The tendons connecting your calf muscles to your foot and ankle bear some of the highest mechanical loads in the body, routinely handling forces several times your body weight with every step. The Achilles tendon alone stores and releases enough elastic energy during running to measurably reduce how much fuel your muscles burn, while the posterior tibial tendon quietly maintains the arch of your foot through decades of walking. When these structures fail, the consequences range from chronic pain and progressive foot deformity to sudden ruptures that can sideline an athlete for months. Understanding which tendons live below your knee, how they get injured, and what actually helps protect them turns out to be more nuanced than the standard advice of “stretch more and ice it.”
The Key Tendons Below the Knee
Your lower leg contains several tendons, but three matter most for everyday movement and injury risk. The Achilles tendon is the largest and strongest tendon in the human body, connecting the calf muscles (the gastrocnemius and soleus) to the heel bone. In a typical adult, the Achilles is roughly 6 to 8 millimeters thick and about 15 centimeters long, though dimensions vary with activity level and body size.1PubMed Central. Comparative analysis of patellar tendon, achilles tendon and plantar fascia structure in indoor and outdoor football players: a novel cross-sectional pilot study It is the primary driver of push-off when you walk, run, or jump.
The posterior tibial tendon runs behind the inner ankle bone and fans out along the underfoot, serving as the main structural support for the foot’s arch. Dysfunction of this tendon is the most common cause of adult-acquired flatfoot, a condition that often goes unrecognized until the arch has already collapsed.2PubMed Central. Posterior tibial tendon dysfunction: an overlooked cause of foot deformity The peroneal tendons travel along the outer ankle, stabilizing the foot against rolling outward. They are less commonly discussed but play a critical role in lateral ankle stability.
The patellar tendon, while technically spanning the knee rather than the ankle, connects the kneecap to the shinbone and is essential for jumping, landing, and deceleration. Athletes in sports that demand repeated vertical loading, like volleyball and basketball, stress this tendon heavily.
How Tendons Power Your Movement
Tendons are not just passive cables. They function as biological springs, absorbing mechanical energy when they stretch under load and snapping it back when the load releases. This spring-like behavior is especially important in the Achilles tendon during running. During the propulsion phase of each stride, the Achilles stores and then releases roughly 8 to 11 joules of elastic strain energy, a contribution that meaningfully reduces how hard your calf muscles have to work.3PubMed Central. Quantifying mechanical loading and elastic strain energy of the human Achilles tendon during walking and running Depending on the runner and the speed, total tendon energy storage and release can range from about 10 to 70 joules per stride.4PubMed Central. Achilles tendon strain energy in distance running: consider the muscle energy cost
This energy recycling is a big part of why running is metabolically affordable for humans. Your calf muscles can operate over smaller ranges of motion and at slower contraction speeds because the tendon handles so much of the mechanical work. The benefit scales with speed: the faster you run, the more energy the Achilles stores and returns, which is consistent with the idea that evolutionary pressure from our ancestors’ running habits may have shaped the tendon’s unusual length in humans.5PubMed. More than energy cost: multiple benefits of the long Achilles tendon in human walking and running
The Blood Supply Problem
One of the things that makes lower leg tendons vulnerable is their blood supply, or lack of it. The Achilles tendon has a notoriously sparse vascular zone in its midsection, roughly 2 to 6 centimeters above where it inserts into the heel bone. Research on the tendon’s blood supply has suggested that this poorly vascularized area may be unable to repair itself adequately after microtrauma, leading to progressive weakening.6PubMed. Blood supply of the Achilles tendon It is not a coincidence that most Achilles ruptures and chronic degenerative problems occur right in this watershed zone.
The posterior tibial tendon has a similar vulnerability. It bends sharply around the inner ankle bone, and the section under the most mechanical stress also receives limited blood flow. Both tendons are essentially fighting the same battle: they handle enormous loads in areas where the body’s repair systems have limited access.
Tendinopathy Is Not Really Inflammation
One of the most persistent misconceptions about tendon problems is that they involve inflammation. The “-itis” suffix in “tendinitis” implies an inflammatory process, but when researchers have examined tissue from chronically painful tendons under a microscope, they consistently find something different. The hallmark of true tendinopathy is tendinosis, a degenerative breakdown of the collagen matrix without the inflammatory cells you would expect if the problem were truly inflammatory. Instead, the tissue shows disordered collagen fibers, increased blood vessel growth into normally avascular regions, and an accumulation of ground substance between fibers.7PubMed Central. Tendinopathy: why the difference between tendinitis and tendinosis matters
This distinction matters because the treatment implications are different. If the problem were inflammation, anti-inflammatory drugs and rest would be the logical fix. But since tendinopathy is a failure of the collagen repair cycle, the more effective approach involves gradually reloading the tendon to stimulate healthy collagen production, something that rest alone does not accomplish. Persistent use of anti-inflammatory medications for a degenerative condition may even be counterproductive, since whatever small inflammatory response does occur in the early stages may be part of the healing signal.
How Achilles Ruptures Happen
Achilles tendon rupture is one of the most dramatic lower-limb injuries, and it tends to follow a specific mechanical pattern. Video analyses of ruptures in athletes have found that the typical mechanism involves the foot being forced upward (dorsiflexion) while the knee is near full extension and the calf muscles are contracting to resist that stretch.8British Journal of Sports Medicine. 041 A systematic video analysis of the mechanism of injury of achilles tendon ruptures This eccentric loading pattern, where the muscle is trying to shorten while being forcibly lengthened, generates the peak tendon forces that can exceed the tissue’s tolerance.9PubMed Central. The Effect of a Novel Achilles Brace on Concentric and Eccentric Achilles Tendon Loading During Tendon Tear Mechanisms
In practice, this often looks like a sudden push-off or change of direction in court sports such as basketball, tennis, or badminton. The classic patient profile is a recreational athlete in their 30s or 40s who has been relatively sedentary and then jumps into intense activity. But the tendon does not fail simply because of one bad moment. In most cases, the tissue has already undergone years of silent degenerative changes in that poorly vascularized midsection, and the acute rupture is the final event in a long chain of accumulated damage.
Posterior Tibial Tendon Dysfunction and Flatfoot
While the Achilles gets most of the attention, posterior tibial tendon dysfunction (PTTD) is arguably the more insidious lower leg tendon problem. It is the leading cause of adult-acquired flatfoot, and it progresses through stages if untreated.10PubMed Central. Posterior Tibial Tendon Dysfunction: An Overview In the early stage, you might notice pain and swelling along the inner ankle, with difficulty standing on your toes on one foot. As the tendon continues to degenerate, the arch gradually flattens, and the foot begins to roll inward. Eventually the deformity becomes rigid, and the surrounding joints develop arthritis.2PubMed Central. Posterior tibial tendon dysfunction: an overlooked cause of foot deformity
PTTD is often missed because it develops slowly and the early symptoms are easy to dismiss. Clinicians recommend using the single-leg heel raise as a screening test: if you cannot rise onto your toes while standing on one foot, or if doing so causes pain along the inner ankle, that is a warning sign worth investigating.11PubMed Central. Reported selection criteria for adult acquired flatfoot deformity and posterior tibial tendon dysfunction: Are they one and the same? A systematic review MRI can confirm the diagnosis when physical examination is ambiguous.12PubMed. Posterior tibial tendon dysfunction as a cause of acquired flatfoot in the adult: value of magnetic resonance imaging Catching it early, when the foot is still flexible, opens up conservative treatment options like orthotic support and targeted strengthening. Waiting until the deformity is fixed limits options to surgery.
Risk Factors That Have Nothing to Do With Exercise
Overuse gets most of the blame for tendon injuries, but some of the strongest risk factors are things people rarely associate with their tendons. Aging is the most fundamental. As tendons age, their stem cells become fewer and less functional, collagen bundles become fragmented, and the tissue accumulates materials that reduce its mechanical competence.13PubMed Central. Effect of Aging on Tendon Biology, Biomechanics and Implications for Treatment Approaches Research on equine tendons, which share structural similarities with human tendons, has found that partially degraded collagen accumulates in the matrix with age because the body cannot remove it efficiently, progressively weakening the tissue.14PubMed Central. Aspartic acid racemization and collagen degradation markers reveal an accumulation of damage in tendon collagen that is enhanced with aging
Certain medications pose a well-documented threat. Fluoroquinolone antibiotics (such as ciprofloxacin and levofloxacin) roughly triple the risk of Achilles tendon rupture, and that risk can persist for up to 60 days after stopping the drug. When fluoroquinolones are taken alongside oral corticosteroids, the risk climbs dramatically, with one study finding nearly a 20-fold increase in Achilles rupture risk for the combination.15PubMed Central. Relative and Absolute Risk of Tendon Rupture with Fluoroquinolone and Concomitant Fluoroquinolone/Corticosteroid Therapy: Population-Based Nested Case-Control Study If you are prescribed a fluoroquinolone, it is worth asking your doctor whether an alternative antibiotic is appropriate, especially if you are also taking steroids or are over 60.
Hormonal factors add another layer of complexity. Estrogen influences tendon structure in ways that are not straightforwardly beneficial. While estrogen increases collagen content in connective tissues, it also decreases tendon stiffness, which can reduce power output and may increase vulnerability to certain injuries.16PubMed Central. Effect of Estrogen on Musculoskeletal Performance and Injury Risk In postmenopausal women using estrogen replacement therapy, tendon collagen turnover rates are higher at rest, but the collagen response to exercise is actually blunted, suggesting a more complex hormonal interaction than simply “more estrogen equals stronger tendons.”17PubMed. Effect of estrogen on tendon collagen synthesis, tendon structural characteristics, and biomechanical properties in postmenopausal women
Landing Technique and Patellar Tendon Stress
For athletes in jumping sports, how you land may matter more than how much you jump. Research comparing volleyball players with and without patellar tendinopathy found that players who developed the condition used a stiffer landing strategy, absorbing less shock through their knees and ankles upon landing. Players who stayed healthy tended to use a softer, more flexed landing technique that distributed force more gradually.18PubMed Central. Relationship between landing strategy and patellar tendinopathy in volleyball The practical takeaway is that training athletes to land with more knee and hip flexion, rather than stiffly absorbing the impact, could help reduce patellar tendon loading over time.
Why Tendons Never Fully Recover
Tendon healing follows three phases: an initial inflammatory response, a proliferative phase where new tissue fills the gap, and a prolonged remodeling phase where the repair tissue matures. Despite this process, healed tendon tissue does not recover the mechanical properties of the original.19PubMed. Biology and physiology of tendon healing The fundamental problem is that tendons heal by forming fibrovascular scar tissue rather than regenerating their original organized collagen structure. The scar is weaker, less elastic, and differently organized than healthy tendon.20PubMed Central. Tendon Regeneration and Scar Formation: The Concept of Scarless Healing
Researchers are actively investigating ways to improve on this outcome. One promising direction involves reducing scar tissue formation during the healing process itself. Animal studies have shown that blocking the migration of certain scar-forming cells into the healing tendon results in better-organized collagen and a higher proportion of the stronger type I collagen fibers that characterize healthy tendon, rather than the weaker type III collagen that dominates scar tissue.21PubMed Central. Mitigating Scar Tissue Formation in Tendon Injuries: Targeting HMGB1, AMPK Activation, and Myofibroblast Migration All at Once These findings are still in early stages, but they suggest that future treatments could potentially shift tendon repair away from scarring and toward something closer to true regeneration.
Surgery Versus Conservative Treatment for Achilles Ruptures
The question of whether to operate on a ruptured Achilles tendon has generated decades of debate, and the answer has shifted over time. A large randomized trial published in the New England Journal of Medicine compared nonoperative treatment, open surgical repair, and minimally invasive surgery. The patient-reported outcomes were essentially the same across all three groups, with no meaningful differences in function or physical performance. The key difference was in re-rupture rates: about 6% of those treated without surgery re-ruptured, compared with under 1% in either surgical group.22PubMed. Nonoperative or Surgical Treatment of Acute Achilles’ Tendon Rupture
Meta-analyses have confirmed this general pattern. Surgery reduces the re-rupture rate but introduces a higher risk of wound complications such as infection and nerve damage.23PubMed Central. Comparing Surgical and Conservative Treatment on Achilles Tendon Rupture: A Comprehensive Meta-Analysis of RCTs Functional outcomes and range of motion tend to be similar regardless of approach.24PubMed. Surgical versus non-surgical treatment for acute Achilles’ tendon rupture. A systematic review of literature and meta-analysis The current consensus has therefore moved toward a more individualized decision. Younger, more active patients who need the lowest possible re-rupture risk may still benefit from surgery, while older or less active patients often do well with structured nonoperative rehabilitation that avoids the risks of surgery.
Rehabilitation and the Power of Eccentric Loading
Whether you are recovering from a rupture or managing chronic tendinopathy, controlled loading is the backbone of tendon rehabilitation. The specific approach that has the strongest evidence behind it is eccentric exercise, in which the muscle and tendon are loaded while lengthening rather than shortening. For the Achilles, this typically means slowly lowering your heel off the edge of a step; for the patellar tendon, it might involve slow decline squats.
A scoping review of eccentric training for tendinopathies in athletes found that it consistently improved pain and function, with high return-to-sport rates and minimal adverse events. Heavy slow resistance training and certain adjunct therapies showed comparable or additive effects.25PubMed Central. Eccentric Training for Tendinopathies in Athletes: A Scoping Review and Evidence Gap Map Modern rehabilitation protocols for surgical Achilles repair emphasize well-controlled early loading rather than prolonged immobilization.26PubMed Central. The role of mechanical loading in tendon development, maintenance, injury, and repair
This shift toward early movement after surgery has produced measurable benefits. A systematic review comparing early functional rehabilitation to traditional casting after Achilles repair found that early rehabilitation led to higher patient satisfaction and faster return to pre-injury sports levels, without increasing the risk of major complications.27PubMed Central. Early functional rehabilitation versus traditional immobilization for surgical Achilles tendon repair after acute rupture: a systematic review of overlapping meta-analyses The days of spending six weeks in a rigid cast after Achilles surgery are increasingly being replaced by protocols that introduce controlled weight-bearing and gentle motion much earlier.
Platelet-Rich Plasma and Other Emerging Treatments
Platelet-rich plasma (PRP) injections have become popular in sports medicine as a treatment for tendon injuries. The idea is to concentrate growth factors from your own blood and inject them into the damaged tendon to accelerate healing. The reality is that results have been highly inconsistent. A commentary reviewing the state of the evidence found that PRP’s effectiveness on injured tendons remains controversial, with many patient- and preparation-related factors influencing outcomes in ways that are not yet well understood.28PubMed Central. Can PRP effectively treat injured tendons? PRP is not a scam, but it is also not the reliable fix it is sometimes marketed as. If you are considering it, understand that the evidence does not clearly support it over structured rehabilitation alone for most tendon conditions.
Practical Ways to Protect Your Lower Leg Tendons
Prevention strategies for lower leg tendons are less glamorous than surgical advances, but they are where you get the most return on effort. Gradual load progression is the single most important principle. Tendons adapt to stress, but they do so more slowly than muscles. When you ramp up running mileage or jump training intensity faster than your tendons can remodel, you create a gap between what the tissue can handle and what you are asking it to do.
For runners dealing with Achilles tendon problems, heel lifts are a simple biomechanical intervention worth knowing about. An 18-millimeter heel lift reduced peak Achilles tendon strain from about 7.4% to 5.5% during running, and cut peak tendon force by roughly 300 newtons. The mechanism is straightforward: the heel lift reduces ankle dorsiflexion, which lengthens the tendon’s mechanical lever arm and reduces the strain placed on it.29PubMed. The effects of orthotic heel lifts on Achilles tendon force and strain during running This is not a permanent solution, but it can be a useful tool during rehabilitation or periods of increased training load.
Maintaining calf strength through regular resistance training, choosing footwear appropriate to your activity, and paying attention to early warning signs like morning stiffness in the Achilles or pain along the inner ankle all reduce the odds of a tendon problem becoming a serious injury. If you are over 40, taking a fluoroquinolone antibiotic, or returning to sport after a long layoff, your tendons deserve extra caution during the ramp-up period.
When the Uninjured Side Is Not Truly Healthy
One finding that underscores how gradually tendon problems develop: when researchers used ultrasound-based tissue characterization and shear-wave elastography to examine people with Achilles tendinopathy on only one side, the supposedly healthy opposite tendon also showed subclinical structural changes compared to tendons from young, healthy controls.30PubMed. Intra individual comparison of unilateral Achilles tendinopathy using B-mode, power Doppler, ultrasound tissue characterization and shear wave elastography In other words, by the time one Achilles tendon has become symptomatic, the other is often already on its way. This is consistent with the understanding that tendinopathy is a systemic problem driven by age, loading patterns, and individual biology rather than purely a local overuse injury. Treating only the painful side while ignoring the other may leave you vulnerable to a second episode down the road.