6.4 Muscle and tendon properties
6 min read•july 30, 2024
Muscles and tendons are crucial components of our body's movement system. They work together to generate force, store energy, and transmit power to our bones. Understanding their properties is key to grasping how our bodies perform in sports and everyday activities.
This section dives into the structure and function of muscles and tendons. We'll explore how they contract, stretch, and adapt to different loads. We'll also look at how their properties affect athletic performance and injury risk, giving us insights into training and injury prevention strategies.
Skeletal Muscle Structure and Composition
Hierarchical Organization and Contraction Mechanism
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- Skeletal muscle composed of hierarchical structures including myofibrils, sarcomeres, and myofilaments (actin and myosin)
- Sliding filament theory explains muscle contraction through interaction of actin and myosin filaments
- Myosin heads form cross-bridges with actin filaments
- ATP hydrolysis powers the sliding motion
- Muscle-tendon unit includes muscle belly, aponeurosis, and tendon working together to transmit force to skeleton
- Aponeurosis serves as broad, flat tendon-like structure within muscle
Fiber Types and Extracellular Matrix
- Muscle fiber types have distinct characteristics in contraction speed, force production, and fatigue resistance
- Type I (slow-twitch): Slow contraction, low force, high fatigue resistance (marathon running)
- Type IIa (fast-twitch oxidative): Fast contraction, moderate force, moderate fatigue resistance (sprinting)
- Type IIx (fast-twitch glycolytic): Fastest contraction, highest force, low fatigue resistance (powerlifting)
- Extracellular matrix in muscle plays crucial role in force transmission and tissue integrity
- Composed of collagen, proteoglycans, and glycoproteins
- Provides structural support and facilitates force transfer between muscle fibers
Tendon Composition and Structure
- Tendons dense connective tissue structures primarily composed of collagen fibers arranged in parallel fashion
- Collagen type I predominant (90% of dry weight)
- Fibers organized into fascicles surrounded by endotenon
- Tendon structure optimized for force transmission
- Crimped collagen fibers allow for initial elongation without damage
- Parallel arrangement of fibers maximizes
- Blood supply and innervation limited, contributing to slow healing process after injury
Force-Length and Force-Velocity Relationships
Force-Length Relationship
- Force-length relationship describes how muscle force production varies with muscle length based on overlap of actin and myosin filaments
- Optimal length for force production occurs at resting length of sarcomere with maximal overlap of actin and myosin filaments
- Too short: Filaments interfere, reducing cross-bridge formation
- Too long: Insufficient overlap, fewer cross-bridges formed
- Three distinct regions in force-length curve:
- Ascending limb: Force increases as muscle lengthens from shortened position
- Plateau region: Optimal force production at resting length
- Descending limb: Force decreases as muscle lengthens beyond optimal length
Force-Velocity Relationship
- illustrates inverse relationship between speed of muscle shortening and force produced
- Hill's equation mathematically describes force-velocity relationship in muscle contraction: Where F = force, V = velocity, F_0 = maximum isometric force, a and b = constants
- Concentric contractions: As velocity increases, force decreases
- Eccentric contractions: Muscle can produce greater force while lengthening
- Power output (force × velocity) peaks at approximately 1/3 of maximum shortening velocity
Applications and Implications
- Force-length and force-velocity relationships interact to determine power output of muscle during dynamic contractions
- Important implications for muscle performance in various sporting activities and rehabilitation exercises
- Plyometric training utilizes force-velocity relationship to enhance power output
- Resistance training at different muscle lengths targets specific portions of force-length curve
- Understanding these relationships crucial for:
- Optimizing exercise selection and technique
- Injury prevention strategies
- Rehabilitation program design
Muscle Mechanical Properties
Passive and Active Tension
- Passive tension in muscle primarily due to elastic properties of connective tissues and titin
- Titin large protein spanning sarcomere, contributes to passive tension at longer muscle lengths
- Active tension generated by cross-bridge cycling between actin and myosin filaments during muscle contraction
- Regulated by calcium release and ATP availability
- Total muscle tension sum of both passive and active components varying with muscle length and activation level
- At short lengths: Active tension dominates
- At long lengths: Passive tension becomes more significant
Muscle Stiffness and Elastic Components
- Muscle stiffness defined as resistance to length change influenced by both active and passive properties
- Active stiffness modulated by number of attached cross-bridges
- Passive stiffness determined by connective tissue and titin properties
- Series elastic component and parallel elastic component contribute to overall mechanical behavior of muscle
- Series elastic component: Tendon and aponeurosis
- Parallel elastic component: Connective tissue surrounding muscle fibers
- Muscle-tendon unit modeled as combination of contractile and elastic elements (Hill's muscle model)
Time-Dependent Properties
- Time-dependent properties of muscle affect its mechanical response during prolonged loading or stretching
- Creep elongation of muscle over time under constant load
- Relevant in activities requiring prolonged muscle tension (rock climbing)
- Stress relaxation decrease in muscle tension over time when held at constant length
- Important consideration in stretching exercises and flexibility training
- Thixotropy temporary change in muscle stiffness based on recent movement history
- Contributes to warm-up effects and morning stiffness
Tendon Viscoelasticity and Energy Storage
Viscoelastic Properties of Tendons
- Tendons exhibit viscoelastic properties combining both elastic (spring-like) and viscous (time-dependent) behaviors
- Stress-strain curve of tendons shows distinct regions each with different mechanical implications:
- Toe region: Initial low-stiffness region as crimped collagen fibers straighten
- Linear region: Elastic deformation with constant stiffness
- Yield point: Onset of microscopic damage leading to tendon failure
- Time-dependent behaviors:
- Creep: Continued elongation under constant load
- Stress relaxation: Decrease in stress when held at constant length
- Rate-dependent stiffness: Increased stiffness at higher loading rates
Energy Storage and Transmission
- Tendons store and release elastic energy contributing to efficient movement in activities like running and jumping
- Up to 35% of mechanical work in running stored in Achilles tendon
- Stiffness and compliance of tendons affect energy storage capacity and rate of force transmission to bones
- Stiffer tendons: Faster force transmission, less energy storage
- More compliant tendons: Greater energy storage, slower force transmission
- Hysteresis in tendons represents energy loss during loading and unloading cycles impacting overall movement efficiency
- Typically 5-10% energy loss in healthy tendons
- Higher hysteresis indicates less efficient energy return
Tendon Adaptation and Mechanical Properties
- Tendon mechanical properties can adapt to chronic loading altering energy storage capacity and injury resistance
- Adaptations to training:
- Increased cross-sectional area
- Enhanced collagen synthesis and alignment
- Improved material properties (e.g., increased stiffness)
- Factors influencing tendon properties:
- Age: Decreased and increased stiffness with aging
- Gender: Typically lower stiffness in females compared to males
- Training history: Specific adaptations based on loading patterns
Muscle and Tendon Properties in Performance and Injury
Performance Implications
- Balance between muscle strength and influences performance in explosive activities
- Optimal stiffness varies depending on specific sport requirements
- Muscle fiber type composition affects individual's capacity for power production versus endurance performance
- Sprinters: Higher proportion of Type II fibers
- Endurance athletes: Higher proportion of
- Muscle-tendon unit's ability to absorb and generate force impacts performance in high-impact activities and change-of-direction movements
- Efficient energy storage and release in tendons enhances performance in jumping and sprinting
Injury Risk Factors
- Eccentric overload can lead to muscle damage and delayed onset muscle soreness (DOMS) impacting subsequent performance and recovery
- Particularly prevalent in unaccustomed exercise or downhill running
- Tendon mechanical properties influence risk of tendinopathies and ruptures particularly in sports involving repetitive loading
- Achilles tendinopathy common in runners
- Patellar tendinopathy ("jumper's knee") frequent in volleyball and basketball players
- Imbalances in muscle strength or flexibility can increase injury risk
- Hamstring-to-quadriceps strength ratio important for knee injury prevention
Training and Injury Prevention Strategies
- Training interventions can modify muscle and tendon properties to enhance performance and reduce injury risk in specific sporting contexts
- Eccentric training effective for improving tendon stiffness and reducing tendinopathy risk
- Plyometric training enhances muscle-tendon unit function for explosive activities
- Age-related changes in muscle and tendon properties affect performance capabilities and injury susceptibility in older athletes
- Decreased muscle mass and force production (sarcopenia)
- Reduced tendon elasticity and increased stiffness
- Injury prevention strategies:
- Progressive loading to allow for tissue adaptation
- Proper warm-up to enhance muscle-tendon unit function
- Balanced strength training addressing agonist-antagonist muscle groups
Key Terms to Review (18)
Contractility: Contractility refers to the ability of muscle fibers to shorten and generate force during contraction. This property is fundamental for muscle function, as it enables movement and force production across joints. The efficiency of contractility is influenced by factors such as the length-tension relationship, muscle fiber type, and neural activation patterns.
Eccentric Contraction: Eccentric contraction refers to a type of muscle contraction where the muscle lengthens while generating force, typically while resisting an external load. This process plays a vital role in various physical activities, allowing for controlled deceleration and absorption of forces during movements like landing or descending. It also contributes to muscle development, energy efficiency, and injury prevention, linking it to crucial aspects such as power generation, coordination, muscle properties, strength training, and muscle actions.
Elasticity: Elasticity is the ability of a material to return to its original shape after being deformed when a force is applied. This property is crucial in understanding how muscles and tendons work, how equipment behaves during sports, and how flexibility affects performance. It’s also significant in designing protective gear that needs to absorb impacts while returning to its original form.
Extensor tendon: An extensor tendon is a fibrous connective tissue structure that connects extensor muscles to the bones of the fingers, wrist, and other joints, facilitating the extension movement. These tendons play a vital role in allowing muscles to exert force on bones to produce motion, particularly the straightening of joints during activities such as grasping and releasing objects.
Flexor tendon: A flexor tendon is a fibrous connective tissue that connects muscle to bone, specifically allowing for the bending movement of joints by transmitting the force generated by flexor muscles. These tendons play a crucial role in activities that require gripping and lifting, as they enable the flexion of various joints, particularly in the fingers and toes. Their structure and properties significantly influence both performance and injury susceptibility in physical activities.
Force-Velocity Relationship: The force-velocity relationship describes how the force a muscle can produce is inversely related to the velocity of contraction; as the speed of contraction increases, the maximum force that can be exerted by the muscle decreases. This relationship is crucial for understanding how muscles generate force during various types of movement and is influenced by several factors, including muscle and tendon properties, biomechanics during strength and plyometric training, and measurement techniques for assessing force production.
Hypertrophy: Hypertrophy refers to the increase in the size of muscle fibers, resulting in greater muscle mass and strength. This process occurs as a response to resistance training, where mechanical tension and metabolic stress on the muscles lead to cellular adaptations. Over time, hypertrophy plays a crucial role in improving athletic performance and enhancing physical appearance.
Isometric contraction: Isometric contraction is a type of muscle contraction where the muscle exerts force without changing its length, meaning there is no visible movement of the joint involved. This process is crucial in maintaining posture and stabilizing joints, connecting it to aspects of neuromuscular control, muscle properties, strength measurements, and training biomechanics.
Kinematics: Kinematics is the branch of mechanics that deals with the motion of objects without considering the forces that cause the motion. This field focuses on parameters such as displacement, velocity, acceleration, and time, which are critical for understanding how athletes move and perform in various sports.
Kinetics: Kinetics is the branch of mechanics that deals with the forces acting on and within a body, and the resulting motion caused by these forces. This concept is essential in understanding how various forces influence movement, providing insights into athletic performance, injury prevention, and equipment design.
Muscle strain: A muscle strain occurs when muscle fibers are stretched or torn due to excessive force or overuse. This injury can impact muscle function, strength, and flexibility, and is closely related to the properties of muscles and tendons, which can affect how they respond to stress and load during physical activities.
Tendon stiffness: Tendon stiffness refers to the ability of a tendon to resist deformation when subjected to mechanical loads. This property is crucial because it influences how effectively tendons transmit forces from muscles to bones during movement. Higher tendon stiffness can enhance the efficiency of muscle contractions and contribute to overall athletic performance, while lower stiffness may lead to a greater risk of injury.
Tendonitis: Tendonitis is an inflammation or irritation of a tendon, which is the thick fibrous cord that attaches muscle to bone. It often results from repetitive motion or overuse, leading to pain and discomfort in the affected area. This condition can significantly impact muscle and tendon properties by affecting their elasticity, strength, and overall function.
Tensile Strength: Tensile strength is the maximum amount of tensile (pulling) stress that a material can withstand before failing or breaking. It is a crucial property in understanding how muscles and tendons behave under load, as well as how connective tissues contribute to the overall stability and functionality of the musculoskeletal system. This measure helps in evaluating not just the strength but also the resilience of these biological materials when subjected to forces during physical activity.
Type I Fibers: Type I fibers, also known as slow-twitch muscle fibers, are specialized for endurance and sustained activity due to their high oxidative capacity. These fibers are rich in mitochondria, myoglobin, and capillaries, allowing them to efficiently utilize oxygen for aerobic metabolism. They play a crucial role in activities that require prolonged muscle contractions, such as long-distance running and cycling, and are connected to the properties of muscles and tendons, the mechanics of force production, and muscle actions during physical activities.
Type IIA fibers: Type IIA fibers, also known as fast-twitch oxidative fibers, are a category of muscle fibers that exhibit a combination of both fast contraction speeds and moderate endurance capabilities. These fibers are well-suited for activities that require both strength and endurance, making them crucial for various sports and high-intensity workouts. Their ability to generate force quickly while also utilizing aerobic metabolism allows them to play a significant role in muscle performance, especially during activities that involve bursts of speed or power followed by sustained efforts.
Type IIx fibers: Type IIx fibers, also known as fast-twitch glycolytic fibers, are a category of muscle fibers that are characterized by their high contraction speed and capacity for powerful, explosive movements. These fibers primarily utilize anaerobic metabolism for energy production, allowing them to generate quick bursts of force but with limited endurance. Understanding Type IIx fibers is essential for grasping muscle and tendon properties, how muscles produce force and respond to different velocities, and the overall muscular system and its actions during various physical activities.
Viscoelasticity: Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. This means that viscoelastic materials can deform under stress like a liquid (viscous behavior) but also return to their original shape once the stress is removed, akin to solid behavior. In the context of biological tissues, this property is crucial for understanding how muscles and tendons function during activities, how protective equipment can absorb and dissipate energy, and how connective tissues contribute to overall stability and flexibility in the body.