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cofactor · 2
The body adapts only to a stimulus beyond what it is used to, and it builds what you train, so to keep getting stronger you have to raise the demand step by step; the American College of Sports Medicine position stand on strength training states that continued adaptation requires progressive training, with frequency rising from 2–3 days a week for beginners to 4–5 for advanced lifters. What health needs does not rise with it: the WHO guidelines found no evidence that more strength training brings more health benefit.
When you raise the demand to improve, you also need lighter stretches in cycles so recovery can keep up. A short hard block after which you come back stronger with rest is normal functional overreaching; when load stays above recovery for long, it slides into non-functional overreaching, where recovery drags on and you stop getting stronger, and at worst into overtraining syndrome, where performance keeps falling and no amount of rest restores it.
regulates · 3
When you raise the demand to improve, muscle and the heart and lungs catch up with a new training load within weeks, while tendon and bone are much slower: the core of the adult Achilles tendon is barely renewed for decades, and after a session collagen breakdown in a tendon peaks earlier than its synthesis. So the danger is not training a lot but suddenly training a lot more; an International Olympic Committee consensus lists rapid changes in training and competition load as a major risk factor for injury.
The tendons around a joint become stiffer and stronger under load; in a meta-analysis this adaptation depended mainly on how heavy the load was, and it takes months. Years of moderate loading do not use a joint up: recreational runners actually have lower rates of hip and knee osteoarthritis than sedentary people; trouble usually comes when load rises faster than the tissues adapt.
Long endurance training and heavy strength training send muscle down two different molecular paths, one building more mitochondria and the other more contractile protein. When you try to push performance up in both at once, a large amount of endurance work slows strength and muscle gains somewhat, more with running than with cycling; the drag matters mainly to people chasing their limits.
contrast · 2
Training a lot with legs that are always sore is often taken for overtraining, but the two are different signals. Delayed-onset muscle soreness comes a day or two after a new movement or a sudden jump in load, fades within a few days and gets milder each time the same exercise is repeated; overtraining shows as performance falling for weeks that no amount of rest restores, often with worse sleep, low mood and frequent illness.
The two stories cover the two ends of the same dose curve. At the starting-from-zero end, the WHO 2020 guidelines say some activity is better than none and bouts of any length count toward the total; at the upper end sits the weekly range WHO recommends for adults, where each extra amount adds less, though no one can say exactly where that tapering begins.