Musculoskeletal Forces: Impact on the Body

Kinetics (4)  Musculoskeletal Forces Nader Ibrahim El-Sayed     Lecturer of orthopaedic  physical therapy (AHUC)

• Kinetics is a branch of the study of mechanics that describes  the  effect of forces on the body . • Force can be considered  as  a push or pull  that  can  produce ,  arrest , or  modify  movement.

Musculoskeletal Forces IMPACT OF FORCES ON THE  MUSCULOSKELETAL  SYSTEM: INTRODUCTORY CONCEPTS AND TERMINOLOGY: • A force that acts on the body is often referred to generically as a load. • Forces  or  loads  that  move,  fixate,  or  otherwise  stabilize  the body also have the potential to deform and injure the body. 

The loads most frequently applied to the  musculoskeletal system are:

• Healthy tissues are typically able to partially resist changes in their structure and shape. •  The  force  that  stretches  a  healthy  ligament,  for  example,  is met  by  an  intrinsic  tension  generated  within  the  elongated (stretched) tissue.  • Any tissue weakened by disease, trauma, or  prolonged disuse may  not  be  able  to  adequately  resist  the  application  of  the loads.  for  example,  The  proximal  femur  weakened  by  osteoporosis, may fracture from the impact of a fall secondary to  compression  or  torsion  (twisting),  shearing,  or  bending  of the  neck  of  the  femur.  Fracture  may  also  occur  in  a  severely osteoporotic hip after a very strong muscle contraction.

• One  method  of  measuring  the  ability  of  a  connective tissue  to  tolerate  a  load  is  to  plot  the  force  required  to deform an excised tissue(stress -strain curve). Stress -strain curve relationship of ligament or tendon : • The vertical (Y) axis of the graph is labelled  stress, a term  that  denotes  the  internal  resistance  generated  as  the  ligament  resists  deformation,  divided  by  its  cross-sectional area.        stress  (pressure)=force/cross sectional area. (N/mm2) • The  horizontal  (X)  axis  is  labelled  strain,  which  in  this  case is the percent increase in a  tissue’s stretched length  relative to its original, pre-experimental length.

Stress strain curve of the ligament or tendon: It  consists of three distinct regions : Toe region : this is where “stretching  out” or "un-crimping" of crimped tendon fibrils. This region is responsible for nonlinear stress/strain curve, because the slope of the toe region is not linear reflects the fact that the collagen fibers within the tissue are initially wavy or  crimped and must be  drawn  taut before significant tension is measured.

• Linear region :  the collagen fibrils orient themselves  in the direction of tensile mechanical load and begin to stretch. The tendon deforms in  a linear fashion  due  to the inter-molecular sliding of collagen. In this  region the tendon will return to its original length when unloaded, therefore this portion is  elastic  and  reversible  and the slope of the curve represents the  Young's modulus. The ratio of the stress (Y) caused by an applied strain (X) in the ligament is a measure of its  stiffness (often  referred to as Young’s modulus

• Yield and failure region :  this is where the tendon  stretches beyond its physiological limit  and  intramolecular cross-links between collagen fibres fail. •  If  micro-failure  continues to accumulate, stiffness is  reduced and the tendon     begins to fail, resulting in  irreversible plastic deformation . If the tendon more stretched, macroscopic  failure  soon follows.

• Tissues  in  which  the  physical  properties  associated with the stress-strain curve change as a function of time are considered   Viscoelastic.   • Most  tissues  within  the  musculoskeletal  system  demonstrate at least some degree of viscoelasticity.   •    A  viscoeleastic  material  is  more  deformable  at  low strain rates but less deformable at high strain rates. • One phenomenon of a viscoelastic material is  Creep.

Creep • Creep  describes  a  progressive  strain  of  a  material when exposed to a constant load over time.  • Unlike plastic deformation, creep is reversible . 

• The  phenomenon  of  Creep  helps  to  explain   why  a  person is taller in the morning than at night?.  • Answer:  The  constant  compression  caused  by  body  weight  on  the  spine  throughout  the  day  literally squeezes  a  small  amount  of  fluid  out  of  the intervertebral discs.  • The  fluid  is  reabsorbed  at  night  while  the  sleeping person is in a non–weight-bearing position.

Ref: • Kinesiology of musculoskeletal system Foundation and rehabilitation,2 nd  ed.2010. • Kinesiology, the skeletal system and muscle function2nd ed.2011

Thank you