Muscle Stiffness and Thixotropy

Muscle Stiffness

• Muscle stiffness is the resistance to change in length.
• Muscle is resistant to lengthening forces, especially over small displacements (short-range stiffness).
• Muscle stiffness is crucial for maintaining posture and limb position.
• Stiffness decreases with larger changes in length and repeated stretching.
• Stiffness returns with time at rest.

Muscle Thixotropy

• Muscle thixotropy is the dependence of muscle stiffness on the history of muscle length change.
• Stable bonds between actin and myosin are broken during stretching or activity, decreasing stiffness.
• Inactivity increases the number of stable bonds, increasing stiffness.
• Muscle activity increases short-range stiffness significantly (up to 100x) due to increased bonds.
• Changes in titin protein elasticity within the sarcomere also contribute to muscle thixotropy.
• Titin stiffness decreases when the sarcomere lengthens and returns during inactivity.

Effects of Stretching

• Stretching temporarily changes joint and muscle stiffness, initially reducing stiffness.
• Stiffness increases rapidly as the time between stretches increases, especially short-range stiffness.
• Stretching can increase muscle length.
• Repeated stretches decrease muscle force, known as stress relaxation.
• A constant force applied to a muscle/tendon unit causes elongation, known as creep.

Stretching Techniques

• Stretching can be categorized as passive or active.
• Passive stretching includes static stretching (holding a position) and ballistic stretching (bouncing).
• Active stretching includes dynamic stretching (controlled movement) and proprioceptive neuromuscular facilitation (PNF) stretching.
• PNF stretching techniques include contract-relax, antagonist-contract, and combining both.

Contract-Relax Technique

• This technique utilizes autogenic inhibition.
• Tension in muscle/tendon activates Golgi tendon organs (GTOs).
• GTO activation inhibits the contracting muscle for approximately 5 seconds.
• This reduces tonic muscle activity, which would otherwise resist lengthening.
• Contraction disrupts stable bonds, leading to thixotropic action and decreased stiffness.

Antagonist-Contract Technique

• This technique requires assistance from a partner or strap.
• It is based on reciprocal inhibition theory.
• Contraction of the antagonist muscle inhibits the agonist (muscle being stretched).
• The stretched muscle is actively inhibited, reducing unwanted activity.

Effects of Stretching on Reflexes

• Stretching decreases the sensitivity of the stretch reflex pathway as measured by reduced H-reflex and T-reflex.
• Cortical stimulation (MEP) remains unchanged, suggesting alpha motor neurons are not inhibited by stretching.
• The lack of T-reflex return to baseline after stretching suggests tendon stiffness is decreased.

Long-Term Effects of Stretching

• A study involving 30 static stretching sessions (5/week for 6 weeks) of plantarflexors showed significant increases in dorsiflexion range of motion and decreases in passive dorsiflexion stiffness in the stretched leg, compared to the control leg.
• These changes persisted for 30 days without stretching, with approximately 74% of flexibility gains and 78% of stiffness reduction retained.
• Reductions in H-reflex occurred after 20-30 sessions and returned to baseline with no stretching.
• T-reflex decreased throughout the stretching program and did not return to baseline 30 days later.

Summary of Long-Term Effects

• Regular static stretching leads to progressive increases in flexibility and reductions in joint stiffness.
• It also depresses stretch reflex expression, particularly during the stretching program.
• Initial flexibility increases are primarily driven by changes in muscle mechanics, while later gains involve changes in reflex sensitivity.

This quiz explores the concepts of muscle stiffness and thixotropy, focusing on how muscles resist elongation and the impact of activity and inactivity on stiffness. It covers the mechanisms involved, especially the roles of actin, myosin, and titin in muscle function. Test your knowledge on these essential physiological principles.