Tendons & Your Cycle: The Hidden Link

You stretched the same way you always do. You warmed up properly. And yet, something pulled. If you have ever rolled an ankle, strained a knee ligament, or felt a nagging ache in your Achilles that seems to come and go with no obvious cause, your menstrual cycle might be part of the story nobody told you.

The relationship between female sex hormones and connective tissue is one of the most under-discussed topics in women's fitness, and it has real consequences for how you train, recover, and protect your body across the month.

What are tendons and connective tissue, and why do hormones affect them?

Tendons are the tough, fibrous bands that connect muscle to bone. Ligaments connect bone to bone. Both are made primarily of collagen, a protein whose structure, stiffness, and repair rate are directly influenced by estrogen, progesterone, and relaxin throughout your menstrual cycle.

Connective tissue is not inert scaffolding. It is metabolically active, meaning it responds to hormonal signals just like your muscles, skin, or gut lining do. Estrogen receptors have been found on tenocytes (the cells that build and maintain tendons), on ligament fibroblasts, and in the synovial tissue lining your joints. When hormone levels shift, so does the behavior of these tissues.

This matters enormously for athletes, gym-goers, and anyone who moves their body regularly. Research published by the National Institutes of Health has documented that women sustain ACL injuries at two to eight times the rate of men, and hormonal fluctuations across the menstrual cycle are considered a significant contributing factor.

How does estrogen affect tendon stiffness and laxity?

Estrogen has a relaxing effect on collagen cross-links, the molecular bonds that give tendons and ligaments their stiffness. When estrogen rises, connective tissue becomes more pliable and extensible. This sounds like a good thing, but in load-bearing structures, too much laxity increases injury risk.

Studies have found that tendon stiffness decreases measurably in the follicular phase as estrogen climbs, and reaches its lowest point around ovulation when estrogen peaks. A landmark study from the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology demonstrated that the mechanical properties of the anterior cruciate ligament (ACL) changed significantly across the menstrual cycle in women, with the lowest stiffness coinciding with the pre-ovulatory estrogen surge.

This does not mean estrogen is harmful. Estrogen also supports collagen synthesis over the long term and has protective effects on tendon health. The issue is the acute fluctuation, not the hormone itself.

"Estrogen has a biphasic effect on connective tissue. At physiological levels it supports collagen production, but the rapid pre-ovulatory spike temporarily reduces mechanical stiffness in ligaments. Women need to be aware of this window, particularly in high-impact or pivoting sports."

Dr. Malachy McHugh, PhD, Director of Research, Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital

What happens to connective tissue during the luteal phase?

After ovulation, progesterone rises and partially counteracts the laxity induced by estrogen. Tendon and ligament stiffness tends to recover in the early to mid-luteal phase. However, as progesterone and estrogen both decline in the late luteal phase before menstruation, some women experience a secondary window of increased joint instability.

Progesterone also influences inflammation pathways. Because connective tissue repair depends on controlled inflammatory signaling, the progesterone-dominant luteal phase is actually a key window for tendon remodeling and adaptation. This means the luteal phase, often dismissed as a time to take it easy, is in fact a productive window for building connective tissue strength, provided loads are managed appropriately.

Late luteal phase is also when prostaglandins rise in preparation for menstruation. These inflammatory compounds can increase generalized tissue sensitivity, which may explain why some women notice more joint aches or a greater sense of tightness in the days before their period arrives.

What is relaxin and how does it affect your joints?

Relaxin is a peptide hormone produced by the corpus luteum after ovulation. It is best known for its role in pregnancy, where it loosens pelvic ligaments to prepare for birth. But even outside of pregnancy, relaxin fluctuates across the menstrual cycle and can contribute to temporary joint hypermobility.

Relaxin receptors are found throughout the musculoskeletal system, including the knee, ankle, and wrist. Research from the American Journal of Sports Medicine found that women with naturally higher relaxin levels had significantly greater ACL laxity than those with lower levels, independent of estrogen, suggesting relaxin plays its own distinct role in cycle-related joint instability.

For most women, this effect is subtle and non-problematic in daily life. For competitive athletes or women doing heavy lifting, explosive training, or contact sport, understanding the timing of relaxin peaks may meaningfully change how they program their most demanding sessions.

Which phase of your cycle carries the highest injury risk?

The pre-ovulatory window, roughly days 10 to 14 of a 28-day cycle, is consistently identified in research as the period of highest connective tissue laxity and therefore greatest injury risk. Estrogen is at or near its peak, relaxin has risen, and tendon stiffness is at its monthly low.

This does not mean you should stop moving. It means this is the phase to:

• Prioritize warm-up quality over length. Spend extra time activating stabilizing muscles before high-impact or lateral movement.
• Reduce maximal effort attempts in heavy lifts, especially those that load the knee or ankle.
• Avoid trying personal records in explosive movements like Olympic lifting, box jumps, or sprinting if you are not fully recovered.
• Focus on technique and body mechanics rather than pushing output.

"The research is clear enough that sports medicine professionals should be incorporating menstrual cycle phase into injury prevention screenings for female athletes. It is not about limiting women. It is about giving them information that men have never needed to account for."

Dr. Stephanie Kliethermes, PhD, ATC, Director of Research, American Medical Society for Sports Medicine

How should you adapt your training to protect your connective tissue?

The key is to match training stimulus to connective tissue readiness across the four phases. During the menstrual and follicular phases, when estrogen is low and tendons are stiffer, you can handle higher mechanical loads more safely. As estrogen rises toward ovulation, shift focus to skill work, stability, and moderate intensity. The luteal phase supports collagen remodeling and is ideal for progressive tendon loading.

Menstrual phase (days 1 to 5, approximately)

Hormones are at their lowest. Tendon stiffness is relatively high. Energy may be lower, but connective tissue is mechanically more predictable. Gentle movement and restorative mobility work support blood flow without unnecessary joint stress. This is not the time to push through pain, but the tissue itself is not particularly vulnerable.

Follicular phase (days 6 to 9, approximately)

Estrogen is rising but has not yet peaked. This is a sweet spot for building strength and power. Tendons are responding well to progressive loading and this phase is ideal for adding volume or intensity to resistance training. Collagen synthesis is upregulated, meaning the stimulus you apply now has a strong repair and adaptation window ahead of it.

Ovulatory phase (days 10 to 14, approximately)

Highest injury risk window. Prioritize neuromuscular control, warm-up quality, and movement accuracy over raw output. If you are a runner, this is the week to dial back mileage slightly or ensure surfaces are forgiving. If you lift, keep technique impeccable and avoid maximum loads in knee-dominant or ballistic exercises.

Luteal phase (days 15 to 28, approximately)

Progesterone supports the collagen remodeling response to training. Early to mid-luteal phase is a strong window for tendon-focused training such as isometric holds, eccentric loading, and loaded carries. Late luteal phase may bring increased tissue sensitivity for some women, so listening to your body and adjusting volume is wise in the final days before menstruation.

Can you strengthen your tendons through cycle-aware training?

Yes. Tendons adapt to mechanical loading through a process called mechanotransduction, where physical stress signals tenocytes to produce more collagen and reorganize existing fibers. Aligning your highest tendon-loading sessions with the follicular and early luteal phases, when estrogen supports collagen synthesis and progesterone supports remodeling, can meaningfully improve tendon resilience over time.

Eccentric loading protocols are well-established for tendon health. Exercises like slow eccentric heel drops for the Achilles, Nordic curls for the hamstring tendons, and slow-tempo squats for the patellar tendon all stimulate tendon adaptation. Scheduling these protocols in the follicular and mid-luteal phases, and reducing intensity in the periovulatory window, is a practical application of cycle-aware training that does not require overhauling your entire program.

Collagen synthesis also requires vitamin C as a cofactor. Some practitioners recommend timing vitamin C-rich foods or supplementation around connective tissue training sessions, particularly in the follicular phase when estrogen is already supporting collagen production.

What other factors affect connective tissue health across your cycle?

Hormones set the backdrop, but several other variables modulate how your connective tissue behaves:

• Hydration: Tendons and cartilage are highly sensitive to hydration status. Dehydrated connective tissue is less resilient and more prone to microdamage. Your fluid needs increase after ovulation due to progesterone's effect on thermoregulation, so staying on top of hydration in the luteal phase matters more than most women realize.
• Sleep: Most collagen synthesis and connective tissue repair happens during deep sleep. Disrupted sleep in the late luteal phase, a common hormonal side effect, can compromise recovery from training loads placed on tendons and ligaments.
• Protein intake: Collagen is a protein. Adequate dietary protein, particularly glycine-rich sources like bone broth, skin-on poultry, or gelatin, provides the substrate for tendon repair. Protein needs are slightly higher in the luteal phase.
• Chronic stress: Elevated cortisol impairs collagen synthesis and can slow tendon healing. Managing stress load across your cycle is not just a mental health concern, it is a physical tissue health concern too.