<s>[INST] I was doing calf raises in the gym and heard a pop along with pain behind my right knee. Immediately I had to stop. I can straighten my knee out and hold body weight. I cannot climb a step putting any weight at all on the same leg. There s no swelling in the knee area. Weakness and terrible pain in the center of the back portion of the knee! [/INST] Hi, thank-you for providing the brief history of you. As you have pain in the center of the knee back side and climbing down the stairs or up is difficult I suspect It to be a meniscus strain. Also, taking a thorough clinical examination will help rule out the knee injury. Also, an MRI will help understand the grade of injury. Having physical therapy after a proper rest should provide you better knee stability and mobility.in my clinical practice such cases are treated slowly and Gradually keeping in mind to avoid future problems coming up. Regards Jay In Chat Doctor.  </s>