What is the difference between eccentric and concentric loading in the rehabilitation of tendon-related issues, such as patellar tendinopathy?

Eccentric vs Concentric Loading in Tendon Rehabilitation

Direct Answer

For patellar tendinopathy, eccentric exercises remain the evidence-based cornerstone of treatment, but recent high-quality evidence shows that combined eccentric-concentric loading (heavy slow resistance training) produces equivalent clinical outcomes and may offer practical advantages. 1, 2

Understanding the Difference

Eccentric Loading (Muscle Lengthening Under Load)

• Involves controlled lengthening of the muscle-tendon unit while resisting force (e.g., lowering phase of a squat) 1
• Stimulates collagen production and guides normal alignment of newly formed collagen fibers 1
• Has the strongest available evidence base for patellar tendinopathy treatment according to systematic reviews 1

Concentric Loading (Muscle Shortening Under Load)

• Involves muscle contraction that shortens the muscle-tendon unit (e.g., rising phase of a squat) 2
• When combined with eccentric loading in heavy slow resistance (HSR) protocols, produces equivalent or superior outcomes 2

Clinical Evidence Comparison

Eccentric-Only Protocols

• The traditional decline board protocol (25-degree angle) has been the standard for two decades 1, 3
• Produces significant improvements in VISA-P scores, pain reduction, and functional outcomes 4, 3
• Critical caveat: Up to 45% of patients may not respond to isolated eccentric training 2
• The decline angle can be reduced to 17 degrees without affecting outcomes, which may improve patient compliance 3

Combined Eccentric-Concentric (Heavy Slow Resistance)

• A 2013 systematic review found equivalent or higher-level evidence for HSR loading compared to isolated eccentric training in patellar tendinopathy 2
• HSR was associated with reduced Doppler area, decreased anteroposterior tendon diameter, and greater evidence of collagen turnover—benefits not consistently seen with eccentric-only protocols 2
• The only mechanism consistently associated with improved clinical outcomes in both Achilles and patellar tendons was improved neuromuscular performance (torque, work, endurance), which HSR achieves effectively 2

Adding Stretching to Eccentric Protocols

• Combining eccentric training with static stretching of quadriceps and hamstrings produces superior outcomes compared to eccentric training alone 5
• At 6-month follow-up, the combined approach showed significantly better VISA-P scores (difference of +19 points) 5

Practical Treatment Algorithm

Initial Phase (Weeks 1-4)

• Implement eccentric decline board exercises (either 25° or 17° based on patient comfort) twice weekly 4, 3
• Add daily static stretching of quadriceps and hamstrings 5
• Apply relative rest by reducing jumping and stair navigation that reproduces pain 1
• Use cryotherapy (10-minute periods through wet towel) for acute pain relief 1

Progressive Phase (Weeks 4-12)

• Consider transitioning to or incorporating heavy slow resistance training (eccentric-concentric combined) if eccentric-only shows inadequate response by week 4-6 2
• Continue stretching program throughout 5
• Monitor tendon stiffness changes, which correlate with clinical improvement 6

Expected Timeline

• Significant improvements in pain and function typically occur within 6 weeks 3
• Additional modest improvements continue through 12 weeks 3
• Approximately 80% of patients recover completely within 3-6 months with appropriate conservative treatment 1

Mechanisms of Action

Why Loading Works

• Improved neuromuscular performance (increased torque, work capacity, endurance) is the only mechanism consistently associated with clinical improvement across both loading types 2
• Eccentric loading specifically promotes collagen fiber realignment and production 1
• HSR loading demonstrates superior tendon structural adaptation (reduced thickness, improved vascularity) 2

Important Mechanistic Finding

• Most high-quality studies found no association between improved imaging findings (reduced anteroposterior diameter, Doppler signal) and clinical outcomes in Achilles tendinopathy, though this relationship appears stronger in patellar tendinopathy with HSR 2

Critical Pitfalls to Avoid

• Never isolate eccentric training dogmatically—the evidence for superiority over combined loading is limited and conflicting 2
• Avoid complete immobilization, which causes muscular atrophy and deconditioning 1
• Do not proceed to surgery without 3-6 months of well-managed conservative treatment 1, 7
• Never inject corticosteroids directly into tendon substance due to rupture risk 1

Clinical Decision Point

Given equivalent clinical outcomes and potentially superior tendon adaptation, clinicians should consider eccentric-concentric loading (HSR) alongside or instead of isolated eccentric loading, particularly in patients who fail to respond to eccentric-only protocols within 4-6 weeks. 2