Muscle strain or overuse injury: This is the most likely diagnosis given the context of knee pain and inability to bend after exercise. Muscle strains or overuse injuries are common in individuals who engage in physical activities, especially if they have not warmed up properly or have overexerted themselves.

Ligament sprain: A sprain in one of the knee ligaments (such as the ACL, MCL, or LCL) could cause pain and limit mobility, especially if the sprain occurred during the exercise.
Meniscal injury: An injury to the meniscus, which is the cartilage in the knee joint, could cause pain, swelling, and difficulty bending the knee.
Tendinitis: Inflammation of the tendons around the knee, such as patellar tendinitis, could cause pain and stiffness, particularly after exercise.
Bursitis: Inflammation of the bursae, which are fluid-filled sacs that cushion the knee joint, could cause pain and swelling, making it difficult to bend the knee.

Fracture: Although less likely, a fracture of one of the bones in the knee (femur, tibia, fibula, or patella) could cause severe pain and inability to bend the knee. Missing this diagnosis could lead to improper healing and long-term disability.
Septic arthritis: A bacterial infection within the knee joint could cause severe pain, swelling, and stiffness. This condition requires prompt medical attention to prevent serious complications.
Osteonecrosis: A condition where the bone tissue dies due to lack of blood supply could cause knee pain and limited mobility. Early diagnosis and treatment are crucial to prevent further damage.

Osgood-Schlatter disease: A condition that affects the tibial tubercle, causing knee pain and swelling, typically in adolescents.
Sinding-Larsen-Johansson syndrome: A condition that affects the patellar tendon, causing knee pain and swelling, typically in adolescents.
Knee osteochondritis dissecans: A condition where a piece of cartilage and bone in the knee joint becomes damaged, causing pain and limited mobility.