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