Chapter 3 - Anatomical Kinesiology

What is biomechanics?

Biomechanics = study of mechanics as it relates to the functional and anatomical analysis of biological systems, especially humans.

What is mechanics, and what are its two divisions?

Mechanics = study of the physical actions of forces.

Two divisions:

1. Statics

2. Dynamics

What is the difference between statics and dynamics?

Statics

• No acceleration

• At rest OR moving at constant velocity

• Forces are balanced

• Body is in equilibrium

Dynamics

• Movement with acceleration

• Forces are unbalanced/unequal

Does statics always mean the body is motionless?

No.

Statics includes:

• Body at rest

• Body moving at constant velocity without acceleration

Key: No acceleration + balanced forces

What is the difference between kinematics and kinetics?

Kinematics = description of motion

• Time

• Displacement

• Velocity

• Acceleration

• Space

Kinetics = study of forces associated with motion

Kinematics = motion
Kinetics = forces

Types of Machines + Mechanical Advantage…

What is mechanical advantage?

Mechanical advantage = Load ÷ Effort

Allows a relatively small effort to move a greater resistance, or a small movement to produce a larger movement elsewhere.

What are the 4 functions of machines in the musculoskeletal system?

1. Balance forces

2. Enhance force / reduce force required

3. Enhance ROM and speed

4. Change direction of force

What 3 types of machines are found in the musculoskeletal system?

Levers — most common
Wheel and axles
Pulleys

Not found: inclined planes, screws, wedges.

levers…

What is a lever?

A rigid bar that rotates around an axis/fulcrum.

In the body:

• Bones = levers

• Joints = axes

• Muscles = apply force

What are the 3 components of a lever?

A = Axis → point of rotation
F = Force → effort, usually at muscle insertion
R = Resistance → load being moved

How are the 3 classes of levers arranged?

1st = FAR → Axis in middle
2nd = ARF → Resistance in middle
3rd = AFR → Force in middle

First- Class Levers…

How does axis position affect a 1st-class lever?

Axis midway → balance; MA = 1

Axis closer to force → speed/ROM; MA < 1

Axis closer to resistance → force; MA > 1

What does the mechanical advantage value tell you?

MA = 1 → effort = resistance
MA < 1 → effort > resistance
MA > 1 → effort < resistance

What is the lever arrangement during triceps elbow extension?

1st-class lever

A = elbow
F = triceps insertion on olecranon
R = unsupported forearm

How does triceps elbow extension change during a push-up?

Normally → 1st-class lever

Hand fixed on floor during push-up → 2nd-class lever

What characterizes a 2nd-class lever?

Resistance is in the middle (resistance is closer to the axis of rotation so this is why it produces force movements).

A–R–F

Produces force movements

MA > 1 → effort < load

What is the classic 2nd-class lever example in the human body?

Rising onto the toes

A = ball of foot
R = body weight at tibiofibular articulation with talus
F = plantar flexors pulling on calcaneus

Are 2nd-class levers common in the human body?

No. Relatively few 2nd-class levers occur in the body.

What characterizes a 3rd-class lever?

Force is in the middle

A–F–R

Favors speed + ROM

MA < 1 → effort > load

Most common lever in the body

During biceps elbow flexion, what are A, F, and R?

A = elbow joint

F = biceps insertion at radial tuberosity

R = forearm resistance at its center of gravity

→ 3rd-class lever

Why is the brachialis considered a “true” 3rd-class lever?

It pulls directly on the ulna below the elbow, and the ulna cannot rotate.

What are examples of 3rd-class levers in the body?

Biceps/brachialis → elbow flexion

Hamstrings → knee flexion while standing

Iliopsoas → hip flexion

When classifying a muscle as part of a lever system, do you use the location of the muscle belly or its insertion?

Use the muscle insertion, NOT the muscle belly.

The muscle belly may be on one side of the joint, but force is applied where the muscle inserts on the bone.

Example: Hamstrings

• Muscle belly = posterior thigh

• Insertion = below the knee

• The insertion is where F (force) is located.

Think: Where do the hamstrings PULL the lower leg? → at their insertion below the knee → that is where F goes.

Why is the brachialis considered a true/direct 3rd-class lever compared with the biceps brachii?

Brachialis: inserts on ulna, which cannot rotate → direct elbow flexion.

Biceps brachii: produces elbow flexion + forearm supination.

→ Its 3rd-class leverage applies specifically to its elbow flexion function.

Torque, Force Arm + Resistance Arm…

What is torque?

Torque = turning effect of an eccentric force.

T = Force × Force arm

What does eccentric force mean when discussing torque?

Force applied off-center/not in line with the center of rotation.

It does NOT mean eccentric muscle contraction.

What is the force arm?

Perpendicular distance from the axis to the line of action of the force.

Also called moment arm/torque arm.

↑ Force arm → ↑ Torque

What is the resistance arm?

Distance from the axis to the point where resistance is applied.

How can you distinguish force arm from resistance arm?

Force arm = Axis → Force

Resistance arm = Axis → Resistance

Force Arm, Resistance Arm + Mechanical Advantage…

This is what I would really know for the exam:

↑ Force arm → ↑ torque → ↓ force needed

↑ Resistance arm → ↑ force needed

Resistance closer to axis → easier

Force farther from axis → easier

Force closer to axis → more speed/ROM, but requires more force

How does changing the force arm affect the amount of force required?

↑ Force arm → ↓ force required

↓ Force arm → ↑ force required

How does changing the resistance arm affect the force required?

↑ Resistance arm → ↑ force required

↓ Resistance arm → ↓ force required

If resistance or the resistance arm increases, what must happen?

Need greater force and/or a longer force arm.

In a 2nd-class lever, what happens when resistance moves closer to the axis?

↑ Mechanical advantage

↓ Distance resistance moves

In a 3rd-class lever, what happens when force is applied closer to the axis?

↑ Speed and ROM

But more force is required.

In a 3rd-class lever, what happens when force is applied closer to the resistance?

Less force is required.

What equation is used to determine the force required in the biceps curl examples?

Force × Force Arm = Resistance × Resistance Arm

What happens to force required when the force arm increases?

↑ Force arm → ↓ Force required

Example:
Force arm 0.10 → 0.15 m
Force required 112.5 → 75 N

This produced the greatest decrease in force required in the examples.

What happens to force required when the resistance arm decreases?

↓ Resistance arm → ↓ Force required

Example:
Resistance arm 0.25 → 0.20 m
Force required 112.5 → 90 N

What happens to force required when the resistance/load decreases?

↓ Resistance → ↓ Force required

Example:
Resistance 45 → 44 N
Force required 112.5 → 110 N

Most important comparison:
Increasing the force arm had the greatest effect on decreasing the force required in these examples.

Human Leverage/ Sport Applications…

What is human leverage generally designed to favor?

Speed and ROM at the expense of force

Short force arms + long resistance arms → greater muscular force required.

What is the advantage of a longer lever in sports?

Longer lever → greater velocity / linear force

Example: Straight-arm tennis swing → longer lever → harder hit

Also useful in baseball, hockey, golf, and field hockey.

What is the advantage of a shorter lever in sports?

Shorter lever → greater quickness

Examples:

• Catcher brings hand near ear → quicker throw

• Sprinter flexes knee, bringing heel toward glute → quicker leg movement