Blog 3: Biomechanics of Athletes Explained.
Part 3: Force, Load & Impact - Understanding Stress on Your Body
Welcome Back to Biomechanics of The Athlete Explained!
In Parts 1 and Part 2, Dr. Keirstyn covered what biomechanics is and how your body works as a connected system (the kinetic chain). Now let's talk about something every athlete experiences but rarely thinks about: force.
Every time you move, your body experiences forces. Running creates impact forces 2-3x your body weight with every step. Jumping can create forces 4-6x your body weight on landing. Even walking generates force.
Understanding how forces affect your body is critical for injury prevention and performance. Let's break it down.
What is Force in Biomechanics?
Simple definition: Force is a push or pull that causes your body to accelerate, decelerate, or deform.
In sports, forces come from:
Ground reaction force: The ground pushing back when you land or push off
Muscle force: Your muscles generating tension to create movement
External forces: Impacts from other players, equipment, or objects
The key principle: Your body must absorb, redirect, or generate these forces. How well you do this determines performance and injury risk.
Ground Reaction Force: The Invisible Impact
Every time your foot hits the ground, the ground pushes back with equal force (Newton's Third Law). This is ground reaction force (GRF).
Examples:
Walking: 1.2x body weight
Running: 2-3x body weight per step
Sprinting: 3-5x body weight
Jumping/Landing: 4-8x body weight
What This Means:
A 150 lb runner experiences 300-450 lbs of force through their leg with every step. Over a 5K run (roughly 3,000 steps), that's millions of pounds of cumulative force.
The Question: Can your body handle that force, or will it break down?
How Your Body Handles Force
The Body's Strategy: Distribute force across multiple joints and tissues.
Good Force Distribution:
Ankle, knee, hip all flex slightly on landing (triple flexion)
Muscles absorb and control the force eccentrically
Force spreads across bones, cartilage, tendons, and muscles
No single structure is overloaded
Poor Force Distribution:
Stiff-legged landing (force goes straight to joints)
One joint compensates for another's lack of movement
Single structure (tendon, cartilage) takes excessive load
Tissue breaks down over time
Example: Landing from a Jump
Good Mechanics:
Land on balls of feet
Ankles, knees, hips flex simultaneously
Glutes and quads absorb force eccentrically (controlling descent)
Force distributed across entire lower body
Poor Mechanics:
Land flat-footed or on heels
Stiff knees (minimal bending)
Force goes directly to knee joint and patellar tendon
Patellar tendinopathy (jumper's knee) develops
The Difference: Injury vs. no injury from the same activity.
Load: How Much Stress Your Tissues Can Handle
Load is the cumulative stress placed on your body over time.
Acute Load: What you do in a single session or week
Chronic Load: What you've been doing over the past 3-4 weeks
The Balance:
Too little load: Tissues don't adapt, you don't get stronger
Appropriate load: Tissues adapt, you get stronger and more resilient
Too much load: Tissues break down faster than they can repair
Example: The 10% Rule
Runners are often told not to increase weekly mileage by more than 10%. Why?
Biomechanical Reason: Tissues (tendons, bones, cartilage) adapt to load slowly. Sudden increases create stress faster than tissues can adapt, leading to injury.
What Happens:
Week 1: Run 20 miles → tissues stressed but recover
Week 2: Run 30 miles (50% increase) → tissues overloaded
Week 3: Shin splints, stress fractures, or tendinopathy develop
The Fix: Gradual load increases allow tissues to adapt.
Why Some Movements Hurt More Than Others
Not all forces are equal. The type, direction, and speed of force matter.
Eccentric Loading (Lengthening Under Tension):
What It Is: Muscle lengthens while contracting (controlling a load)
Examples:
Lowering phase of a squat
Landing from a jump
Running downhill
Decelerating during a cut or change of direction
Why It's Harder: Eccentric contractions create more force and more muscle damage than concentric (shortening) contractions.
Result: Soreness, fatigue, and higher injury risk if not trained properly.
Repetitive Low-Load vs. Single High-Load:
Repetitive Low-Load:
Running (2-3x body weight, thousands of times)
Swimming (low force, high repetition)
Cycling (moderate force, high repetition)
Injury Type: Overuse injuries (tendinopathy, stress fractures)
Single High-Load:
Olympic lifting (massive force, few reps)
Sprinting (very high force, short duration)
Jumping (high force, moderate reps)
Injury Type: Acute injuries (muscle strains, ligament tears) or overuse if volume is high
The Lesson: Both create risk, but through different mechanisms. Training must prepare for your sport's specific demands.
Impact Absorption: The Key to Longevity
Athletes who can absorb force well stay healthy. Those who can't break down.
What Determines Force Absorption Capacity?
1. Muscle Strength
Stronger muscles absorb more force before tissues fail
Weak muscles → joints and tendons take excessive stress
2. Eccentric Strength
Ability to control lengthening (lowering, landing, decelerating)
Most injuries happen during eccentric phases
3. Joint Mobility
Mobile joints distribute force across larger range
Stiff joints concentrate force in small areas
4. Neuromuscular Control
Brain's ability to coordinate muscles quickly
Poor control → delayed muscle activation → poor force absorption
5. Fatigue
Tired muscles can't absorb force well
Late-game injuries often stem from fatigue affecting mechanics
Training to Handle Force
Principle 1: Build Tissue Capacity Gradually
Your tissues adapt to load over weeks and months, not days. Increase training volume progressively.
Principle 2: Train Eccentrically
Most injuries happen during eccentric phases. Train your body to control force.
Examples:
Eccentric squats (slow 3-5 second descent)
Nordic hamstring curls (eccentric hamstring loading)
Single-leg step-downs (eccentric quad and glute control)
Principle 3: Practice Landing Mechanics
Landing is a skill. Practice soft, controlled landings.
Drill: Box jump step-offs (step off box, land softly, hold position for 3 seconds)
Principle 4: Don't Ignore Fatigue
When fatigued, mechanics deteriorate and injury risk spikes. Recognize when to back off.
Real-World Application: The Runner
Scenario: A runner increases mileage from 20 to 35 miles per week and develops shin splints.
Biomechanical Explanation:
Ground reaction force: 2.5x body weight per step
35 miles ≈ 35,000 steps per week
Total force: 87,500x body weight per week
Shin muscles and tibial bone stressed beyond adaptive capacity
Microdamage accumulates faster than repair
Shin splints (or stress fracture) develops
The Fix:
Reduce mileage to allow tissue recovery
Strengthen calf and shin muscles (increase force absorption capacity)
Improve landing mechanics (reduce peak force)
Gradually rebuild mileage (allow adaptation)
How Dr. Keirstyn Can Help
At Endurance Therapeutics, Dr. Keirstyn helps athletes understand how their bodies handle force and where breakdowns occur. Through movement assessment and education, she identifies:
How you absorb and distribute force during sport-specific movements
Where compensations create excessive stress
Which tissues are overloaded and why
How to train your body to handle higher loads safely
Treatment and education include:
Joint mobilization to improve force distribution
Soft tissue work to reduce overloaded muscles
Exercise prescription to build eccentric strength and force absorption capacity
Load management guidance to prevent overtraining injuries
Whether you're dealing with an overuse injury or want to prevent one, understanding how your body handles force is critical.
Book an assessment to learn how your movement patterns are affecting tissue stress and what you can do to stay healthy under load.
What's Next
In Part 4, we'll dive into movement patterns — the fundamental movements your body performs (squat, hinge, lunge, push, pull) and what "good form" actually means. We'll break down common faults and how to fix them.
📍 Endurance Therapeutics | Oakville, Ontario
📞 905-288-7161