What you call age-related stiffness is actually tight hip flexors from sitting. Shortened hip flexors tilt your pelvis, kill glute activation, and hurt your knees. Sprinting fixes it by forcing hips into full extension and retraining your nervous system for speed.
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Your doctor calls it age-related stiffness. It's really not. It's actually a muscle you've been ignoring for a long time, and sometimes you barely even train it, which is slowly shutting down your entire lower body. And to make it even worse, you add decades of sitting for prolonged hours, which actually shortens your hip flexors as well too, which tends to tilt your pelvis forward and actually shut off your glutes from firing and jams your lower back in constant tension, which is bad. All that pain from the hip flexors will also affect your knees, just dragging your lower back along with it too. Crazy part about all of this is that there's something almost nobody does over 40, Which is sprint. Sprinting is movement, and it literally forces your hips into full extension, the exact opposite position of actually sitting down. That single movement does 2 things stretching alone never will. It lengthens the hip flexors under load, and it trains your nervous system to move fast again. Yes, even with weights too as well. If hip flexors aren't just tight, they're literally slow as hell too. And unfortunately, slow hips are what turn a misstep into a torn muscle or a blown-out knee, God forbid. So sprint work is what keeps you agile, fluid, and not just flexible, ladies and gentlemen.