If you've stretched, you've relaxed, therefore lengthened your muscles. Essentially, you've decreased your strength for a short while. I used to read a physical therapy blog where they discussed such things (no longer active or I'd post the link). So if you're trying to increase muscle strength with your weights, skip the pre-weight stretch of the muscles you're going to work with weights and stretch after. I wouldn't even bother doing the cool-down unless it feels really good, just jump into the weights. But I'm not a patient exerciser.
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