Abstract

Hamstring muscle injuries represent one of the most prevalent problems in professional soccer and other high-demand sports. Recurrence rates are high, and return-to-play times are often prolonged, which highlights the need to optimize rehabilitation protocols. Eccentric training with flywheel devices has emerged as a promising tool to enhance muscle healing, support tissue regeneration, and reduce the risk of re-injury. This article analyzes the scientific evidence available on the effects of eccentric training with flywheel devices on the healing process of hamstring muscles, highlighting physiological mechanisms, clinical benefits, and practical applications.

Introduction

Hamstring injuries account for 12% to 37% of all injuries in professional soccer. The long head of the biceps femoris is the most commonly affected, particularly during sprinting and high-intensity decelerations. These injuries present a high recurrence rate, which can reach up to 30% within the first two months after return-to-play. Traditional rehabilitation protocols include concentric strengthening, flexibility work, general conditioning, and progressive return to activity.

Over the past two decades, however, rehabilitation approaches have increasingly incorporated eccentric training, with solid evidence showing that it improves muscle and connective tissue resilience and induces protective structural adaptations. Within this framework, flywheel inertial devices provide a unique eccentric stimulus characterized by continuous, adaptive loading proportional to the applied force, which enhances neuromuscular activation and tissue repair processes.

Physiological Mechanisms of Muscle Injury and Healing

Initial damage

A hamstring injury is characterized by rupture of muscle fibers, disruption of sarcomeres, and damage to the surrounding connective tissue. Hemorrhage, inflammation, and necrosis occur in the affected area, followed by the activation of satellite cells.

Phases of healing

1. Inflammatory phase (0–72h): infiltration of inflammatory cells and removal of necrotic tissue.
2. Regeneration phase (3–10 days): activation of satellite cells, synthesis of myofibrils, extracellular matrix production.
3. Remodeling phase (>10 days): fiber alignment, collagen maturation, functional integration.

Throughout this process, controlled mechanical loading is essential to orient fiber regeneration and prevent excessive scar tissue formation, which would compromise muscle elasticity and strength.

The Role of Eccentric Training

Eccentric exercise has demonstrated superior effects in both prevention and rehabilitation of muscle injuries through several mechanisms:

• Increased fascicle length of the hamstrings, particularly the long head of the biceps femoris, reducing strain at elongated muscle positions.
• Greater stiffness and resilience of tendon and extracellular matrix, decreasing the likelihood of re-injury.
• Stimulation of type I collagen synthesis, crucial for connective tissue repair.
• Activation of satellite cells and growth factors, enhancing muscle regeneration.
• Improved neuromuscular capacity to absorb high forces during deceleration phases.

   

Flywheel Devices in Rehabilitation

Flywheel technology, initially developed for aerospace applications, allows for variable, self-regulated resistance: the kinetic energy stored in the flywheel is transformed into eccentric load during the return phase of movement.

Key features:

• Maximal eccentric overload at the end of the range of motion.
• No fixed load limit—the resistance depends on the athlete’s applied force.
• Increased recruitment of fast-twitch fibers and core activation.
• Multiplanar, functional, and sport-specific stimulus.

These characteristics allow practitioners to replicate the demands of high-speed running and deceleration—the typical mechanisms of hamstring injury—in a progressive and controlled manner.

Scientific Evidence 

Several studies have evaluated the impact of inertial flywheel training in hamstring rehabilitation:

• Askling et al. (2013): demonstrated that eccentric rehabilitation protocols accelerated return-to-play and reduced recurrence compared to conventional methods.
• Tous-Fajardo et al. (2016): reported that flywheel training increases eccentric activation and neural adaptation, enhancing muscle healing.
• Morales-Artacho et al. (2017): found increases in biceps femoris fascicle length after six weeks of flywheel eccentric training.
• Timmins et al. (2016): identified short fascicle length in the biceps femoris as a strong predictor of injury risk, and showed that eccentric training could reverse this condition.

Collectively, the evidence suggests that flywheel eccentric training promotes muscle regeneration and improves hamstring architecture during the healing process.

Practical Applications in Hamstring Healing

1. Early stage (regenerative phase)

• Objective: promote activation without excessive stress.
• Exercises: bilateral flywheel hamstring curls with low inertia, partial range of motion, controlled speed.

2. Intermediate stage (early remodeling)

• Objective: increase fascicle length and tolerance to eccentric load.
• Exercises: unilateral hip extensions, single-leg flywheel hamstring curls, with emphasis on controlled deceleration.

3. Advanced stage (return-to-sport)

• Objective: transfer adaptations to sport-specific tasks.
• Exercises: flywheel split squats, multiplanar eccentric actions, combined with submaximal sprints and decelerations on the field.

Progression should be individualized based on pain, tissue healing, and tolerance to eccentric loading.

Clinical Benefits

1. Reduced return-to-play time: up to 25% faster in some studies.
2. Lower recurrence rates: attributed to improved structural adaptations.
3. Better scar tissue quality: more aligned fibers and higher type I collagen ratio.
4. Increased eccentric strength capacity: critical for sprinting and high-speed running.
5. Transfer to sports performance: improvements in power, change of direction, and sprinting.

Limitations and Precautions

While promising, flywheel eccentric training requires careful consideration:

• Risk of overload if introduced too early or with excessive inertia.
• Need for technical supervision to ensure proper execution.
• Lack of standardized rehabilitation protocols (sets, repetitions, progression).
• Limited large-scale clinical trials compared to Nordic hamstring protocols.

Future Research Directions

1. Randomized controlled trials comparing flywheel vs. Nordic hamstring or conventional rehab.
2. Molecular markers of healing and collagen synthesis in response to flywheel eccentric training.
3. Optimized load progression based on injury severity and individual response.
4. Long-term effects on re-injury rates across full competitive seasons.

Conclusions

Eccentric flywheel training appears to be a highly effective strategy to enhance hamstring muscle healing after injury. Its ability to induce eccentric overload, increase fascicle length, stimulate tissue regeneration, and improve neuromuscular adaptations positions it as a valuable tool in modern rehabilitation.

When implemented progressively, individually, and under professional supervision, flywheel eccentric training not only optimizes the healing process but also reduces recurrence risk and supports athletic performance upon return-to-play.

References

1. Askling, C. M., Tengvar, M., Saartok, T., & Thorstensson, A. (2013). Acute hamstring injuries in Swedish elite football: a prospective randomized controlled clinical trial comparing two rehabilitation protocols. British Journal of Sports Medicine, 47(15), 953–959.
2. Tous-Fajardo, J., et al. (2016). The flywheel leg-curl machine: offering eccentric overload for hamstring development. International Journal of Sports Physiology and Performance, 11(7), 891–895.
3. Morales-Artacho, A. J., et al. (2017). Effects of eccentric overload training using a flywheel device on muscle architecture, strength, and functional performance in soccer players. Journal of Strength and Conditioning Research, 31(5), 1254–1262.
4. Timmins, R. G., et al. (2016). Short biceps femoris fascicles and eccentric knee flexor weakness increase the risk of hamstring injury in elite football (soccer): a prospective cohort study. British Journal of Sports Medicine, 50(24), 1524–1535.