The Achilles tendon is the strongest tendon in the human body. During running, it stores and releases elastic energy like a spring, absorbing 6-8 times your body weight with every stride. This remarkable capacity is what makes running energetically efficient.
It’s also what makes Achilles problems so frustrating. When this tendon gets injured, the high forces it handles mean that even “easy” running creates significant load. You can’t just “take it easy” — the tendon either handles running loads or it doesn’t.
Why It Happens
Tendons adapt to training loads, but they adapt more slowly than muscles — on the order of months, not weeks.[1] Your cardiovascular system adapts in days. Your muscles adapt in 2-4 weeks. Your tendons need 8-12 weeks of consistent loading to meaningfully remodel.
This mismatch is the core of Achilles problems. A runner increases their training — more mileage, more speed work, more hills — and their muscles handle it fine. Their heart and lungs handle it fine. Their Achilles hasn’t caught up. The tendon is receiving forces it hasn’t had time to adapt to, and the collagen structure begins to degrade faster than it can repair.
The Speed Connection
Speed work is the biggest Achilles risk factor in recreational runners. Not because speed is inherently dangerous, but because faster running increases the force on the Achilles with each stride. A tempo run at 7:30/mile puts significantly more load on the tendon than an easy run at 10:00/mile.
Adding speed work to a training plan that was previously all easy running creates a spike in Achilles loading — even if the total volume doesn’t change. This is why Pacewright’s base-building phase is important: easy running at moderate volume gives tendons time to adapt to impact forces before higher intensities are added.
Warning Signs
Morning stiffness. The Achilles feels stiff for the first 5-10 minutes after getting out of bed, then loosens up. This is the earliest sign of tendon irritation. It’s easy to dismiss — “it goes away, so it’s fine” — but the stiffness means the tendon is inflamed and not fully recovering between days.
Warmup pain. The first half mile of running hurts, then the pain fades. This is the endorphin response masking continued tendon stress. The tendon is being loaded beyond its current capacity, but your brain turns down the pain after warming up.
Swelling or thickening. If the tendon looks or feels thicker than the other side, it’s undergoing pathological change — the body is laying down disorganized collagen in response to chronic overload. This is past the “just rest” stage.
Management
Reduce intensity, not necessarily volume. Converting quality sessions to easy runs reduces Achilles load significantly while maintaining cardiovascular fitness. Complete rest often leads to deconditioning that makes the return worse.
Eccentric loading. Heel drops (standing on a step, lowering your heel below the step level slowly) are the gold standard for Achilles tendinopathy rehabilitation. 3 sets of 15 reps, twice daily, both straight-leg and bent-knee. This stimulates organized collagen remodeling.
Address calf weakness. The calf muscles (gastrocnemius and soleus) are the primary shock absorbers for the Achilles. Weak calves transfer more force to the tendon. Calf raise programs — starting with bilateral, progressing to single-leg — build the muscular capacity that protects the tendon.
Don’t stretch aggressively. Stretching a reactive tendon can worsen it. Gentle mobility is fine. Forced stretching or aggressive foam rolling directly on the tendon is counterproductive.
Timeline. Achilles issues that have been present for less than 2 weeks often resolve with 7-14 days of reduced training plus eccentric loading. Issues present for 4+ weeks typically require 8-12 weeks of modified training and rehabilitation. The longer you wait to address it, the longer the recovery.
Healthcare provider note: If anything in this article conflicts with guidance from your healthcare provider, follow your provider’s advice — they know your situation, we don’t.