Tendons rupture on the way down, and high blood sugar triples the risk

The moment a tendon tears is rarely the moment of maximum output. It is the landing, not the jump; the deceleration, not the sprint. Tendons take their heaviest load in the force absorption phase — and most training programmes never train that phase at all.

The scale is not small. About 60% of Achilles ruptures are sport-related, male incidence runs more than three times female and is climbing year over year, and the peak injury window is 30 to 49. The average patient is close to 38 — a recreational athlete, not a professional.

Why are tendons so slow?

Muscle recovers and adapts within 48 hours. Tendon takes weeks to do the same work. An adult has roughly 400 tendons — more than the 206 bones they connect to — and they are not passive rope but living metabolic tissue. The problem is that damage accumulates quietly, over weeks and months, before the athlete or the medical staff notices anything.

What is missing from your week?

Three families of work need to be present. First, heavy eccentrics — a slow single-leg calf lower, loading the tendon at full stretch. Second, low-level force absorption — jump rope, single-leg hops, bounding: repetitive, rhythmic landing. Third, explosive depth work — depth jumps and drop-catch push-ups, trained for quality over volume at no more than three sets of six.

The one that goes missing is usually the second. Programmes train maximal load and they train explosiveness, but almost nobody schedules repeated absorption of ordinary force.

How much does diet matter?

As one molecular exercise physiologist puts it, 95% of tendon quality comes down to the training and about 5% to nutrition. But when that 5% works against you, it works in one direction.

Sustained high blood glucose leaves excess sugar to bind to tendon collagen as advanced glycation end products (AGEs), making the fibres stiffer, more brittle, and more likely to snap under load. A 2021 study put the tendon injury risk of athletes with elevated blood sugar at three times that of athletes with normal glucose. Saturated fat points the same way: high-fat-fed animals showed reduced tendon function and reduced tendon collagen, and elevated blood cholesterol has been reported to impair tendon healing after surgery.

Which numbers should you check?

• A1C below 5.7% — once you cross into pre-diabetic range, the AGE process accelerates
• Vitamin D between 40 and 50 ng/mL — food delivers roughly 400 to 800 IU a day, well short of the target, so 1,000 to 2,000 IU of D3 is the practical route
• Ferritin 40 to 90 ng/mL for athletes, never below 30 — iron is a required cofactor for collagen synthesis alongside vitamin C
• Zinc, manganese and copper — found in oysters, pumpkin seeds and whole grains, and almost nobody checks them

Collagen powder does not let you skip that order. Taking it without vitamin C, vitamin D, iron and the trace minerals means the raw material arrives with no way to be used. The body cannot build what it cannot process.

How this relates to your strength score

A Big 3 log tracks muscle, not tendon. Strength climbs on a scale of weeks; tendon cannot keep that pace, and the gap between the two is where injuries live. The faster your score is rising, the more the landing and deceleration work needs to be in the same week. That gap is covered further in testosterone and tendon.

And your log will warn you before pain does. The first sign of overtraining shows up as a drop in performance, not soreness. An unexplained fall in output — the weight going backwards, the same set feeling unusually heavy — is the moment to adjust, not the moment to wait for a snap.