PHTY103 Lecture Notes - Lecture 15: Catabolism, Neural Adaptation, Tendon Cell
15. The body's response to immobilisation, inactivity,
disuse and training
• Explain the physical changes that occur in body tissues (muscle, bone, cartilage tendon
and ligaments) as a result of immobilisation, inactivity and disuse
o Refers to the process of disturbing the normal function of tissue by decreasing or
eliminating load placed on it
o Immobilisation can occur in many various forms and often is used in order to
protect a damaged tissue from further aggravation or injury to promote healing
o May also come in the form of disuse or inactivity in the absence of injury or
pathology
• Cast immobilisation
• Spinal cord transection
• Space/weightlessness
• Bed rest
o Bone
• Dynamic loading in "normal" state triggers modelling-remodelling to adapt to
new stresses
• Wolff's law: remodelling of bone is influenced and modulated by mechanical
stresses
• Load usually relates to either muscle activity of gravity (weight-bearing)
• Hence profound reduction of mechanical loading dramatically affects bone
turnover rates and compromises the structure of bone and hence its
mechanical properties
• One explanation proposed is mechanostat, i.e. bony change is determined by
strain thresholds
▪ Large or high loads detected above threshold range activates modelling
to increase periosteal formation and remodelling to replace fatigue-
damaged bone
▪ Small or low disproportionate loads below threshold range activates
remodelling to decrease bone mass and subsequently increases strain
level
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• Bone not subjected to mechanical stress (load) which leads to
▪ Loss of bone mineral density
▪ Reabsorption of periosteal and subperiosteal bone
• This causes a change in mechanical properties
▪ Decrease to both strength and stiffness
▪ 3-fold reduction in load to failure (ultimate failure point)
• Bed rest induces bone mass decrease 1% week
• Loss can be very rapid - within first few months
• Paralysis and bone loss
▪ Individuals with spinal cord injury
• 20-40% reduced cortical thickness
• 1-5% reduced cortical density
• 20-50% reduced trabecular density
• Space flight
▪ 4-6 month missions at International Space Station
• 8-9% reduction in CSA at femoral neck and proximal femoral region
• 2% reduction in cortical density at tibia and femur (not in radius)
• Trabecular density reduced by 5% tibia, 12% femur, 4% lumbar spine
(not in radius)
• 7-14% reduction in vertebral compressive strength
• Relative disuse
▪ e.g. implant/plate
• Plate and bone share load in proportions determined by geometry
and properties of each structure
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