Why Your Lower Back Hurts After Spring Running—And It’s Not Your Hamstrings

That first run of spring feels wonderful, right up until the moment it doesn’t. You’ve dusted off your trainers, headed out into the pale April sunshine, and woken up the next morning with a nagging ache deep in your lower back. So you do what everyone does: you sit on the floor, pull your knee to your chest, and stretch your hamstrings. Except here’s the uncomfortable truth, your hamstrings probably aren’t the problem. The Muscle You’ve Been neglecting, the one that’s almost certainly behind that post-run back pain, is your hip flexor.

The hip flexor problem nobody talks about

The hip flexors are a group of muscles that run from the front of your hip down to your thigh, connecting your lower spine to your legs. The most significant of these is the iliopsoas, a deep muscle that attaches directly to the lumbar vertebrae. Spend most of your week sitting at a desk, as the majority of us do, and this muscle shortens and tightens over months and years. Then you ask it to suddenly extend through a full running stride, and it pulls. Not dramatically, not with a pop or a snap, but with a slow, grinding protest that you feel the next morning in your lower back.

The reason this gets misdiagnosed so often, even by people who consider themselves reasonably body-aware, is that the pain doesn’t announce itself at the front of the hip where the muscle lives. It radiates backward. A tight iliopsoas tugs on the lumbar spine, compressing the vertebrae and straining the surrounding muscles and ligaments. The back aches, so we stretch the back. The back still aches, so we stretch the hamstrings. Meanwhile, the actual culprit sits completely untouched, quietly tightening further with every hour spent in a chair.

Why spring specifically makes this worse

Winter is, for most recreational runners in the UK, a period of reduced activity. Shorter days, colder mornings, the general psychological weight of January and February all conspire to keep us more sedentary than usual. By the time March and April arrive, the hip flexors have had months to adapt to a shortened, seated position. They’ve essentially forgotten what it feels like to fully extend.

A spring jog after a winter of relative inactivity is therefore a fairly significant shock to a muscle that’s spent 12 to 16 weeks in a contracted state. The stride length of running demands a range of motion the hip flexors haven’t experienced since last autumn. The result is predictable, even if it always feels surprising in the moment. This isn’t a weakness or a failure, it’s simply a physiological response to a mismatch between the demands placed on a muscle and the mobility that muscle currently has.

There’s also a subtler issue at play. Most people who do stretch before a run default to static stretches: touching your toes, holding a quad stretch against a wall, maybe a seated twist. These have their place, but they’re poorly suited to preparing a cold, tight hip flexor for dynamic activity. The muscle needs movement to warm up, not prolonged static holding, which can actually reduce force output in the short term according to research published by sports science institutions over the past two decades.

The stretches that actually help

A proper hip flexor stretch requires the hip to move into extension, which means getting the leg behind the body. The kneeling lunge stretch is the most accessible version: kneel on your right knee, left foot forward, and gently drive your hips forward until you feel a pull at the front of the right hip, not a pain, just a sustained tension. Hold for 30 to 45 seconds, breathe through it, and then switch sides. Do this after your run, when the muscle is warm, rather than before.

Before you run, the better approach is dynamic movement. Walking lunges, leg swings (holding a wall or lamppost for balance and swinging each leg forward and back through its natural arc), and slow hip circles all help wake up the hip flexors and take them through a range of motion progressively. These movements increase blood flow, signal to the nervous system that extension is coming, and reduce the abrupt demand that causes that post-run ache. Three to five minutes of this before you set off is worth more than ten minutes of static stretching on a cold pavement.

Strengthening matters too, not just stretching. Weak glutes and core muscles force the hip flexors to compensate during running, taking on work they’re not designed to handle alone. Single-leg glute bridges, dead bugs, and even brisk walking with an exaggerated heel push-off can build the posterior chain strength that takes pressure off the front of the hip. This won’t solve things overnight, but after three or four weeks of consistency, most recreational runners notice a meaningful difference.

When to stop and speak to your GP

Lower back pain after running is usually muscular and resolves within a few days with rest, gentle movement, and the kind of targeted stretching described above. A bag of frozen peas wrapped in a tea towel applied to the area for 15 to 20 minutes in the first 24 hours can help manage inflammation. Paracetamol or ibuprofen, taken according to the packet instructions, can ease Discomfort in the short term.

That said, if the pain is severe, shoots down one or both legs, is accompanied by numbness or tingling, or doesn’t improve after a week of rest, please see your GP. Lower back pain can occasionally indicate something beyond muscle tightness, and those symptoms deserve proper medical assessment rather than a Google-guided diagnosis. (This article, likewise, is information rather than medical advice.)

The hopeful thought here is this: most people who discover their hip flexors are the source of the problem find that relatively small changes make a real difference, fairly quickly. Your spring running season doesn’t have to start with a week on the sofa. It just has to start with a slightly different warm-up routine and a lunge stretch you’ve probably never prioritised before.