Biomechanical Safety: How Time Under Tension Protects and Strengthens the Entire Tendon Matrix
It’s always fun to see our methods validated by scientific studies and other experts in the field. Dr Keith Baar has taken the internet by storm lately with his groundbreaking concepts on tendon safety and strengthening. We have been doing what Baar is teaching now since 1991, when PJ perfected his Time Under Tension training (TUT) methodology, which we still use at the X Gym today (this post was written in September of 2026).
14 years later, in 2005, Dr. Baar began producing groundbreaking research that confirms the effectiveness of our methodology. It is now becoming mainstream, thanks to his many podcast appearances. Listed in the references section below is one of his many great YouTube videos in case you want to do a “deep dive,” along with some of his and other studies validating our methodology.
When PJ was developing the X Gym methods, starting back in 1989, he did so through experimentation with his private training clients, based on scientific studies. He found that training one set to Complete Muscle Fatigue (CMF), using TUT to get there, necessitated light weights and, with controlled movements, made it exponentially safer than traditional training that uses heavy weights, mindless movements, and jerky, ballistic repetitions over multiple sets. Click here to see PJ’s partial bibliography showing some of the studies he used to design his experiments (which he is still doing today).
This post is for those who want to understand why the X Gym methods are safer and so much more effective for tendons and connective tissue than traditional training.
In traditional strength training, performance is almost universally equated with lifting heavy loads through dynamic sets and repetitions. From a mechanical and physiological standpoint, however, traditional weightlifting carries a high risk of connective tissue damage. Recent research into tendon physiology—such as the molecular and mechanical insights detailed by Dr. Keith Baar—highlights the specific vulnerabilities of connective tissue and illustrates why conventional training methods frequently lead to injuries ranging from microtears to full tendon ruptures.
Understanding these mechanisms of injury demonstrates why a high-intensity, controlled approach using continuous time under tension provides a far safer and superior stimulus for structural remodeling.
The Biomechanics of Tendon Injury in Traditional Training
Traditional strength protocols rely on heavy external resistance moved over multiple sets and reps. This introduces two major structural hazards to connective tissue: force spikes from ballistic or jerky movements, and localized stress concentration across isolated tendon fibers.
Tendons are composed of parallel bundles of collagen fibers embedded in a viscoelastic matrix. During heavy, fast, or jerky movements, force curves exhibit sharp, unpredictable spikes. Because momentum and acceleration dictate the movement, force is not distributed evenly across the cross-sectional area of the tendon. Instead, the majority of the tensile load is focused onto narrow strips or specific bundles of fibers.
Over repetitive traditional sets and reps, these isolated strips undergo rapid mechanical fatigue. When high peak force is applied to a fatigued tendon—especially at the turnaround point of a heavy repetition—the fatigued fibers fail to shield the adjacent tissue. The load suddenly shifts onto unconditioned or weaker collagen strands. Under this sharp, localized strain, the tissue ruptures, operating much like a sheet of paper tearing once a single point of concentration fails.
How 2-3 Minutes of Time Under Tension Drastically Increases Safety
To eliminate mechanical force spikes and prevent connective tissue degradation, X Gym utilizes 2 to 3 minutes of continuous time under tension (TUT) in a single set per exercise, performed with light weights and slow, mindful control.
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Light Weights are Required by the Stimulus: Maintaining continuous muscle and tendon engagement for 2 to 3 minutes makes heavy weights impossible to use. The light load removes the severe compression and shear forces that heavy weights impose on joints and articular cartilage.
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Elimination of Momentum: Moving slowly and mindfully removes acceleration from the equation. Without jerky movements or sudden directional changes, peak force spikes are eliminated. Force transfer remains predictable and within a safe physiological envelope.
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Zero Stress Shielding or Tearing Risk: Controlled movement prevents the rapid mechanical fatigue of isolated fibers, ensuring that the structural threshold of the tissue is never violently exceeded.
Strengthening the Whole Tendon vs. Isolated Strips
The primary biological advantage of long, continuous time under tension lies in how force is distributed throughout the extracellular matrix of the tendon.
Under fast, ballistic, or heavily loaded conditions, the fluid inside the viscoelastic tendon matrix does not have sufficient time to shift. The strain remains localized, pulling heavily on specific strips while leaving other parts of the matrix unengaged.
Conversely, sustained, unbroken tension allows fluid to express evenly throughout the tissue matrix. This uniform mechanical strain triggers mechanotransduction across the entire structural cross-section of the tendon:
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Uniform Fiber Recruitment: Unbroken loading for 2 to 3 minutes recruits every fiber bundle across the entire breadth of the tendon, ensuring equal work distribution.
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Matrix-Wide Collagen Remodeling: Because the entire tendon experiences uniform mechanical strain rather than localized shearing, cell signaling drives collagen synthesis across the whole structure rather than trying to repair damaged, isolated strips.
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Long-Term Structural Density: The result is a uniformly thicker, stronger, and more resilient tendon that resists injury in everyday movement.
By abandoning heavy loads, sets, reps, and ballistic motions in favor of a single, mindful, 2-to-3-minute set under continuous tension, it is possible to maximize functional strength and matrix remodeling while maintaining complete biomechanical safety. This is why people who do traditional training are frequently injured, and many X Gym members have enjoyed consistent exercise for years without any injuries.
References
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