6.4 Muscle and tendon properties

Skeletal Muscle Structure and Composition

Hierarchical Organization and Contraction Mechanism

Skeletal muscle is built like a series of nested tubes. The whole muscle contains bundles of myofibrils, which are made up of repeating units called sarcomeres. Each sarcomere contains two key myofilaments: actin (thin) and myosin (thick).

Muscle contraction follows the sliding filament theory:

1. A nerve impulse triggers calcium release inside the muscle fiber.
2. Calcium exposes binding sites on actin filaments.
3. Myosin heads attach to actin, forming cross-bridges.
4. ATP hydrolysis powers the myosin heads to pull actin inward, shortening the sarcomere.
5. The cycle repeats as long as calcium and ATP are available.

The full muscle-tendon unit includes the muscle belly, the aponeurosis (a broad, flat sheet of connective tissue within the muscle), and the tendon itself. Together, these structures transmit contractile force from the muscle fibers to the skeleton.

Fiber Types and Extracellular Matrix

Muscle fibers aren't all the same. They fall into three main types, each suited to different demands:

The extracellular matrix surrounding muscle fibers is composed of collagen, proteoglycans, and glycoproteins. It provides structural support and plays a direct role in transmitting force laterally between fibers, not just along the length of the muscle.

Tendon Composition and Structure

Tendons are dense connective tissue structures built primarily from type I collagen, which makes up about 90% of their dry weight. The collagen fibers are arranged in parallel bundles called fascicles, each wrapped in a thin sheath of connective tissue called endotenon.

Two structural features make tendons well suited for force transmission:

• Crimped collagen fibers allow a small amount of initial elongation without damage, acting as a built-in buffer.
• The parallel fiber arrangement maximizes tensile strength along the pulling direction.

Tendons have limited blood supply and innervation. This is why tendon injuries heal slowly compared to muscle injuries, and it's a major factor in managing conditions like Achilles tendinopathy.

Force-Length and Force-Velocity Relationships

Force-Length Relationship

The force-length relationship describes how the amount of force a muscle can produce depends on its current length. This comes down to how much actin and myosin overlap within each sarcomere.

At the sarcomere's resting length, overlap between actin and myosin is maximized, so the greatest number of cross-bridges can form. Move away from that optimal length in either direction and force drops:

• Too short: Actin filaments from opposite sides collide and interfere with cross-bridge formation.
• Too long: There's not enough overlap for many cross-bridges to attach.

The resulting force-length curve has three regions:

1. Ascending limb — Force increases as the muscle lengthens from a shortened position toward optimal length.
2. Plateau region — Peak force production near resting length.
3. Descending limb — Force decreases as the muscle stretches beyond optimal length.

Force-Velocity Relationship

The force-velocity relationship captures the trade-off between how fast a muscle shortens and how much force it can produce. During concentric (shortening) contractions, faster movement means less force.

This relationship is described mathematically by Hill's equation:

$(F + a)(V + b) = (F_0 + a)b$

where $F$ = force, $V$ = velocity, $F_0$ = maximum isometric force, and $a$ and $b$ are experimentally determined constants specific to the muscle.

Key points from this relationship:

• Concentric contractions: Force decreases as shortening velocity increases.
• Eccentric contractions: The muscle can produce greater force than its isometric maximum while being lengthened. This is why lowering a heavy weight feels more controllable than lifting it.
• Peak power output (force × velocity) occurs at roughly one-third of maximum shortening velocity, not at maximum force or maximum speed.

Applications and Implications

These two relationships interact during real movement. A muscle's actual output depends on both its current length and how fast it's changing length.

Practical applications include:

• Plyometric training exploits the force-velocity relationship by using a rapid eccentric stretch before a concentric contraction, increasing power output through the stretch-shortening cycle.
• Resistance training at varied joint angles targets different portions of the force-length curve, which matters for sport-specific strength.
• Rehabilitation programs use these principles to select exercises at appropriate muscle lengths and speeds for recovering tissues.

Muscle Mechanical Properties

Passive and Active Tension

Muscle tension has two sources. Active tension comes from cross-bridge cycling between actin and myosin during contraction. It's regulated by calcium release and ATP availability. Passive tension comes from the elastic properties of connective tissues and a giant protein called titin that spans the sarcomere like a molecular spring.

The total tension in a muscle is the sum of both components, and their relative contributions shift with muscle length:

• At short lengths, active tension dominates.
• At long lengths, passive tension becomes increasingly significant as titin and connective tissues are stretched.

This is why you feel resistance when stretching a muscle even without any voluntary contraction.

Muscle Stiffness and Elastic Components

Muscle stiffness is the resistance to a change in length. It has both active and passive contributors:

• Active stiffness depends on how many cross-bridges are currently attached. More activation means more stiffness.
• Passive stiffness depends on connective tissue and titin properties.

The classic Hill muscle model represents the muscle-tendon unit as a combination of elements:

• Series elastic component (SEC): The tendon and aponeurosis, arranged in series with the contractile element. When the muscle contracts, it stretches the SEC before force reaches the bone.
• Parallel elastic component (PEC): Connective tissue surrounding muscle fibers (epimysium, perimysium, endomysium). This resists stretch even when the muscle is relaxed.
• Contractile element: The actin-myosin cross-bridge machinery itself.

Time-Dependent Properties

Muscle doesn't respond the same way to a quick pull as it does to a slow, sustained stretch. Three time-dependent properties matter here:

• Creep: Under a constant load, muscle gradually elongates over time. This is relevant in activities requiring prolonged tension, like holding a grip in rock climbing.
• Stress relaxation: When held at a constant length, the tension in the muscle gradually decreases. This is why holding a stretch becomes easier after several seconds.
• Thixotropy: Muscle stiffness temporarily changes based on recent movement history. A muscle that has been still for a while feels stiffer until it's moved through a few cycles. This contributes to both warm-up effects and the stiffness you feel first thing in the morning.

Tendon Viscoelasticity and Energy Storage

Viscoelastic Properties of Tendons

Tendons are viscoelastic, meaning they behave partly like a spring (elastic) and partly like a viscous fluid (time-dependent). This dual nature shows up clearly in the tendon stress-strain curve, which has three distinct regions:

1. Toe region (0–2% strain): Crimped collagen fibers straighten out. Stiffness is low, and the tendon elongates easily.
2. Linear region (2–4% strain): Fibers are straightened and bear load uniformly. The tendon behaves elastically with relatively constant stiffness.
3. Beyond the yield point (>4% strain): Microscopic collagen damage begins. Continued loading leads to partial or complete tendon rupture.

Tendons also display time-dependent behaviors similar to muscle:

• Creep: Continued elongation under constant load.
• Stress relaxation: Gradual decrease in stress at constant length.
• Rate-dependent stiffness: Tendons are stiffer when loaded quickly, which is why rapid movements produce different mechanical responses than slow ones.

Energy Storage and Transmission

One of the tendon's most important roles in sport is acting as an energy storage device. During activities like running and jumping, tendons stretch under load, store elastic energy, and then release it during recoil. The Achilles tendon alone can store up to 35% of the mechanical work during running, significantly reducing the metabolic cost of locomotion.

Tendon stiffness determines the balance between energy storage and force transmission speed:

• Stiffer tendons transmit force to bone more quickly but store less energy. This benefits activities requiring rapid, precise force application.
• More compliant tendons store more elastic energy but transmit force more slowly. This benefits cyclical activities like distance running.

Hysteresis refers to the energy lost as heat during each loading-unloading cycle. Healthy tendons lose only about 5–10% of stored energy per cycle. Higher hysteresis means less efficient energy return and can indicate tendon pathology.

Tendon Adaptation and Mechanical Properties

Tendons adapt to chronic loading, though more slowly than muscle due to their limited blood supply. Training-induced adaptations include:

• Increased cross-sectional area
• Enhanced collagen synthesis and improved fiber alignment
• Greater material stiffness

Several factors influence baseline tendon properties:

• Age: Tendons lose elasticity and become stiffer with aging, reducing energy storage capacity.
• Sex: Females typically have lower tendon stiffness than males, which may relate to differences in injury patterns.
• Training history: Tendons adapt specifically to the type of loading they experience. Heavy slow resistance training, for example, promotes different adaptations than plyometric training.

Muscle and Tendon Properties in Performance and Injury

Performance Implications

The balance between muscle strength and tendon stiffness directly affects performance in explosive activities. A sprinter benefits from stiffer tendons for rapid force transmission, while a distance runner benefits from more compliant tendons for energy storage. Optimal stiffness is sport-specific.

Fiber type composition also shapes performance capacity. Elite sprinters tend to have a higher proportion of Type II fibers, while elite endurance athletes have more Type I fibers. Training can shift Type IIx fibers toward Type IIa, but the overall slow-to-fast ratio is largely genetic.

The muscle-tendon unit's ability to absorb and release energy efficiently is critical in high-impact activities and change-of-direction movements. Efficient elastic energy return in the tendons directly enhances jump height and sprint acceleration.

Injury Risk Factors

• Eccentric overload can cause microscopic muscle damage and delayed onset muscle soreness (DOMS), particularly during unaccustomed exercise or downhill running. DOMS typically peaks 24–72 hours after exercise.
• Tendinopathies develop from repetitive overloading that exceeds the tendon's capacity to repair. Common examples include Achilles tendinopathy in runners and patellar tendinopathy ("jumper's knee") in volleyball and basketball players.
• Muscle imbalances increase injury risk. The hamstring-to-quadriceps strength ratio is a well-studied example: a ratio below approximately 0.6 is associated with increased risk of knee and hamstring injuries.

Training and Injury Prevention Strategies

Training can modify both muscle and tendon properties to improve performance and reduce injury risk:

• Eccentric training (e.g., Nordic hamstring curls, heavy slow resistance protocols) is effective for increasing tendon stiffness and reducing tendinopathy risk.
• Plyometric training enhances the stretch-shortening cycle function of the muscle-tendon unit, improving power output in explosive activities.
• Progressive loading is essential to allow tissues time to adapt. Tendons adapt more slowly than muscle, so rapid increases in training volume or intensity can outpace tendon remodeling.

Age-related changes compound these concerns. Older athletes experience decreased muscle mass (sarcopenia), reduced force production, and diminished tendon elasticity. Training programs for older populations should account for these changes with appropriate loading progressions.

Core injury prevention strategies include:

• Gradual, progressive increases in training load
• Proper warm-up to reduce thixotropic stiffness and enhance muscle-tendon unit function
• Balanced strength training that addresses both agonist and antagonist muscle groups
• Monitoring for early signs of tendon overload (localized pain, morning stiffness in the tendon)