To answer this we have to look at strength tests carried out on some of the leading athletes over the last 50 years. His event is power oriented. Runners don't require that type of strength. Well, it alters as the running distance gets farther. We know that a world-class sprinter has to be strong.
Speed is defined as the rate of stride times the length of stride.
The latter relies on flexibility and the strength of the final push-off from the toes. Some runners have a maximum stride-length at full speed of only 5 feet, while others have a span of 8 feet. The former has weak leg strength, the latter, strong. We have answered one question already. If you want to improve your basic speed you need powerful legs.