Your Tendons Keep Getting Injured. Your Genes May Be Why.

Collagen is the structural protein in your tendons, ligaments, and fascia. It’s the rope that holds everything together. COL5A1 codes for collagen type V, which is a critical regulator of collagen fiber diameter and arrangement. Think of it as the master architect that decides whether your collagen fibers are thick and tightly organized or thin and loose.

Here’s the problem: the rs12722 variant in COL5A1, carried by roughly 30-35% of people, alters how type V collagen is assembled into the larger collagen scaffolding. People with the T allele have thinner, less densely packed collagen fibers, which means their tendons are literally weaker at the molecular level. Your tendons have less structural integrity before training even begins. Every rep, every sprint, every jump damages fibers that are already compromised.

This shows up in real life as tendon problems that seem disproportionate to your training load. You might get patellar tendinopathy from training that your friends handle fine. Or your Achilles stays irritated for months after a minor increase in running volume. You’re not imagining it; your tendons genuinely are more fragile.

Collagen type I is the dominant structural protein in your tendons and ligaments. It makes up roughly 85-90% of tendon dry weight. While COL5A1 sets up the structural blueprint, COL1A1 provides the sheer load-bearing capacity. It’s the difference between a rope made of strong fibers versus weak fibers.

COL1A1 has several polymorphisms that affect collagen expression levels and fiber quality. Variants associated with lower collagen type I expression, present in roughly 25-30% of the population, reduce the total amount of type I collagen your tendons can produce. Your body literally has less raw material to work with when repairing training-induced microtrauma. The repair process runs slower and builds back less robustly. This is especially problematic in your 30s and 40s when collagen synthesis naturally declines anyway.

You notice this as a pattern of persistent tendon complaints. You’ll get better for a few weeks, then one slightly harder training session re-injures the same tendon. The healing never quite gets you back to fully robust. It’s because you’re building back thinner, less densely cross-linked collagen each time.

Vitamin D doesn’t work just because you have it in your blood. Your cells need a receptor to actually use it. VDR, the vitamin D receptor, sits on the surface of your muscle and tendon cells. It’s the lock that vitamin D fits into. Without it, vitamin D is just floating around doing nothing. The BsmI and FokI polymorphisms in VDR determine how efficiently your cells can capture and respond to vitamin D signaling.

People with certain VDR variants, affecting roughly 30-50% of the population depending on ancestry, have less efficient vitamin D signaling at the cellular level. Even if your serum vitamin D levels look good, your muscle and tendon cells cannot synthesize protein effectively or regulate calcium signaling properly without robust VDR function. This directly impairs training adaptation and recovery. Your tendons stay weaker because the cells simply cannot access the repair signals they need.

This manifests as slow recovery that baffles you because your vitamin D levels are normal. You supplement vitamin D, your doctor checks your blood, everything looks fine on paper. But your tendons still feel fragile and your muscles don’t grow as fast as they should. The problem is not the vitamin D, it’s that your cells can’t use it.

Every time you exercise, you create free radicals. This is normal and actually triggers beneficial adaptation. But your cells need to neutralize these radicals quickly, or they spiral into excessive oxidative stress. SOD2, superoxide dismutase 2, is your mitochondrial antioxidant. It sits inside the powerhouse of your cells and converts the reactive oxygen species generated during exercise into harmless water and oxygen.

The Val16Ala variant in SOD2, present in roughly 40% of people as the homozygous form, reduces mitochondrial antioxidant efficiency. People with the Ala16 variant clear oxidative stress roughly 30-40% slower during and after exercise. The free radicals linger longer, damaging mitochondrial proteins and collagen fibers. This accelerates the accumulation of microtrauma in your tendons and increases systemic inflammation in response to training.

You experience this as disproportionate muscle soreness (DOMS lasting 4-5 days instead of 2-3), tendons that feel achy even on rest days, and slower recovery from hard training sessions. You might feel like you’re chronically under-recovered, despite sleeping well and having adequate rest days. The issue is that your cells are taking longer to neutralize the oxidative damage from training.

Inflammation after training is not a bug; it’s a feature. Controlled inflammation triggers the repair cascade that makes you stronger and more resilient. The problem is controlling it. IL6, interleukin-6, is a pro-inflammatory cytokine. It’s supposed to rise after training, orchestrate the initial immune response, and then come back down. But if your genetics make you a high IL6 responder, the inflammation stays elevated longer.

Certain IL6 promoter variants, present in roughly 25-35% of the population, increase baseline IL6 expression and amplify the IL6 surge in response to training stress. Your immune system overreacts to training stimulus, causing inflammation to persist rather than resolve efficiently. This delays muscle protein synthesis, extends tendon soreness, and can tip minor training stress into actual injury. You’re fighting persistent inflammation instead of using it as a repair signal.

This shows up as tendons that feel perpetually irritated, recovery taking dramatically longer than it should, and tendon pain that spikes unpredictably even with reasonable training loads. You might notice that one hard workout sets your tendon issues back by a week. It’s because your inflammation response cannot turn off efficiently.

TNF (tumor necrosis factor alpha) is the master pro-inflammatory cytokine. It’s the alarm that gets pulled when your body detects threat or stress. A little TNF is necessary; it initiates the cascade that makes you stronger. But if your genes predispose you to high TNF production, the alarm gets pulled too hard and stays active too long. Certain TNF promoter variants (like -308G>A, rs1800629) are present in roughly 10-20% of people and increase baseline TNF levels and amplify TNF response to stress.

High-TNF-responder genetics mean your body perceives training stress as more threatening than it actually is, triggering a disproportionately large inflammatory cascade. This is particularly problematic for tendon health because tendons are less vascularized than muscle and depend on controlled inflammation for repair signaling. Excessive TNF keeps the tendon in a pro-inflammatory state, preventing the transition from initial immune response to actual tissue rebuilding. You’re stuck in the damage phase rather than progressing to repair.

You notice this as tendon issues that feel worse after training, taking days to settle down, and a sense that your body is treating training like a threat instead of an adaptation stimulus. Tendon pain might increase even when you reduce training load, because the inflammatory tone itself is the problem.