--- module: 032-02 language: en chapter: 32 title: "Skeletal Muscle, Bone, Joint, and Connective Tissue Physiology" module_title: "Load, tissue adaptation, movement economy, and rehabilitation physiology" source_sha256: 41eeb620842aa1acb1f75c85a1789c1619a2ff5ed17e74c95b389b9d76e1fa54 --- # Load, adaptation, and rehabilitation ## Force transfer and the limiting variable ### Neural command through muscle, tendon, joint, bone ### Loss at any link feels like weakness #### Pain inhibits recruitment #### Stiffness shortens usable range ### Identify the limiting variable ## Motor units and recruitment ### One alpha motor neuron and its fibres ### Size principle: small low-threshold units first #### Economical activity, high-force reserve kept ### Discharge frequency summates force ### Synchronisation and rate coding adapt early #### Strength before hypertrophy ### Denervation sprouting: larger units, less control ## Sarcomere mechanics and leverage ### Isometric force peaks near optimal overlap ### Concentric force falls as velocity rises ### Eccentric resists more force at lower cost #### Lowering possible when lifting is not #### Unfamiliar eccentric work causes soreness ### Torque equals force times moment arm ### Co-contraction: stability, energy, compression ### One test angle samples one mechanical state #### Restore force through range and task speed ## Power and ageing ### Power equals force times velocity ### Independence needs power more than static strength ### Ageing loses fast units and type-two area #### Power falls before manual grades change ### Chair rise, stairs, gait speed as tests ### Moderate resistance moved with rapid intent ## Energy systems and fatigue ### Phosphocreatine, glycolysis, oxidation overlap ### Lactate shuttles carbon and reducing equivalents #### Level reflects production, oxidation, clearance ### Fatigue is task-specific, central or peripheral ### Heat, dehydration, glycogen depletion limit work ### Record heart rate, speed, force, recovery time #### Name the limiting system #### Match recovery interval to that system ## Hypertrophy and unloading ### Mechanical tension, synthesis exceeds breakdown ### Satellite cells donate nuclei ### Protein, energy, age, anabolic resistance ### Bed rest: muscle, insulin, bone, orthostasis #### Brief frequent loading preserves function ## Bone strength and remodelling ### Toughness needs collagen and mineral together ### Cortical geometry drives bending resistance ### Trabecular perforation weakens beyond mineral loss ### Density predicts population, not individual strength ### Osteocyte fluid flow lowers sclerostin #### Novel dynamic strain favours formation #### Rest restores mechanosensitivity ### Resorption precedes refilling #### High turnover creates temporary weakness ### Glucocorticoids: fragile bone and weak muscle ### Intermittent parathyroid hormone builds bone ## Fracture repair and weight bearing ### Haematoma provides signals and matrix ### Relative stability: bridging callus ### Absolute stability: direct remodelling ### Excess motion disrupts vessels and mineral ### Weight bearing not set by pain alone #### Fixation, bone quality, wound, falls risk #### Progress from below gait deterioration ### Train other limbs, trunk, cardiopulmonary system ## Cartilage, joints, and tendon ### Interstitial fluid first carries compression #### Sustained load shifts it to solid matrix ### Ligaments: end-range restraint and sensation ### Laxity and altered proprioception persist ### Joint swelling inhibits the quadriceps ### Tendon pain does not gauge damage #### Symptom-guided progressive loading #### Complete rest and sudden return both harm ## Return and exercise prescription ### Return not set by absent rest pain ### Symmetry guarantees nothing if both sides weak ### Connective tissue heals slower than muscle #### Scar restores continuity, not architecture ### Dose: load, speed, frequency, novelty, recovery ### Build reserve so daily life uses less capacity ### State starting dose, progression, review time