Understanding Injury DevelopmentCHAPTER 15Mechanical Stress & Injury►Observed Motion = combination of all forces►Improving Performance Depends on improving application of all forces INJURY = damage to tissue►MECHANICAL STRESS (pressure) Distribution of a force over a body it acts on F/A Force / Area (N/m2)Mechanical Stress►Compressive – squeezing Impact; weight; muscle tension Muscles ~ Rotary and Stabilizing components►Tensile – Pulling►Shear – Sliding►Torsion - Twisting►Complex Loading – combinationTissue Response►Wolff’s Law – tissue adapts to level of stress BONES: adapts by increasing mineralization MUSCLES: increase cross sectional area of individual fibers LIGAMENTS / TENDONS: affected by level of training NEURAL TISSUE: does not benefit from applied stressLevel of stress on tissue changes with activity level (magnitude)Controlling level of imposed stressStress Continuum Active Lifestyle = Physiologic loading zone►Stress Maintained►Tissue maintains current status►Muscle maintains same force generating capabilityPhysiologic Training ZoneIncreased level of stress imposedMicrodamage causedBody’s Response to Microdamage► Remodeling initiated– rebuilding of the tissue► Time required is related to extent of damage► Hypertrophy – result of healing for tissue► Overload Principle Systematic application of loads in the physiologic training zone Causes cellular/structural changes within a tissue called the Training Effect Systematic Loading – imposed tissue loading purposely exceeds the tissue’s yield threshold BUT adequate rest time is allowed prior to next overload sessionPathologic Underload Zone►Low loading applied to a tissue►Inactivity; Zero gravity►De-training effect occurs How do the tissues respond in this situation according to Wolff’s Law?Pathologic Overload Zone►Substantial damage done to tissue►Traumatic / Acute Injury►Overuse Injury Page 344 Fig. 15.4 Theoretical relationship between stress magnitude, number of repetitions, & injury Page 345 Fig. 15.5 What determines whether imposed stress will cause a training effect or lead to injury?Individual Differences►Simple Model Injury Prevention = ↓ magnitude & frequency of loading and ↑ rest for remodeling►Tissues have different thresholds Bone>Tendon>Ligament>Cartilage Direction of stress also affects threshold►Recovery rateIntrinsic / Extrinsic Factors►Intrinsic Anthropometrics Skeletal alignment Fitness History of injury►Extrinsic Task-related►Nature of task; Individual execution; Movement  Environment►Surface; Rules; Skill levelPage 347 Table 15.1I/E Factors & Running►Knee – most common site of injury►Why are some runners injured and others not?►Why is injury often unilateral?►Are some individuals predisposed to injury or at greater risk?Loading Patterns ►Cyclical activity Approximately 300-900 steps per mile►Stance Downward motion of runner stopped & then reversed to propel runner upward and forward Lower extremity absorbs energy then generates energy Approx. 200-300ms of foot contact►Striking patterns Rearfoot (most common) Forefoot / Mid-foot►Ground reaction force Reaction force applied to the body during contact with the ground Center of Pressure – Point of application of the single vector representing the GRF applied to footGround Reaction Force►Components FzActs upward FXActs anterior-posteriorly FYActs mediolaterally►Impulsive load►Page 352 Fig. 15.9►Identify  Extrinsic Factors (group A) Intrinsic Factors (group B) Physiologic considerations (group