Muscle and Tendon Remodeling with Use and Disuse Overview (04:27)

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Carl Foster introduces Dr. Mario Narici. Muscle strength, neural drive, and hypertrophy increases during resistance training due to neural adaptations. Hypertrophy has a delayed onset but continues growing after a neural drive plateau.

Rapid Cellular Adaptation (02:26)

Myogenic adaptations to resistance training occur within 12 hours—much earlier than observed from muscle structure. Satellite cells donate myonuclei to multi-nuclear skeletal cells to maintain a nuclear domain constant. These changes are sustained by an increase in muscle protein synthesis.

Hypertrophy Study (03:06)

Suspecting muscle hypertrophy occurs faster than previously described; Dr. Narici's team trained 30 individuals for 35 days with concentric and eccentric contractions. Knee extensors hypertrophied and increased in force and torque. Fascicle length and pennation angle increased after just 20 days.

Muscle and Tendon Hypertrophy (02:23)

Sarcomere addition in series occurs faster than in parallel. Dr. Narici's team also measured patella tendon mechanical properties by ultrasound. After training, tendon stiffness increased by 23%, showing that resistance training conditions both muscle and tendon.

Force-Velocity Relationship Study (04:00)

One group trained concentrically, and the other trained eccentrically. Pennation angle increased more in the concentric group and fascicle length increased more in the eccentric group. Differing myogenic responses to eccentric and concentric loading may reflect sarcomere addition differences.

Musculoskeletal Changes in Response to Chronic Inactivity (03:44)

One group on complete bed rest had decreased calf muscle cross-sectional area and shortened fascicles. Another group trained lower limb extensor muscles, protecting against quadriceps atrophy. Both groups had decreased triceps surae strength.

Muscle Fiber Length and Tendon Mechanical Properties (03:12)

Muscles with long fibers have wider length-force relationships and greater force-velocity relationships than short fiber muscles. Muscle mechanical output depends on muscle-tendon interaction; tendon stiffness decreased more in the bed rest group than in the partial exercise group.

Chronic Inactivity and Muscle Force (02:43)

If the tendon becomes more compliant, fascicles shorten upon contraction. Subjects on bed rest had decreased specific force of single fibers, correlating with decreased myosin content.

Lower Limb Unloading Study (01:57)

Wearing a contraption and lifting one foot, participants walked with crutches for 23 days. They had decreased cross-section areas, fascicle length and pennation angles; these results correlated with a 30 day bed rest study. Muscle atrophy proceeds at 0.4% per day.

What Regulates Sarcomere Remodeling? (02:18)

With inactivity, sarcomeres are likely removed from the muscle periphery. Translocator proteins involved in mechanotransduction include focal adhesion kinase (FAK), which is sensitive to mechanical loading. Vastus lateralis biopsies of individuals walking with crutches for 23 days showed decreased FAK activity.

Muscle Atrophy Mechanisms (02:44)

FAK is an upstream modulator of protein synthesis. After ten days of lower limb suspension, myofibrillar protein synthesis was reduced by 50%; collagen synthesis also decreased. The ubiquitin proteasome system activation increases in inflammatory atrophy cases, but data is inconclusive for non-inflammatory atrophy.

Effects of Prolonged High Heel Use on Muscle-Tendon Units (03:53)

Calf muscles in long term high heel users showed shortened gastrocnemius fascicles, greater cross-sectional tendon area, increased Achilles tendon stiffness, reduced ankle active ROM, and nearly double resting plantar flexion angle; there were no functional consequences.

Conclusions (01:46)

Hypertrophy or atrophy onset occurs within two weeks and molecular and cellular processes occur within hours; use or disuse changes tendon structure; combined muscular and tendinous changes change muscle mechanical properties; and sarcomere remodeling is associated with protein synthesis changes from mechano-transductory protein alterations.

QA: Minimizing Disuse Atrophy (00:60)

A minimum load to protect against atrophy depends on which muscle is being trained. Dr. Narici encourages full resistive training to prevent atrophy.

QA: Protein Synthesis Loss (01:23)

Rats with high limb suspension have pre-mRNA and myosin heavy chain and actin transcriptions repressed within 12 hours.

QA: Treating Tendon Injuries (03:20)

Traditional treatments like anti-inflammatories, cortisone injection, platelet rich plasma, and mechanotherapy have limited success. Tendons will increase collagen to self-heal, but not necessarily increase stiffness. Dr. Narici recommends measuring their mechanical properties and increasing their size.

QA: Remodeling the Muscle vs. Tendon (01:27)

Dr. Narici found tendon mechanical properties changed faster than muscle atrophy, but he has not done a direct comparison of changes in collagen and protein synthesis.

Credits: Muscle and Tendon Remodeling with Use and Disuse (00:26)

Credits: Muscle and Tendon Remodeling with Use and Disuse