Single Muscle Fibre Contractile Characteristics and Endurance Exercise
Written by Ben Bunting: BA(Hons), PGCert. Sport & Exercise Nutrition. British Army Physical Training Instructor.
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The contractile characteristics of single muscle fibres can be altered in response to endurance exercise. These changes are related to the training effects on calcium sensitivity and the AMPK activation that occurs during repetitive contractions.
Numerous studies have investigated these changes in different muscle groups and with varying training programs. The purpose of this article is to provide a comprehensive overview of the most relevant data regarding single-fiber adaptations in response to training.
Muscle Fibers
The muscles in our body work to control and coordinate movement. They do this by generating force through a process called depolarization, which is triggered by a stimulatory input like a nerve impulse or a heart rate signal. When that happens, your muscles contract and release energy to move your body.
Muscle fibers are a type of cell in the body that synthesizes contractile proteins to cause muscle contraction. They are highly multinucleated, and they are located in the skeletal muscle tissue.
Muscles are made of thousands of specialized muscle fibers wrapped together by connective tissue sheaths. The outermost sheath surrounds the entire muscle; the inner sheath encloses individual muscle fibers.
Skeletal muscles make up about 40% to 50% of the body's weight. They are typically classified into a number of groups based on location and function.
Some muscle groups are arranged in series, while others have parallel fibers. The orientation of the fibers influences how much force a muscle can produce, as well as its overall strength and flexibility.
Each muscle fiber has a specific morphology and is characterized by its own unique set of protein components. There are seven recognized human muscle fiber types, and they are grouped according to their myosin ATPase staining characteristics.
Myosin
Myosin is a protein that forms the core of thick myofilaments, which contain actin binding sites and ATP-binding sites for potential energy storage. Myosin also contains a head that projects out from the core.
When the actin and myosin head come in contact with each other, ATP breaks down into ADP, which releases energy that causes the myosin head to swivel back and forth.
The movement of myosin filaments toward the M line generates a strong pull, and the sarcomere shortens. In the opposite sarcomere, thin filaments are connected back to back at a structure called the Z-line or disk. Thin filaments are about 1.0 mm long, and the length varies with muscle type and species.
Endurance Exercise Adaptations
Endurance exercise is a form of training that focuses on improving your ability to perform at high levels for an extended period of time. It includes both general endurance and specific endurance for a particular sport or activity.
Adaptations of Muscular Endurance
When you perform an endurance exercise, your muscles use oxygen to produce ATP (the energy molecule in the cell). The size and number of the mitochondria (energy factories in the cells) increases as a result. This allows more ATP to be produced and makes it available for your muscles to use, thus increasing force production.
Ligament and Tendon Adaptations
The trained muscle also improves its ability to strengthen ligaments and tendons that connect bones to muscles. This enhances your strength and prevents injury.
Angiogenesis – Growth of New Capillaries
The increased blood flow to your muscles during exercise allows them to deliver more oxygen and nutrients to your tissues, improving your performance. This is known as angiogenesis, and it’s a key adaptation of endurance training.
Types of muscle fibers
Whether you're an all-star athlete, looking to train for a marathon or just trying to increase your strength, understanding the types of muscle fibers in your body is essential. There are two different types of skeletal muscle fibers, slow-twitch (type I) and fast-twitch (type II).
The proportion of each muscle fiber type in your body depends on a variety of factors, including training, age, and genetics. For example, athletes that want to improve their endurance need to focus on training their slow-twitch muscles. Conversely, athletes that want to maximize power or sprint speed need to target their fast-twitch muscle fibers.
It is well documented that people vary tremendously in their proportion of fast and slow muscle fibres within their muscles (Medler & Mykles, 2015; Schiaffino & Reggiani, 2011). Muscles from sprinters or marathon runners, for example, have more slow twitch type I muscle fibres than athletes with low levels of strength training (Klitgaard et al. 1990a; Aagaard et al. 2007).
The distribution of slow and fast muscle fibres differs between people, with the percentage of 'slow' type I fibres varying from under 20% in people who are excellent sprinters to over 95% in marathon runners. These differences are largely genetic, but also influenced by exercise. Different studies have shown that explosive exercise, such as box jumps, can increase the number of 'fast' fibres in skeletal muscles.
There are also significant correlations between the percentage of fast and slow muscle fibres and other skeletal muscle parameters, such as metabolic capacity, that are based on measurements of single muscle fibres. Hybrid fibers, which are the points on this continuum where 'fast' muscle fibres and 'slow' muscle fibres converge, are common in normal muscles (Medler & Mykles, 2010; Blaauw et al. 2013).
Interestingly, there is also evidence to suggest that the size and function of 'slow' fibres can be modified with exercise (Trappe et al. 2001; Williamson et al. 2012).
Lifelong endurance exercise (LLE) increases the size and function of 'slow' muscle fibres in men. These 'endurance' fibres are much larger, more powerful and faster than 'slow' muscle fibres of young exercisers or old healthy non-exercisers (Gries et al. 2019).
These findings support the concept that the ageing process can result in an increase in the size and function of 'slow' 'endurance' fibres with lifelong endurance exercise, which is partly mitigated by the training intensity. These findings complement data from other ageing studies that show the benefits of lifelong endurance exercise on whole muscle function (Chambers et al. 2020).
Muscle fiber force
The force of a single muscle fibre can vary significantly with the type of exercise and endurance training a person undertakes. For example, cardiovascular endurance exercises like running and swimming help to improve a person's VO2 max, or the body's maximum oxygen intake and aerobic capacity. They also develop a person's slow-twitch muscle fibres, assisting muscles to use oxygen more efficiently.
There are three types of skeletal muscle fibre, which produce different degrees of force: slow twitch fibres (Type I), fast twitch fibres (Type II) and hybrid muscle fibres containing both type I and type II fibres. All of these fibre types are suited to endurance training, but the strength of each of them is dependent on a variety of factors.
A study investigating muscle fiber force used a single muscle fibre that was isolated from a muscle bundle using a force transducer and motor arm. This was then transferred to an experimental chamber filled with relaxing solution.
Submaximal isotonic load clamps were performed on each fibre segment and force-velocity parameters and power were determined. The size of the muscle fibre, contractile velocity and force-velocity relationship were recorded at three points along the length of the fibre. The diameter of the fibre was compared to a standard reference, measured in millimetres.
For comparison, two different groups were tested (LLE-F and LLE-P) with a one-way nested ANOVA and Tukey's post hoc test. The results of this analysis showed that the contractile kinetics of slow muscle fibres were 30% larger in LLE-F than in YE and OH participants, suggesting that these lifelong endurance athletes had provided a hypertropic stimulus to their muscle fibres.
However, it is important to note that a large amount of research on the effect of lifelong endurance training on skeletal muscle has been carried out in men and women, so the impact of training intensity on muscle fibre function may be less obvious than it would be in an animal model.
In addition, the effect of chronic endurance training is likely to be complex, with shifts in muscle metabolism accompanied by increased mitochondrial density and oxidative enzymes, as well as changes in fibre type. These effects are thought to amplify the metabolic and physiological responses of muscle to exercise and lead to specific adaptations.
Muscle fiber specific tension
The specific tension of a single muscle fibre contractile characteristic is a measure of its intrinsic force-generating potential. It can be obtained by dividing the tendon force, by the physiological cross-sectional area (PCSA) of a muscle muscle. It is a relatively new measure that has been developed to overcome the limitations of in vivo muscle size and force estimates in children and adults.
A recent study investigated the effect of endurance exercise on specific tension in a muscle that has a high intrinsic force-generating capacity, the quadriceps muscle. During this test, subjects performed knee flexion and extension and leg raises with a range of knee angles. Muscle architecture was recorded during a maximum voluntary contraction (MVC) at a knee angle 10 deg more extended than where quad occurred and also during contraction at only 90% of MVC.
Muscle specific tension was measured in a hypotonic Ringer's solution using paired, two-tailed t-tests to compare to baseline tension and mass measurements. The mass was measured as the ratio of raw muscle weight to slack muscle length, with muscles dabbed on paper towel to remove surface water prior to mass measurement and then soaked at slack length to reduce intramuscular hydrostatic pressures that might impede osmosis.
This approach produced PCSA values that were slightly smaller than those from the instant centre of rotation method, which was used in previous studies. However, these were still significantly higher than those from the tibio-femoral contact point approach. This may be due to the fact that PTMA lengths during contraction were not available in this study.
Furthermore, it is possible that the PTMA lengths were underestimated because of the use of the tibio-femoral touch point rather than the instant centre of rotation, which would result in overestimation of tendon force. This could also explain the large difference in specific tension between the studies of Tsaopoulos et al. (2007) and this study.
Unloaded Shortening Velocity
The functional profile of single muscle fibres (diameter, cross-sectional area, peak force, unloaded shortening velocity, absolute power and normalized power) and their relationship with whole muscle size, strength and power are dependent on neural activation patterns and/or muscle metabolic properties.
The contractile characteristics of individual muscle fibres vary significantly with different modes of muscle activation, a process known as “modulating the F-V curve” by Sasaki and Ishii (2005). While Edman (1979) showed that the unloaded shortening velocity of single muscle fibres was independent of muscle activation level (twitch versus tetanus), Sasaki and Ishii (2005) reported that the F-V relationship of human muscles increased with muscle activation in vivo up to 487 deg s-1 at an activation level of 60% of maximal voluntary contraction.
Thus, the functional adaptations of single muscle fibres with endurance exercise are highly specific to the mode of muscle activation and are often influenced by the presence of fast fibres, which can increase the magnitude of the F-V relationship. However, other factors such as the length of the muscle, type of muscle and its excitation level can also affect this relationship.
Since muscle fibers have a high degree of plasticity, they can undergo a large range of changes with training load, which is often referred to as the “plasticity effect.” The functional adaptations of muscle fibres in response to lifelong endurance exercise are likely to be driven by the presence of fast fibres, which have been found to have robust molecular responses to a single bout of submaximal cycle exercise and easily recruit during submaximal cycles of exercise.
Conclusion
The contractile properties of individual muscle fibres are largely determined by their expression of different myosin heavy chain (MHC) isoforms. These MHC isoforms have a critical role in determining the contractile performance of human skeletal muscles, including speed, force and power output during exercise training.
The functional properties of individual fibers are highly specific to the mode of muscle activation. These include neural activity, muscle metabolic characteristics, and mechanical properties of the fibers themselves.
Training-induced changes in contractile properties of individual fibres are well documented. For example, high-intensity endurance swimming has been shown to induce changes in the contractile properties of the EDL and SOL of adult rats. These include increases in the sensitivity to Ca2+ and Sr2+, and lower thresholds for contraction by these activating ions.
