Hamstring muscle weakness after ACL repair has been reported in studies done comparing one type of ACL graft to another. The hamstring tendon graft doesn't appear to cause any problems with function during daily activities.
Other researchers report decreased stability after surgery for patients with the hamstring tendon graft. It's thought that the way the graft is fixed to the bone may be part of the problem. In many cases the end of the tendon is attached outside of the joint rather than inside the knee. This may put it at a biomechanical disadvantage for movement and strength.
More studies are needed looking at the long-term effects of the tendon grafts used in ACL repairs. Function, strength, and motion should be compared between groups of athletes versus non-athletes.