Recently, someone told me that I should not stretch my hamstrings, why?

Without seeing you it is hard to tell. Most people have lengthened hamstrings and stretching them makes them weaker. Here is a common scenario. Most people sit for many hours. This position can lead to tight hip flexors and weak glutes (butt). Over time, a forward tilted pelvis may develop leading to an arching of the low back. If this is the case, then the hamstrings become stretched from this position. This lengthened muscle will often do extra work for the weak glutes. As such, it gets fatigued and stressed. While it may feel good to stretch, it will actually make the problem worse. Stretching an already stretched muscle can seriously weaken it or lead to injury. In this case, stretching the hip flexors and strengthening the glutes will take the extra load off of the hamstrings.
Hamstrings are caught inbetween joint actions and have many demands.  Many times it may be more important to find the cause of the tension.  If hip flexors are tight and over stimulated (often due to repitition/over use), then glutes can be understimulated.  Glutes have many jobs, one of which is hip extension.  If the glutes are understimulated and not able to fire in a synergistic manner, we start to compensate.  Hamstrings are exellent hip extensors, so they step up and work overtime, as does the low back.  In this scenario, you are better off to start working with the hip flexors and work your way to the hamstrings.  

Also, if you are working from the bottom up, the hamstrings are also great knee flexors.  In this case working with foot mechanics and the gastroc/soleus complex will help the integrity of the hamstrings.  If the muscle itself is honestly in a shortened possition (with trigger points), soft tissue work will be able to reduce the stimulus and help to return normal tension.  

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.