kinesiology

What is the primary purpose of movement analysis?

To understand how the body moves, why it moves, and what stresses tissues experience

A patient demonstrates abnormal gait mechanics. Which part of movement analysis identifies the pattern vs the cause?

Kinematics identifies the pattern; kinetics identifies the cause

What is kinematics?

The description of motion without considering forces

What is kinetics?

The study of forces and torques that cause motion

Why is observing joint angles alone insufficient for clinical reasoning?

Because kinematics does not explain the forces causing the movement

A patient moves normally but reports pain. Which analysis is most important?

Kinetic analysis to assess joint and muscle forces

What is anthropometry?

Measurement of body characteristics including mass, length, COM, and moment of inertia

Why is anthropometry required for dynamic analysis?

Because mass and inertial properties affect acceleration and force demands

How does increased body mass affect joint torque during gait?

It increases external torque and muscle force requirements

Why might two patients perform the same movement but experience different joint stresses?

Differences in anthropometry alter force and torque demands

What is center of mass?

The point where total body mass is considered concentrated

How does COM affect stability?

Stability increases when COM is lower and within the base of support

Why does raising the arms challenge balance?

It elevates and shifts the COM

How can a PT manipulate COM to assist a patient with balance deficits?

By cueing posture or limb position to keep COM over BOS

What is mass moment of inertia?

Resistance of a body to angular acceleration

What factors affect moment of inertia?

Mass and distance of mass from the axis of rotation

Why do runners flex their elbows during running?

To decrease moment of inertia and improve efficiency

Why is distal limb weighting challenging for rehab patients?

It increases moment of inertia and muscular demand

What is a free body diagram?

A simplified drawing showing all forces acting on a body or segment

What forces must be included in an FBD?

Muscle force, gravity, ground reaction force, joint reaction force, and external loads

Why are FBDs critical for understanding joint stress?

They reveal how muscle and external forces interact

How does an FBD help with exercise modification?

It shows how changing position alters torque demands

What is joint reaction force?

The force one joint surface exerts on another

What is the primary contributor to JRF?

Muscle contraction

Why can joint pain occur during low-load activities?

Muscle force can create high joint compression

Why might strengthening initially increase joint pain?

Increased muscle force raises joint reaction force

What is a reference frame?

A system used to describe position and direction of movement

What is a relative reference frame?

Movement of one segment relative to another

What is a global reference frame?

Movement relative to space

Which reference frame is used for arthrokinematics?

Relative reference frame

Why is gait analyzed using a global reference frame?

Because movement is described relative to the ground

What is composition of forces?

Combining parallel coplanar forces into a single resultant force

What is vector resolution?

Breaking a force into perpendicular and parallel components

Which force component causes compression?

The perpendicular component

Which force component causes shear?

The parallel component

Why is shear clinically significant at the knee?

It increases ACL stress

How can a PT reduce shear forces during exercise?

By adjusting joint angle to favor compression

What is bowstringing?

When a tendon is held away from the joint axis, increasing the internal moment arm

What structures commonly create bowstringing?

Bones, retinacula, and pulley systems

How does bowstringing affect torque production?

It increases torque efficiency by increasing the internal moment arm

Does bowstringing increase when a tendon or pulley is lost?

No, bowstringing is lost

Why does patellectomy reduce knee extension strength?

Loss of bowstringing decreases the quadriceps moment arm

What are internal forces?

Forces produced by muscles

What are external forces?

Gravity, external loads, and contact forces

How do internal and external torques relate?

They act in opposite rotary directions

Why must muscle force increase as external load increases?

To counteract greater external torque

When is external torque greatest?

When the external force is perpendicular to the segment

How does changing joint angle affect muscle demand?

It alters moment arms and force components

Why does resistance feel harder at certain joint angles?

External torque is maximized

How can a PT grade exercise difficulty without changing weight?

By changing limb position relative to gravity

What defines a mechanically advantageous joint position?

Long internal moment arm, short external moment arm, ~90° insertion

Why is mid-range strengthening commonly prescribed?

It provides optimal torque with less joint stress

Why might end-range strengthening increase injury risk?

Moment arms and force components are less favorable

What is static analysis?

Analysis of a system in equilibrium with no acceleration

When is static analysis most appropriate clinically?

Isometric exercises and posture analysis

What is dynamic analysis?

Analysis of movement with acceleration

Why does dynamic analysis require more data than static?

Inertial and acceleration forces must be accounted for

Why is gait considered a dynamic analysis problem?

Forces and accelerations change continuously

What tools measure kinematics?

Goniometers, accelerometers, video, motion capture

What tools measure kinetics?

Force plates, dynamometers, isokinetic devices

Why are force plates valuable in gait analysis?

They measure ground reaction forces causing motion

Why does reducing external load not always reduce joint stress?

Muscle force may still be high to maintain torque

Why can two patients perform the same task but load joints differently?

Anthropometric and mechanical differences

Why does leaning the trunk toward the stance leg reduce hip pain?

It shortens the external moment arm

Why does balance training often involve arm movement challenges?

Arm motion shifts COM and inertia

A patient reports knee pain during terminal knee extension with minimal resistance. What mechanical factor most likely explains this?

External torque is maximized near full extension due to a long external moment arm

Why might reducing resistance not reduce joint pain during an exercise?

Muscle force may still be high to counteract external torque, maintaining high joint reaction force

A patient can move through full ROM but fatigues quickly. What kinetic factor may explain this?

Inefficient moment arms requiring increased muscle force

Why does adding ankle weights significantly increase hip flexor demand during gait?

Increased moment of inertia from distal mass placement

A therapist cues a patient to bend their elbows during balance training. What is the primary biomechanical rationale?

Decreasing moment of inertia to improve control

Why does holding an object farther from the body increase low back stress?

External moment arm increases, requiring greater trunk extensor force

A patient demonstrates good kinematics but worsening symptoms. What should the PT analyze next?

Kinetics to assess force and joint loading

Why is static analysis insufficient for evaluating stair descent?

Stair descent involves acceleration and changing forces requiring dynamic analysis

A patient with quad weakness reports more pain during slow controlled movements than fast ones. Why?

Slow velocity requires higher force due to the force–velocity relationship

Why does increasing cadence reduce peak joint loading during gait?

Shorter stance time reduces peak ground reaction forces

A PT chooses isometrics early for a painful joint. What is the mechanical benefit?

Force production without joint motion limits shear

Why can isometric contractions still provoke joint pain?

They generate joint reaction force despite no movement

A patient with hip OA leans toward the stance leg during gait. What mechanical advantage does this provide?

Shortens the external moment arm, reducing abductor force demand

Why does loss of bowstringing increase muscle force requirements?

A shorter internal moment arm requires more force to produce the same torque

Why does patellectomy decrease knee extension efficiency?

Loss of bowstringing reduces quadriceps moment arm

A patient has pain only at specific joint angles during exercise. What explains this?

Moment arms and force components change with joint angle

Why is mid-range often safest for strengthening early in rehab?

Moment arms and length–tension are most favorable

A patient can lift light weight but struggles with holding it statically. Why?

Static equilibrium still requires high muscle force to counter torque

Why might a PT modify joint angle instead of reducing resistance to manage pain?

Joint angle alters torque more effectively than load alone

A patient’s movement speed increases but accuracy decreases. What inertial factor contributes?

Higher angular acceleration increases inertial resistance

Why does distal limb control deteriorate first in neurological conditions?

Higher moment of inertia makes distal segments harder to control

Why does prolonged sitting worsen disc-related symptoms?

Fibrocartilage relies on movement for nutrient diffusion

A patient with weak hip abductors experiences lateral trunk sway. What kinetic imbalance exists?

Insufficient internal torque to counter external torque from body weight

Why do eccentric contractions allow greater external load control?

They produce higher force per crossbridge

Why are eccentric exercises emphasized in tendinopathy rehab?

They improve tissue load tolerance at high force levels

A patient reports pain only during lowering phases of movement. What contraction type is implicated?

Eccentric contraction

Why is gait always considered a dynamic analysis problem?

Acceleration and force magnitudes change continuously

Why are force plates essential for understanding movement cause?

They measure ground reaction forces driving motion

Two patients perform the same squat but have different knee pain levels. Why?

Differences in anthropometry and force distribution

Why does joint compression increase with stronger muscles?

Muscle force contributes directly to joint reaction force

Why does reducing base of support increase postural demand?

COM must be controlled over a smaller area

Why is balance more challenged with arms overhead?

COM shifts upward and moment of inertia increases

Why does fatigue increase injury risk late in exercise sessions?

Higher force required due to reduced mechanical efficiency

Why does slow controlled rehab not always mean low joint stress?

Slow velocity often requires higher force production