Hamstring, calf, or quad issues occur at times when your fitness level is moving forward.
The rate of tissue tightening as your fitness level progresses in any given week may exceed the rate [frequency of stretching] and/or effectiveness of your stretching protocol. Such issues may be independent of the strength or lack there-of…of those tissues. Strength training is the most effective rehab approach but not the primary mechanism of prevention. Stay anabolic and do formal stretch sessions somewhat frequently to keep up with the rate of tissue tightening as your fitness level moves forward. Do these things and you remove the major limitations and road blocks that are embedded in most traditional approach training programs in our sport.

TheETG range of motion —– http://theetgtrackclub.com/documents/TheETGromsMasterOfSport.pdf