Abstract

Knee tendinopathies, particularly patellar tendinopathy, represent one of the most common causes of chronic anterior knee pain in athletes engaged in sports involving repetitive jumping, acceleration, and changes of direction, such as soccer and basketball. Traditional treatment approaches have relied heavily on linear eccentric training, especially decline squats. However, the use of flywheel inertial devices with eccentric overload has emerged as an effective alternative to optimize tendon loading, induce structural adaptations, and improve function. This article reviews the physiological mechanisms, scientific evidence, and practical applications of eccentric overload squats performed on flywheel devices in the rehabilitation of knee tendinopathy.

Introduction

Tendinopathies, particularly patellar tendinopathy (jumper’s knee), are highly prevalent in elite and recreational athletes. Anterior knee pain often becomes a limiting factor, restricting performance and in some cases forcing early withdrawal from sport. The pathophysiology of tendinopathy is not limited to inflammation but involves a degenerative process characterized by collagen disorganization, extracellular matrix (ECM) changes, and abnormal neovascularization.

Therapeutic interventions should aim to restore the tendon’s ability to tolerate mechanical loading, enhance type I collagen synthesis, and optimize musculotendinous function. In this context, eccentric training has consistently been shown to be the most effective method for recovery.

The introduction of flywheel inertial devices represents a major advance, providing load that adapts to the athlete’s effort, emphasizing eccentric overload and promoting tendon-specific adaptations.

Pathophysiology of Tendinopathy

Chronic tendinopathy is characterized by:

• Collagen disorganization and disrupted fibrillar alignment.
• Mucoid degeneration and increased ground substance.
• Neovascularization and ingrowth of aberrant nerves.
• Decreased tendon stiffness and mechanical strength.

This pathological state reduces the tendon’s ability to transmit force and store elastic energy, leading to impaired performance and pain during sports activity.

Rationale for Eccentric Loading in Rehabilitation

Eccentric exercise offers several tendon-specific benefits:

1. Stimulation of type I collagen synthesis, critical for restoring ECM integrity.
2. Increased tendon stiffness and resilience, improving load tolerance.
3. Pain reduction, potentially via modulation of abnormal innervation.
4. Improved eccentric strength and neuromuscular control under high loads.
5. Enhanced energy storage and release capacity, essential for explosive tasks.

These adaptations explain why eccentric protocols have become the cornerstone of tendinopathy rehabilitation.

Flywheel Devices and Eccentric Overload

Flywheel technology is based on the principle of inertia: concentric movement spins the flywheel, storing kinetic energy that must then be absorbed eccentrically. This produces a self-regulated eccentric overload that surpasses what can be achieved with traditional free weights.

Advantages of flywheel training:

• Variable, adaptive load: not limited by fixed weights but scaled to the athlete’s output.
• Greater eccentric stimulus: peak load occurs at the end of the range of motion.
• Functional transferability: replicates the explosive and elastic demands of sport.
• Enhanced muscle activation, including core and stabilizers around the knee.

When applied to the squat exercise, flywheel devices provide a controlled and progressive method of applying eccentric load directly to the knee extensor mechanism.

Scientific Evidence

The literature consistently supports eccentric training for patellar tendinopathy, with growing evidence for flywheel application:

• Gual et al. (2016): reported reductions in pain and improvements in function in athletes with patellar tendinopathy after eight weeks of flywheel squat training.
• Romero-Rodríguez et al. (2011): showed that eccentric overload via inertial devices produced greater neuromuscular adaptations compared to traditional methods.
• Fanchini et al. (2014): found significant improvements in pain (VISA-P scores) and vertical jump performance following progressive eccentric squat programs.
• de Hoyo et al. (2015): demonstrated reduced incidence of tendon-related injuries in soccer players after preventive flywheel eccentric training.

Together, these findings highlight flywheel squats as a highly effective rehabilitation and prevention strategy.

 
Practical Applications of the Flywheel Squat in Tendon Rehabilitation

1. Early stage (initial readaptation)

• Goal: introduce low-intensity eccentric load, stimulate collagen synthesis.
• Execution: bilateral partial squats with low inertia, controlled eccentric phase.

2. Intermediate stage (specific strengthening)

• Goal: increase eccentric tolerance and tendon stiffness.
• Execution: bilateral deep squats with moderate inertia, emphasizing deceleration at end range.

3. Advanced stage (return-to-sport phase)

• Goal: transfer adaptations to explosive sports tasks.
• Execution: unilateral flywheel squats, assisted squat jumps with eccentric overload, integration with plyometrics.

Progression should be individualized according to pain levels, functional assessment, and tolerance to eccentric loading.

Clinical Benefits

1. Pain reduction in daily and sport-specific tasks.
2. Improved functional outcomes, measured with scales such as VISA-P.
3. Increased tendon stiffness and mechanical strength.
4. Enhanced eccentric quadriceps strength.
5. Performance transfer, with improvements in jumping and sprinting capacity.

Limitations and Precautions

Despite its benefits, flywheel eccentric squat training must be applied carefully:

• Risk of tendon irritation if eccentric overload is introduced too early.
• Requires technical supervision to ensure safe execution.
• Lack of standardized protocols (sets, reps, inertia levels).
• Limited long-term longitudinal studies compared to traditional decline squats.

Future Research Directions

1. Comparative trials between flywheel squats and traditional decline squats.
2. Studies defining optimal inertia progression and loading parameters.
3. Research on non-athletic populations with chronic knee pain.
4. Long-term follow-up studies on recurrence rates in competitive athletes.

Conclusions

The eccentric overload squat on a flywheel device is a safe and effective strategy for the rehabilitation of knee tendinopathies, particularly patellar tendinopathy. By providing self-regulated eccentric loading, stimulating collagen synthesis, and improving tendon stiffness, this method offers clear advantages over traditional eccentric protocols.

Its integration should be progressive, individualized, and supervised, forming part of a comprehensive rehabilitation plan that includes strength, motor control, and sport-specific reintegration.

References

1. Gual, G., et al. (2016). Effects of flywheel training on tendinopathy: a randomized controlled trial. Scandinavian Journal of Medicine & Science in Sports.
2. Romero-Rodríguez, D., et al. (2011). Eccentric overload training in sports and rehabilitation: a systematic review. Journal of Sports Science & Medicine.
3. Fanchini, M., et al. (2014). Eccentric squat training in athletes with patellar tendinopathy. Clinical Journal of Sport Medicine.
4. de Hoyo, M., et al. (2015). Effects of eccentric overload training on injury incidence in soccer players. International Journal of Sports Physiology and Performance.

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