Tendinopathy of the insertions of the hamstring muscles.
The hamstring tendons attach below the knee: one attaches to the top of the outer bone in the lower leg (fibula) and the other two attach to the inner side of the upper shin bone. Tendiopathy can come on with over use or a sudden spurt of movement and the patient will complain of pain on walking, running etc at either the inside or outside of the knee, where there will also be local tenderness. Bending the knee against resistance will also bring on the pain.
Physiotherapy can help by using a specific massage technique called frictions across the tender site. Electrotherapy may be used, particularly ultrasound, to help to relieve pain and promote recovery. Resting from the aggravating activity will be advised until the symptoms settle fully to prevent recurrence.