Key Takeaways
- Lever systems in the body consist of fulcrum (joint), effort (muscle force), and resistance (load); most joints are third-class levers.
- Mechanical advantage (MA) = effort arm / resistance arm; values <1 favor speed and range of motion over force.
- The force-velocity relationship shows an inverse relationship: as velocity increases, force production decreases (and vice versa).
- The length-tension relationship identifies optimal sarcomere length for maximum force production.
- Elastic energy stored during eccentric phases (stretch-shortening cycle) enhances force production in subsequent concentric actions.
Biomechanics
Quick Answer: Biomechanics is the study of forces and their effects on living systems. Key concepts include lever systems (most body levers are third-class, favoring speed over force), the force-velocity relationship (inverse relationship between force and velocity), and the stretch-shortening cycle (elastic energy storage enhances force production).
Lever Systems
The human body uses lever systems to produce movement. Each lever consists of:
- Fulcrum: The pivot point (joint)
- Effort force: The force applied by muscle
- Resistance: The load being moved
Three Classes of Levers
| Class | Arrangement | Example in Body | Mechanical Advantage |
|---|---|---|---|
| First | Fulcrum between effort and resistance | Neck extension (atlanto-occipital joint) | Can be >1 or <1 |
| Second | Resistance between fulcrum and effort | Plantarflexion (calf raise) | >1 (force advantage) |
| Third | Effort between fulcrum and resistance | Bicep curl (elbow flexion) | <1 (speed/ROM advantage) |
Third-Class Levers (Most Common)
Most joints in the human body function as third-class levers:
- Effort (muscle) is applied between the fulcrum (joint) and resistance (load)
- Mechanical advantage is less than 1
- Advantage: Greater range of motion and speed of movement
- Disadvantage: Requires more muscle force than the actual load
Example (Bicep Curl): The biceps must generate approximately 7-8 times the force of the weight being lifted due to the short effort arm (biceps insertion) compared to the resistance arm (length to the hand).
Mechanical Advantage
Mechanical Advantage (MA) determines the efficiency of force production:
Formula
MA = Effort Arm / Resistance Arm
Where:
- Effort arm: Distance from fulcrum to point of force application
- Resistance arm: Distance from fulcrum to resistance (load)
| MA Value | Meaning | Trade-off |
|---|---|---|
| MA > 1 | Force advantage | Sacrifices speed and ROM |
| MA = 1 | Equal | Balanced |
| MA < 1 | Speed/ROM advantage | Requires more force |
Practical Application
Different exercises change mechanical advantage:
| Exercise Variation | Mechanical Advantage | Result |
|---|---|---|
| Barbell curl | Standard | Standard difficulty |
| Preacher curl | Decreased | Harder at bottom position |
| Incline curl | Decreased | More stretch, harder |
| Concentration curl | Increased (shortened ROM) | Easier at some positions |
Force-Velocity Relationship
The force-velocity curve describes the inverse relationship between the force a muscle can produce and its contraction velocity.
Key Principles
| Velocity | Force Capability | Muscle Action |
|---|---|---|
| Zero (isometric) | Maximum isometric force | Isometric |
| Slow concentric | High force | Concentric |
| Fast concentric | Low force | Concentric |
| Slow eccentric | Higher than isometric | Eccentric |
| Fast eccentric | Highest force | Eccentric |
Implications for Training
| Goal | Velocity | Load | Application |
|---|---|---|---|
| Maximum strength | Slow | Heavy (>85% 1RM) | Powerlifting, maximal efforts |
| Power development | Moderate-fast | Moderate (30-70% 1RM) | Olympic lifts, jumps |
| Speed | Maximum | Light (<30% 1RM) | Sprinting, throwing |
Exam Tip: Power is maximized at intermediate velocities and loads (approximately 30-70% 1RM), not at maximum load or maximum velocity. Power = Force x Velocity.
The Power Curve
Power = Force x Velocity
Since force and velocity have an inverse relationship, power is maximized somewhere in the middle:
- Too heavy → high force but low velocity → moderate power
- Too light → high velocity but low force → moderate power
- Optimal load → balanced force and velocity → maximum power
Length-Tension Relationship
The length-tension relationship describes how the force a muscle can produce varies with its length.
Optimal Length
| Muscle Length | Actin-Myosin Overlap | Force Production |
|---|---|---|
| Very shortened | Too much overlap (interference) | Low |
| Optimal length | Maximum cross-bridge formation | Maximum |
| Very lengthened | Minimal overlap | Low |
Active vs. Passive Tension
| Component | Source | When Active |
|---|---|---|
| Active tension | Actin-myosin cross-bridges | At all muscle lengths (varies) |
| Passive tension | Elastic components (titin, connective tissue) | When muscle is stretched beyond resting length |
| Total tension | Active + Passive | Sum of both components |
Practical Applications
| Exercise | Muscle Length | Implication |
|---|---|---|
| Incline DB fly | Stretched (lengthened) | More passive tension, potentially more damage |
| Cable crossover | Shortened | Maintains tension at shortened position |
| Preacher curl | Stretched (bottom) | Biceps under significant stretch |
| Concentration curl | Can shorten fully | Less stretch, more focus on contraction |
Stretch-Shortening Cycle (SSC)
The stretch-shortening cycle enhances force production by storing and releasing elastic energy:
Phases
| Phase | Action | What Happens |
|---|---|---|
| Eccentric phase | Muscle lengthens under load | Elastic energy stored in series elastic component |
| Amortization phase | Brief transition | Time between eccentric and concentric |
| Concentric phase | Muscle shortens | Stored elastic energy released, enhancing force |
Key Principle
A concentric action preceded by an eccentric action produces more force than a concentric-only action due to:
- Elastic energy storage in tendons and titin
- Stretch reflex activation (muscle spindles)
- Enhanced muscle activation
Critical: The amortization phase must be short (<0.25 seconds) to maximize elastic energy return. Longer delays allow stored energy to dissipate as heat.
SSC Classifications
| Type | Amortization | Ground Contact | Examples |
|---|---|---|---|
| Fast SSC | <0.25 sec | <0.25 sec | Sprinting, depth jumps, bounding |
| Slow SSC | 0.25-0.50 sec | >0.25 sec | Countermovement jump, loaded jumps |
Training Applications
| Exercise | SSC Type | Purpose |
|---|---|---|
| Depth jumps | Fast SSC | Reactive strength, rate of force development |
| Box jumps | Minimal SSC | Concentric power (no eccentric preload) |
| Countermovement jump | Slow SSC | General power assessment |
| Bounding | Fast SSC | Running-specific power |
| Plyometric push-up | Fast SSC | Upper body reactive strength |
Other Biomechanical Concepts
Moment Arm
The moment arm is the perpendicular distance from the line of force to the axis of rotation:
- Longer moment arm = greater torque for same force
- Explains why stance width and grip width affect exercise difficulty
Torque
Torque = Force x Moment Arm
Torque (rotational force) determines the actual effect of a force on joint rotation.
Ground Reaction Force
Newton's Third Law: For every action, there is an equal and opposite reaction.
- When you push against the ground, the ground pushes back
- Ground reaction force (GRF) can exceed body weight during dynamic activities
- Running GRF: 2-3x body weight
- Jumping GRF: 4-7x body weight (landing)
Impulse
Impulse = Force x Time
- Increasing either force or time increases impulse
- Important for: jumping (maximize impulse to maximize takeoff velocity), throwing, sprinting
- Rate of force development (RFD) is the ability to develop force quickly
Types of Muscle Actions in Movement
| Movement Phase | Muscle Action | Example (Squat) |
|---|---|---|
| Descent (yielding) | Eccentric | Quadriceps lengthen while resisting load |
| Bottom position (pause) | Isometric | Momentary hold at bottom |
| Ascent (overcoming) | Concentric | Quadriceps shorten to extend knees |
Most joints in the human body function as which class of lever?
According to the force-velocity relationship, at what velocity can a muscle produce the GREATEST force?
In the stretch-shortening cycle, why must the amortization phase be kept SHORT?
If mechanical advantage (MA) = effort arm / resistance arm, what does an MA of less than 1 indicate?