Understanding Tendons

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Understanding Tendons: Anatomy, Function, and Injury Prevention

You might be experiencing persistent Achilles tendon pain after your morning jog, or perhaps shoulder discomfort that limits your daily movements. As physiotherapists specializing in musculoskeletal disorders, we regularly see patients concerned about these tendon pains that seem to take forever to heal. You are not alone: tendinopathies account for 30% to 50% of sports and work-related injuries.

Here's the good news: 80% to 90% of tendinopathies improve with appropriate conservative treatment, without requiring surgery. Most tendon pains do not indicate a serious injury, but rather an insufficient adaptation to mechanical demands.

Here's what science reveals about tendons, which often surprises our patients:

Tendons don't always get inflamed: The term "tendinitis" (inflammation) is less accurate than "tendinopathy" (degenerative change). Microscopic studies show that painful tendons exhibit collagen disorganization rather than classic inflammation.
Poor blood supply explains slow healing: Tendons receive 7 to 10 times less blood than muscles, which is why full recovery can take 3 to 12 months, or even 1 to 2 years for complete maturation.
Progressive loading is the best treatment: Unlike complete rest, heavy slow resistance training shows the best scientific evidence for stimulating tendon regeneration.
Tendons can support 17 times your body weight: Their unique Type I collagen composition gives them exceptional strength, provided they are properly conditioned.

This guide explores tendon anatomy, their biomechanical function, common injury mechanisms, and evidence-based prevention and treatment strategies. To learn more about physiotherapy in general, consult our complete guide to physiotherapy.

What is a tendon?

A tendon is a strong band of dense fibrous connective tissue that connects muscles to bones to transmit muscle contraction force and produce joint movement. Primarily composed of collagen fibers organized in a hierarchical structure, a tendon can withstand up to 17 times the body's weight in tension and compression. The adult human body contains approximately 4,000 tendons of varying sizes.

Key Anatomical Differentiation:

Tendons differ from ligaments and muscles in their function and composition:

Tendons: connect muscle to bone, transmit force, composed of dense collagen
Ligaments: connect bone to bone, stabilize joints, also rich in collagen but with more elastin
Muscles: generate force through contraction, composed of contractile muscle fibers and connective tissue

This distinction is important for understanding injury mechanisms and the specific therapeutic approaches for each tissue. If you are experiencing knee pain that may involve tendons, consult our guide on knee pain.

The tendon is an integral part of the musculotendinous unit, where the myotendinous junction (muscle-tendon) and the osteotendinous junction (tendon-bone) represent critical biomechanical transition zones, often vulnerable to injury.

How tendons are made

Tendons are composed of 65% to 80% Type I collagen, arranged in parallel longitudinal fibers that provide their tensile strength. This hierarchical architecture includes two protective layers (the outer epitendon and inner endotendon) and specialized cells called tenocytes. This entire structure is characterized by poor vascularization but rich sensory innervation, including Golgi Type III mechanoreceptors.

Hierarchical Collagen Structure:

The structural organization of a tendon follows a precise hierarchy:

Tropocollagen: basic collagen molecules
Microfibrils: tropocollagen assemblies
Subfibrils: groups of microfibrils
Fibrils: structures visible under an electron microscope
Collagen Fibers: bundles of fibrils (visible under a light microscope)
Fascicles: groups of fibers
Tendon: final assembly of fascicles

This organization allows for efficient force transmission while maintaining some elasticity. Collagen fibers have a characteristic wavy pattern (crimp pattern) that straightens out when put under tension, acting as a shock absorption system.

Protective Layers:

Epitendon: an outer sheath of connective tissue that encloses the entire tendon, facilitates gliding between adjacent structures, and contains the main blood vessels and nerves
Endotendon: a thin layer of connective tissue that surrounds each individual fascicle, allows for the transmission of (limited) nutrients, and coordinates the action of the different fascicles

Cellular Composition:

Tenocytes and tenoblasts make up only 5% of the tendon's volume but play a crucial role:

Tenoblasts: young, active cells that produce collagen and the extracellular matrix.
Tenocytes: mature, star-shaped cells aligned between collagen fibers, responsible for tissue maintenance and repair.

Vascularization and Innervation:

The low vascularization of tendons (7 to 10 times less than muscles) explains their slow healing. Blood vessels come from three sources: the myotendinous junction, the osteotendinous junction, and the paratendon (outer sheath). In contrast, tendons have rich sensory innervation, including:

Golgi mechanoreceptors (tendon organs): detect tension and regulate muscle contraction
Free nerve endings: signal pain
Proprioceptive receptors: provide information about joint position and movement.

This dense nerve supply explains why tendinopathies are painful and how tendons contribute to precise motor control.

What is the purpose of tendons in the body?

Tendons transmit the force generated by muscles to bones to produce joint movement by converting muscle contraction into bone displacement. They also act as biomechanical shock absorbers, absorbing up to 35% of mechanical stress to protect muscles and joints from impact, and provide constant proprioceptive feedback via the Golgi tendon organs, which regulate muscle tension.

Primary Function: Force Transmission

When a muscle contracts, it shortens and pulls on its insertion tendons. This pull is transmitted to the bone, creating joint movement according to the laws of biomechanical leverage. The efficiency of this transmission depends on:

Pennation angle (the angle between muscle fibers and the tendon): affects the maximum force that can be transmitted
Tendon length: shorter tendons transmit more force but allow less movement
Tendon stiffness: a stiffer tendon transmits force more quickly

Secondary Function: Storage and Release of Elastic Energy

During activities like running or jumping, tendons act like biomechanical springs. During the landing phase (or eccentric phase), the tendon stretches and stores elastic energy. In the propulsion phase (or concentric phase), this energy is released, contributing to more efficient movement.

The Achilles tendon perfectly illustrates this phenomenon: during walking, it stores and releases up to 35% of the total mechanical energy, reducing the amount of muscle work required. This energy-saving ability is crucial for endurance activities.

Tertiary function: neuromuscular regulat

ion and proprioception

Golgi tendon organs, located at the muscle-tendon junction, detect changes in tension. When tension becomes excessive, they trigger an inhibitory reflex (inverse myotatic reflex) that reduces muscle contraction, thereby protecting the system from tears.

This proprioceptive function continuously informs the central nervous system about:

The state of muscle tension
Joint position
The speed and range of movement

This information allows for fine motor adjustments and precise coordination, which are essential for complex movements and injury prevention.

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What are the most common tendon injuries?

Tendon injuries include tendinopathy (chronic degeneration of collagen with fiber disorganization), tendinitis (rare acute inflammation), partial tears (rupture of some fascicles), and complete ruptures (total interruption of tendon continuity). Commonly affected tendons are the Achilles (12-18% of runners), the rotator cuff (30% of adults >60 years old), the patellar tendon (14% of jumping athletes), and the adductors (10-15% of pivot sports).

Clarified terminology: tendinopathy vs. tendinitis

For decades, the term "tendinitis" (suffix "-itis" = inflammation) was used for all tendon pain. Advances in histopathology have revealed that most chronic tendinopathies do NOT show classic inflammation but rather:

Disorganization of the collagen matrix
Increase in type III collagen (weaker than type I)
Disorganized neovascularization (growth of new blood vessels)
Presence of pain-sensing nerve fibers accompanying these new blood vessels

The term "tendinopathy" is therefore more accurate to describe this progressive degeneration. True tendinitis (acute inflammation) primarily occurs within the first 48-72 hours after a traumatic injury.

Categories of tendon injuries:

Overload tendinopathy (the most common)
Gradual progression over weeks/months
Pain at the start of activity that lessens with warming up
Characteristic morning stiffness
Caused by an imbalance between mechanical load and the body's ability to adapt
Partial tear
Rupture of some tendon fibers
Acute pain during a specific effort
Partial loss of strength
Diagnosed by ultrasound or MRI
Complete tear
Complete tendon tear
Sensation of a "whip" or "snap"
Immediate functional incapacity
Often requires surgical intervention

Common locations and at-risk populations:

Achilles Tendon: runners, racquet sports, ages 30-50
Rotator Cuff: manual laborers, throwing sports, ages >40
Patellar Tendon: basketball, volleyball, jumping sports
Hip Adductors: soccer, hockey, pivoting sports
Epicondyles (elbow): tennis, repetitive gripping work

For a deeper understanding of specific tendinopathies, consult our guides on Achilles tendon tendinopathy and patellar tendon tendinopathy.

Identified risk factors:

Sudden increase in training volume or intensity (training error #1)
Deficient biomechanics (poor movement technique)
Muscular imbalances (relative weakness in certain muscle groups)
Reduced or excessive flexibility
Age (progressive loss of collagen quality after 30 years old)
Certain medications (fluoroquinolones, corticosteroids)
Metabolic diseases (diabetes, high cholesterol)

Why do tendons hurt?

Tendon pain is caused by a gradual breakdown of the tissue. This process leads to the production of more type III collagen (which is less strong than type I) and the disorganized growth of new blood vessels, accompanied by pain-sensing nerve fibers that release pain-causing substances. Contrary to popular belief, most chronic tendon problems (tendinopathies) do NOT involve classic inflammation. Instead, they are characterized by a structural disorganization triggered by repetitive stress that exceeds the tissue's ability to repair itself.

Degenerative Mechanism (Non-Inflammatory):

When a tendon is subjected to repetitive or excessive loads without adequate recovery, a cascade of tissue changes begins:

Initial Decompensation Phase (0-2 weeks)
Micro-tears in collagen fibers
Activation of tendon cells to repair damage
If the stress continues: the tissue cannot fully repair itself
Disorganization Phase (2-12 weeks)
Disorganization of the collagen matrix
Increased production of type III collagen (which is "softer" and less strong)
Decrease in type I collagen (strong)
Change in the normal wavy structure of the fibers
Chronic Phase (>12 weeks)
Neovascularization (disorganized growth of new blood vessels)
Invasion of sensory nerve fibers accompanying these new blood vessels
These nerve fibers release pain-sensing neurotransmitters (substance P, CGRP)
Result: chronic pain even with moderate loads

Factors Triggering Pain:

Repetitive Mechanical Stress: performing the same movements thousands of times (running, hitting a ball, lifting)
Insufficient Conditioning: tendons not prepared for the imposed loads
Inadequate Warm-up: cold tissues are less elastic and more vulnerable
Insufficient Recovery: inadequate rest time between efforts for regeneration
Prolonged Sedentary Lifestyle: deconditioned tendons lose their ability to adapt
Aging: natural decrease in collagen quality and quantity after age 30

The Role of Genetics:

Recent studies identify genetic polymorphisms associated with an increased risk of tendinopathy, particularly genes coding for:

Collagen synthesis (COL5A1)
Matrix metalloproteinases (MMP3)
Growth factors (TGF-β1)

However, genetics are NOT destiny: they influence susceptibility, but modifiable factors (training, technique, recovery) remain crucial.

Why do tendons heal slowly?

Tendons heal slowly due to their limited blood supply, which restricts the delivery of nutrients, oxygen, and repair cells. Complete healing involves three successive phases: inflammatory (5-7 days), proliferative (several weeks), and remodeling (6-8 weeks up to 1-2 years for complete maturation), with each phase requiring a delicate balance between progressive mechanical loading and protection against excessive stress that could compromise healing.

Impact of Low Blood Supply:

Tendons receive 7 to 10 times less blood than muscles. This anatomical characteristic is explained by their function: to transmit force rather than generate energy (unlike muscles, which require a massive blood supply for contraction).

Consequences of this Low Blood Supply:

Limited nutrient supply (glucose, amino acids)
Limited oxygen supply (slowed energy metabolism)
Slow arrival of immune cells and growth factors
Slow elimination of metabolic waste

The areas where the tendon connects to muscle (myotendinous) and bone (osteotendinous) have a relatively better blood supply, but the central portion of the tendon (midsubstance) is particularly avascular. This explains why tendinopathies in the mid-portion are the slowest to heal.

Phases of Tendon Healing:

Phase 1: Inflammatory (0-7 days)

Goal: to clean the injured area
Process: arrival of inflammatory cells (neutrophils, macrophages)
Production of growth factors
Formation of a provisional fibrin clot
Recommendation: avoid excessive stress, relative protection, no complete rest

Phase 2: Proliferative (7 days to 6 weeks)

Goal: rebuild the tissue
Process: activation of tenocytes and tenoblasts
Massive production of type III collagen (temporary scar tissue)
Start of neovascularization
Recommendation: gradual introduction of controlled loads to guide fiber alignment

Phase 3: Remodeling (6 weeks to 1-2 years)

Goal: strengthen and organize the tissue
Process: gradual replacement of type III collagen with type I collagen
Realignment of fibers according to mechanical stress lines
Decrease in cellularity and vascularization
Progressive increase in mechanical strength
Recommendation: systematic progression of load, patience for complete maturation

Factors influencing healing speed:

Age: slower healing after 40 (reduced cellular activity)
Severity of injury: complete ruptures require 2-3x more time than simple tendinopathies
Location: avascular areas heal more slowly
Mechanical load: too little delays healing, too much causes new damage
Nutrition: adequate intake of protein, vitamin C, zinc
Smoking: reduces vascularization and slows healing by 30-50%
Comorbidities: diabetes, metabolic disorders affect healing

Why Patience is Essential:

Tendon scar tissue reaches:

30% of its normal strength at 3 weeks
50% at 6 weeks
80% at 3 months
100% only after 1 year (or even 2 years for complete maturation)

Resuming activity too early or too intensely can interrupt the remodeling process and turn an acute tendinopathy into a stubborn chronic problem.

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How does physiotherapy help tendons heal?

Physiotherapy primarily promotes tendon healing through Heavy Slow Resistance (HSR) training, which stimulates the production of quality collagen and the realignment of fibers. Complementary treatments include extracorporeal shockwave therapy (which stimulates regeneration through controlled micro-trauma), manual therapy (mobilizations to limit adhesions), ultrasound, and movement re-education to correct biomechanical deficits, all with a minimum protocol of 12 weeks of continuous progressive loading.

Evidence-Based Approach: Heavy Slow Resistance Training

Research shows that Heavy Slow Resistance (HSR) training outperforms all other conservative interventions for tendinopathies. This method involves:

Principles of HSR:

Heavy loads (70-85% of maximum strength)
Slow execution speed (6 seconds per repetition: 3 sec concentric + 3 sec eccentric)
Moderate volume (3-4 sets of 6-8 repetitions)
Frequency: 3 times per week
Gradual progression over 12+ weeks

Mechanisms of Action:

Mechanotransduction stimulation: tenocytes detect the load and increase type I collagen synthesis
Fiber realignment along stress lines
Increase in tendon cross-sectional area
Improvement in viscoelastic properties
Reduction of pathological neovascularization

Comparison with classic eccentric training:
HSR produces similar or superior improvements compared to pure eccentric training, with less pain during exercises and better patient compliance.

Shockwave Therapy (ESWT - Extracorporeal Shockwave Therapy):

Mechanism: Shockwaves create controlled micro-traumas that:

Stimulate neo-angiogenesis (formation of new healthy blood vessels)
Trigger the release of growth factors
Fragment calcium deposits (if present)
Induce an analgesic effect by hyperstimulation of nerve fibers

Typical protocol: 3-6 sessions, 1 week apart, 2000-2500 shocks per session.

Evidence: Meta-analyses show 60-70% improvement for Achilles and shoulder tendinopathies.

Manual therapy and mobilizations:

Goals:

Prevent or treat tissue adhesions
Improve tendon gliding
Restore joint mobility
Reduce biomechanical compensations

Techniques: deep transverse frictions (Cyriax), joint mobilizations, myofascial release of adjacent muscles.

Other modalities (variable evidence):

Therapeutic Ultrasound: thermal and mechanical effect, limited evidence
Low-Intensity Laser: may reduce pain and acute inflammation
Neuro-Proprioceptive Taping: mechanical support, sensory feedback
Electrotherapy (TENS, Interferential): temporary pain relief

Movement re-education:

Identify and correct deficits:

Deficient biomechanics (running technique, sports movements)
Muscle imbalances (weakness, inhibition)
Motor control deficits
Joint mobility limitations

Video analysis and functional assessment help identify contributing factors specific to each patient.

Treatment duration and progression:

Realistic Timeline:

Weeks 0-2: reduce irritability, introduce minimal load
Weeks 3-6: progress HSR load, begin return to modified activities
Weeks 7-12: progress towards specific sports activities
Weeks 12+: consolidation, recurrence prevention

Key points: 12 weeks is the absolute minimum, 6 months is a more realistic timeframe for a full return to sport.

How to care for your tendons daily?

To protect your tendons, gradually increase your training load following the 10% rule (maximum 10% increase in weekly volume). Warm up properly for 10-15 minutes before intense activities. Allow 48-72 hours of recovery between sessions that stress the same tendons. Practice preventive strength training 2-3 times a week. Avoid repetitive movements without prior progressive conditioning, and maintain regular physical activity to counteract the harmful effects of a sedentary lifestyle.

Principle of Gradual Progression (The 10% Rule):

The #1 training mistake causing tendinopathies is increasing the load too quickly. The 10% rule states:

Increase training volume by a maximum of 10% per week
Example: if you run 20 km this week, a maximum of 22 km next week
Applies to: distance, intensity, frequency, total workload

Deload weeks (recovery weeks):
Every 3-4 weeks, reduce your training volume by 20-30% to allow for supercompensatory adaptation. Tendons adapt more slowly than muscles, so respecting this timeframe is crucial.

Appropriate Warm-up:

An effective warm-up to protect your tendons includes:

General aerobic phase (5-7 min): brisk walking, light jogging, cycling
Goal: increase tissue temperature, blood circulation
Dynamic joint mobilizations (3-5 min): controlled movements through the full range of motion
Goal: joint lubrication, neuromuscular activation

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Specific activation (2-3 min): progressive movements similar to the planned activity
Goal: prepare specific movement patterns

Avoid: prolonged static stretches before exercise (they reduce strength and power)

Recovery and Rest:

Tendons need 48-72 hours to recover from intense activity:

Plan rest days or cross-training
Avoid consecutive intense sessions that stress the same tendons
Adequate sleep (7-9 hours): essential for tissue repair
Appropriate nutrition: proteins (1.6-2.2 g/kg), vitamin C, omega-3

Preventive Strength Training:

Muscle strength protects tendons by:

Reducing the relative stress on the tendon for a given load
Improving motor control and joint stability
Increasing the capacity to absorb impact forces

Minimum program:

2-3 sessions per week
Focus: stabilizing muscles and complete muscle chains
Include eccentric exercises (superior protection)
Gradual load progression

Recognize early warning signs:

Consult a physiotherapist if:

Tendon pain lasting >2 weeks
Morning stiffness lasting >30 minutes
Pain gradually increasing despite relative rest
Pain interfering with daily activities
Sensation of the tendon "cracking" or "grinding"

Early intervention = faster healing and prevention of chronicity.

Lifestyle factors:

Avoid smoking: reduces vascularization and collagen quality
Maintain a healthy weight: overweight = increased stress on weight-bearing tendons (Achilles, patellar)
Manage stress: chronically elevated cortisol affects tissue regeneration
Regular physical activity: a sedentary lifestyle degrades tendon quality

What should you remember about tendons?

Tendons are remarkable structures primarily composed of hierarchically organized type I collagen. They play an essential biomechanical role by transmitting muscle force, absorbing shocks, and providing constant proprioceptive feedback. Their limited blood supply explains why tendon injuries require patience and appropriate treatment, often taking 3 to 12 months for complete recovery.

Modern physiotherapy, focused on heavy and slow resistance training (HSR) combined with shockwave therapy and movement re-education, offers the best scientific evidence to promote tendon regeneration. Prevention relies on gradual progression, adequate warm-up, sufficient recovery, and preventive strength training.

If you are experiencing persistent tendon pain or want to optimize your training program to prevent injuries, our physiotherapists can assess your specific condition and create a personalized, evidence-based treatment plan. Book an appointment at one of our Greater Montreal clinics for a complete assessment.