What causes the leg muscles to lock during running?

Answer 1

For anaerobic running (sprinting) use of glycogen as a substrate for energy will result in a high concentration of hydrogen ions and blood lactate. Although you may hear that blood lactate is responsible for "locking up" during these events, it is actually the [H+] that lowers the Ph of the blood and in turn causes your body to start protecting itself by blocking actin from attaching to myosin, thereby preventing muscle contraction.

Answer 2

well they "rip" and then you should let them be for a day to recover and when they do the rips heal and get bigger also, thus increasing the size and ability of the muscle