Researchers have found a correlation between injuries and frequent race efforts. This connection also extends to speedwork, since intervals also require a near-maximal effort. So if you train fast once or twice a week and then race on the weekend, that’s a lot of hard efforts without sufficient rest, particularly if you follow this pattern week after week. Regular speed training is fine for those aiming for podium placements or age-group awards, but it is not recommended for the average runner.
(Source: active.com)
