Muscle Properties - Speed and Force

Muscle Properties - Speed

• As muscles shorten faster, maximum force decreases, following the force-velocity relation.
• Power is calculated as F x v, leading to a power-velocity relation.
• There is a trade-off between speed and force, meaning we can either move slowly and lift heavy or move quickly and lift light, but not both.

Muscle Properties - Length

• Shortening and lengthening of sarcomeres changes the overlap of myosin and actin, affecting the number of bound cross-bridges and ultimately sarcomere and muscle force.
• Optimum length is approximately 2.7 µm, which is about 1/10 the width of a fine hair.
• Changes in actin-myosin overlap largely underpin the force-length relation.
• Within a muscle, sarcomeres can be at different lengths.
• The force-length relation shifts as activation changes, with a shift in optimum length with frequency (force).

Muscle Structure - Myosin

• Myosin has a heavy chain (head, neck, and tail) and light chains that influence function.
• The head/neck (motor) domain bends to pull on actin, working in low gear.
• Myosin molecules arrange themselves with tails together and heads at the ends.

Muscle Structure - Sarcomere

• Actin and myosin are the major constituents of the fundamental unit of muscle: the sarcomere.
• The sarcomere has a lattice structure.

Muscle Properties

• Myosin heads rotate, detach, recover, and reattach to actin in a process called cross-bridge cycling.
• The recover-reattach time is constant, so faster cycle rates result in less time for myosins to be in contact with actin, reducing force production.

Muscle Architecture

• Different muscle architectures include pennate and parallel-fibred muscles.
• Long fibres/fascicles allow for large range of motion and more work, but have higher metabolic costs.
• Short fibres have lower metabolic costs.
• Longer fibres have more sarcomeres in series, which don't increase isometric force but use more ATP.

Fibre Length and O2/ATP

• Longer fibres have more sarcomeres in series, using more ATP to generate tension.
• Shorter fibres have lower metabolic costs.

Fibre Length and Speed

• Longer fibres have more sarcomeres in series, resulting in slower shortening velocity.
• Shorter fibres are better suited for high-speed movements.

Fibre Length and Work

• Longer fibres have a greater range of shortening, performing more work.
• Longer fibres require less sarcomere shortening, leading to slower velocity.

Muscle Properties - Speed

• As muscles shorten faster, maximum force decreases, known as the force-velocity relation
• Power is the product of force and velocity, resulting in a power-velocity relation
• There is a trade-off between moving slowly and lifting heavy, or moving quickly and lifting light, but not both

Muscle Properties - Length

• Shortening and lengthening of sarcomeres changes the overlap of myosin and actin, affecting sarcomere and muscle force
• Optimum length for muscle force generation is around 2.7 µm, similar to 1/10 the width of a fine hair
• Changes in actin-myosin overlap underpin the force-length relation
• Within a muscle, sarcomeres can be at different lengths
• In vivo operating length relative to optimum differs between muscles

Muscle Properties - Length (continued)

• Force-length relation shifts as activation changes, with electrically-stimulated forces at low frequencies being good for lengthening the muscle, but not at high frequencies
• Actin and myosin are closer together at long lengths, making interaction more likely for a given activation level

Muscle Structure - Myosin

• Myosin has a heavy chain (head, neck, and tail) and light chains that influence function
• The head/neck (motor) domain bends to pull on actin, working in low gear like a bike or car going up a hill
• Myosin molecules arrange themselves with tails together and heads at the ends, allowing them to 'walk' along actin to cause muscle contraction

Muscle Structure - Sarcomere

• Actin and myosin are the major constituents of the sarcomere, the fundamental unit of muscle
• The lattice structure of sarcomeres is important for muscle function

Muscle Contraction

• Myosin heads rotate, detach, recover, and reattach to actin in a process called cross-bridge cycling
• The faster the cycle rate, the less time myosins are in contact with actin, resulting in less force being produced

Pennation

• Some muscles have high angles of pennation, allowing more contractile tissue to attach to the tendon/aponeurosis
• This increases the physiological cross-sectional area (PCSA) of the muscle, allowing for more force production
• Fibres also rotate as they shorten, reducing the amount of shortening required and increasing force production

Pennation and Rotation

• Sarcomeres shorten slower and generate more force due to the force-velocity relationship
• Fibres can remain closer to optimum length, examples include gastrocnemius, vastus lateralis and medialis, and triceps brachii

Viscoelasticity

• Muscles exhibit viscoelastic properties, with creep (increased strain under constant stress) and stress relaxation (decreased stress at constant strain)
• Viscoelasticity provides protection from injury and over-elongation when loaded rapidly
• Tendon stiffness and hysteresis influence movement capacity

Muscle Properties - Speed

• As muscles shorten faster, maximum force decreases, known as the force-velocity relation
• Power is the product of force and velocity (P = F x v), so there is also a power-velocity relation
• We can either move slowly and lift heavy, or move quickly and lift light, but we can't do both (heavy and fast)

Muscle Properties - Length

• Shortening and lengthening of sarcomeres change the overlap of myosin and actin, affecting sarcomere and muscle force
• At short lengths, there are fewer bound cross-bridges and actin-myosin collisions, resulting in decreased force
• At optimum length (~2.7 µm), there are most bound cross-bridges, resulting in maximum force
• At long lengths, there are again fewer bound cross-bridges and actin-myosin collisions, resulting in decreased force
• The force-length relation is largely, but not completely, underpinned by changes in actin-myosin overlap
• Within a muscle, sarcomeres can all be at different lengths
• The "best" length is not always the "optimum" length determined during a maximal contraction

Muscle Properties - Length (continued)

• When a muscle connects to a long tendon, higher force stretches the tendon, causing the muscle to work at a shorter length
• Alternatively, reducing force allows the tendon to shorten, stretching the muscle anyway
• The optimum "muscle-tendon unit" length, and therefore the optimum joint angle, probably changes little with changes in force
• Passive forces also contribute when a muscle is stretched, including the parallel elastic component (PEC)
• The PEC includes membranes surrounding fibers, fascicles, and whole muscles, keeping the muscle from overstretching
• However, the PEC passive force decreases or becomes zero during contraction as the fibers shorten and stretch the series elastic component (SEC)

Muscle Architecture

• Muscles can have varying architectures, including pennate and parallel fibers
• Long fibers/fascicles allow for a large range of motion (ROM) and perform more work (F x d)
• Long fibers/fascicles have high shortening speeds
• Short fibers have lower metabolic cost (less ATP use)
• Longer fibers have more sarcomeres in series, generating good force at higher shortening speeds
• Examples of muscles with different architectures include the vastus lateralis, hamstrings, and gluteus maximus

The Tendon

• Muscles transfer their forces through tendons to the bones
• Understanding the tendon is key to understanding functional movement
• Tendons have a hierarchical structure, including molecules, fibrils, fascicles, and tendons
• The fundamental component is collagen, which is elastic due to covalent bonds between amino acids
• Cross-linking of collagen fibrils increases stiffness and reduces breaking of fibrils
• Load sharing occurs between fibrils and fascicles, and shear between these constituents allows for tendon elongation

Tendon Force-Length Relation

• Tendons stretch when loaded, but properties vary
• The force-length relation has a toe region, linear region, and failure region
• When loaded, collagen stretches, and some collagen stretches, resulting in damage to collagen and filamentous/fascicular shear

Tendon Stress-Strain Relation

• To compare tendons of different sizes and types, we normalize force and length
• Stress is the force change per cross-sectional area (N/mm²)
• Strain is the length change per initial length (%)
• The stress-strain relation shows that tendons dissipate energy partly through shear between fibrils and fascicles during relaxation

Viscoelasticity

• Biological tissues, including tendons, exhibit viscoelastic behavior, combining viscous and elastic properties
• Viscoelasticity is important for injury prevention and braking, but not for propulsion/locomotion
• Creep occurs when there is increased strain under constant stress, and stress relaxation occurs when there is decreased stress at constant strain
• Fibril sliding causes viscoelasticity, providing greater protection from injury and over-elongation when loaded rapidly