Why Heavy Slow Loading Is Crucial to Tendon Repair

Heavy slow loading (HSL) is crucial for tendon repair because it drives specific physiological adaptations within tendon tissue—improving both the quality and quantity of tendon healing, and restoring mechanical strength, alignment, and pain resilience.

Why Heavy Slow Loading Is Essential

• Stimulates Collagen Production: Mechanical signals from heavy slow loading trigger molecular pathways that increase the production and realignment of collagen fibres—the structural backbone of healthy tendons. This is more pronounced compared to lighter loading or rest, which leads to a decline in collagen density and tendon health.

• Improves Tendon Stiffness and Strength: High-load exercises increase tendon stiffness, which reduces the strain placed on the tendon during movement. This is important because less strain at a given load helps prevent microdamage and further injury.

• Minimises Shock and Promotes Alignment: Slow, controlled movements during resistance exercise reduce rapid loading and 'shock' forces, allowing the collagen fibres to heal and align correctly. This leads to improved structural integrity of the tendon.

• Engages Mechanotransduction Pathways: Both mild and strong loading activate mechanotransduction—cell signalling in response to mechanical force. However, strong (heavy) loading may also induce beneficial microdamage and influence inflammation in a way that accelerates repair and tissue reorganisation, whereas mild loading mainly works through mechanotransduction alone.

• Pain Reduction: Heavy slow resistance has consistently shown to help reduce tendon pain in clinical studies, outperforming rest or light loading, especially during the early and middle stages of rehabilitation.

How Heavy Slow Loading Is Applied

• Begin with double-leg or bilateral exercises like squats or calf raises, using slow, controlled movement for sustained time under tension (~45 seconds per set).

• Progressively increase resistance, aiming to reach loads approaching 1.5x body weight for double-leg and ~70% body weight for single-leg exercises.

• Transition to single-leg variations as strength and pain tolerance improve.

• Higher loads (>70% maximum effort) and slower tempos are key to achieving sufficient tendon strain for adaptation.

Clinical Practice Note

While heavy slow loading is powerful for tendon repair, pain levels should be closely monitored (typically kept below 4/10), and the load must be progressed in a structured way under professional guidance to avoid re-injury or excessive microdamage.

Heavy slow loading is not just a strengthening tool—it's an essential stimulus for tendon healing and remodelling, outperforming rest and lighter activity by directly targeting the physiological mechanisms responsible for tendon repair.