercises and contract-relax stretches. It is important to note that muscles do not stretch like rubber bands but rather grow longer like retractable antennas.
The Flexibility Growth Factor (FGF) refers to the ability of muscles to grow in length and become more supple. Studies show that a muscle can contract up to 70% of its normal resting length or stretch up to 130% of its normal resting length.
Lengthening muscle fibers can improve muscle performance and increase the range of motion. It can also help prevent injuries and improve flexibility.
