Biomechanics

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1 Introduction

About the CSCS Exam Structure and Content Breakdown Pass Rates and Scoring How to Use This Guide

2 Exercise Science

Muscular and Neuromuscular Systems Skeletal and Cardiovascular Systems Bioenergetics and Metabolism Biomechanics Physiological Adaptations to Training

3 Sport Psychology & Nutrition

Motivation and Goal Setting Arousal, Anxiety, and Performance Mental Skills and Team Dynamics Macronutrients and Energy Hydration, Timing, and Supplements

4 Exercise Technique

Resistance Training Technique Olympic Lifting Technique Plyometric and Speed Training Flexibility and Warm-up Spotting and Equipment Safety

5 Program Design

Needs Analysis and Training Goals Training Variables Periodization Models Resistance Training Program Design Speed, Agility, and Conditioning Programs

6 Testing, Evaluation & Administration

Pre-Participation Screening Cardiovascular and Body Composition Testing Strength, Power, and Speed Testing Flexibility Assessment Facility Management and Legal Issues

Key Takeaways Explain This Topic AI

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.

Last updated: January 2026

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