Joint movement

What can the terms torque and moment be used as in biomechanics?

They can be used interchangeably

What is tensile force?

An external pulling/stretching force that produces internal tension

What is shear force?

Forces applied parallel to a surface or cross section of tissue

What is valgus?

Distal end moves away from the midline (knock knees)

What is varus?

Distal end moves toward the midline (bow legged)

What is a retinaculum?

A band of thickened connective tissue that often holds ligaments in place

What is an open kinetic chain?

A movement where the distal end is free

What is a closed kinetic chain?

A movement where the distal end is fixed

What is plastic deformation?

Permanent damage or change to tissue created by force or stress

What is elastic deformation?

Reversible stretch or compression of tissue

What is a lever arm?

The distance from the force to the pivot point

What is force?

A push or pull on an object that causes it to accelerate

What is the equation for force?

Force = mass x acceleration

What is translation in biomechanics?

Movement where every point in the bone moves the same distance in the same direction

What type of forces occur in the sagittal plane?

Anterior and posterior shear

What type of forces occur in the frontal plane?

Medial and lateral glides (shear)

What type of forces occur in the longitudinal plane?

Compression and distraction/tension

What is an example of anterior shear at the knee?

Forward translation of the tibia relative to a fixed femur

How is anterior shear used clinically at the knee?

ACL diagnosis using anterior tibial translation

What does lack of resistance during anterior tibial translation suggest?

The ACL may not be intact

How do the quadriceps influence shear at the knee?

They create anterior translation (shear) of the tibia

How do the hamstrings influence shear at the knee?

They create posterior translation (shear) of the tibia

How do the hamstrings help protect the ACL?

They restrict anterior tibial shear

What is torque?

A rotational force produced by a force acting at some distance from the center of rotation

What is the equation for torque?

Torque = distance x force

In a knee extension machine what torque does the weight create?

flexion torque

What must the quadriceps do during a knee extension machine exercise?

Produce an equal extensor torque to counter the flexion torque

How does the lever arm change as the knee extends in a knee extension machine?

The lever arm increases

How does increasing the lever arm affect quadriceps demand?

The quadriceps must generate more force and torque

In a squat when is the knee moment arm greatest?

90 degrees of knee flexion (bottom of the movement)

When does the highest knee torque occur during a squat?

At maximum knee flexion

Why do the quadriceps work harder in deeper knee flexion during squats?

The moment arm increases

Why does raising a chair help elderly people with weak quadriceps?

It decreases the torque demand at the knee

How does raising the chair decrease torque demand?

The center of gravity moves closer to the knee pivot point

What torque occurs at the hip during single leg stance?

Adduction torque

What creates adduction torque during unilateral stance?

Upper body weight and the lifted leg

Approximately how far from the hip joint does body weight act during single leg stance?

About 10 cm (0.1 m)

What effect does body weight have on the pelvis during single leg stance?

It causes pelvic rotation/adduction torque

What must the hip abductors do during single leg stance?

Produce torque equal to the adduction torque

Why must hip abductors generate high force during single leg stance?

Their lever arm is half the distance of the body weight lever arm

How much force must hip abductors generate relative to body weight?

About twice the body weight force

What effect do the adductor and abductor forces have on the hip joint?

They create large compression forces

How large can hip compression forces become during single leg stance?

Approximately 2.5-3 times body weight

How does trunk lateral shift reduce hip pain?

It decreases the adductor lever arm and torque

What happens to hip compression forces when trunk shift compensation occurs?

Compression forces decrease

How much can hip compression decrease with trunk shift compensation?

Approximately 40-50%

What is a negative effect of trunk shift compensation?

Hip abductors become weaker

How can trunk shift compensation negatively affect the knee?

It creates abduction torque on the knee

What structures may be stressed by knee abduction torque?

The MCL and lateral knee compression structures

How is hip abductor torque commonly assessed?

Side lying with clinician-applied adduction force and dynamometer measurement

Which side should a cane be used on for greatest reduction in hip compression?

Contralateral side

Why is a contralateral cane more effective?

It creates a large abduction torque far from the hip pivot

How much can a contralateral cane reduce hip compression forces?

Up to 40%

How much can an ipsilateral cane reduce hip compression forces?

Approximately 8-10%

How does manual therapy reduce hip pain?

By creating distraction and reducing compression

What is distraction?

Separation of joint surfaces/tissue

What is the normal tibiofemoral alignment angle in standing?

Approximately 3-8 degrees valgus

Which knee compartment normally experiences slightly greater compression?

The medial condyle

What is genu varum?

Bow legged alignment

What tibiofemoral angle is seen in genu varum?

Less than 0 degrees

What torque increases in genu varum?

Adduction torque

Which side of the knee experiences greater compression in genu varum?

The medial side

Which side experiences greater tensile forces in genu varum?

The lateral side

What conditions are associated with genu varum?

Medial arthritis and lateral ligament laxity

What is genu valgum?

Knock knee alignment

What tibiofemoral angle is seen in genu valgum?

Greater than 10 degrees

What torque increases in genu valgum?

Abduction torque

Which side of the knee experiences greater compression in genu valgum?

The lateral side

Which side experiences greater tensile forces in genu valgum?

The medial side

What conditions are associated with genu valgum?

Lateral arthritis and medial ligament laxity

What is assessed during the varus stress test?

Lateral collateral ligament integrity

What is assessed during the valgus stress test?

Medial collateral ligament integrity

How is the valgus stress test performed?

By creating tibial abduction and medial distraction

What structures exert lateral forces on the patella?

Lateral retinaculum

What structures exert medial forces on the patella?

Medial retinaculum

What structure applies superior force to the patella?

Quadriceps tendon

What structure applies inferior force to the patella?

Patella ligament via tibial tuberosity

How does the patella improve quadriceps function?

It increases the quadriceps lever arm

Why does the patellofemoral joint experience high compressive forces?

Opposing pull from the quadriceps and patella tendon

What patellofemoral stress occurs during 0-30 degrees of squat?

Low stress

What happens to patellofemoral stress above 50 degrees of squat?

Stress increases

When is patellofemoral stress very high during a squat?

Around 90 degrees knee flexion

What happens to the patellar lever arm near terminal knee extension in open chain exercises?

It decreases

Why does the lever arm decrease near extension?

The patella moves superiorly and sits more shallowly in the trochlear groove

How does a shorter lever arm affect quadriceps torque production?

Less torque can be produced

Why might patients with weak quadriceps struggle with terminal knee extension?

Torque production decreases near 0 degrees

What knee flexion range is recommended for irritable patellofemoral joints?

0-45 degrees

Why should deep knee flexion be avoided in irritable patellofemoral joints?

Compression forces become too high

What squat technique increases patellofemoral compression?

Knees moving in front of the toes

Why does knees-over-toes squatting increase compression?

The line of gravity moves further from the knee

What other force increases with knees-over-toes squatting?

Anterior shear on the ACL

What squat technique decreases knee compression forces?

Keeping knees behind the toes

Why does knees-behind-toes squatting decrease compression?

The line of gravity stays closer to the knee

Which muscles are recruited more when knees stay behind the toes?

Hamstrings and glutes

How does lateral trunk tilt during landing affect the knee?

It shifts the center of gravity laterally

What forces increase with lateral trunk tilt during landing?

Lateral compression and medial tension

What can repetitive abnormal landing mechanics cause at the patella?

Patellar rotation and altered pull

What muscular weakness contributes to poor landing mechanics?

Hip abductor weakness

How can posterior hip abductors be targeted during rehabilitation?

Using a wall as a guide during exercises

What is an example of an internal focus cue?

Land with flexed hips and knees