The tendon works hard whenever the knee bends and straightens under load. It works hardest when knee movements involve a full bend under your body weight and gravity, for instance if you do a full squat exercise in weightlifting, or when you take off from one leg in high jumping or long jumping. The tendon also generates a strong pull when you go up and down stairs, when you run on hilly ground, and when you kick a football. Because the tendon plays such an important part in all movements at the knee, it may be strained simply by overwork. An overuses strain is more likely to occur if the tendon is working inefficiently, because it is fatigued through overwork, or tight due to cold or previous excessive exercise.
Bad shoes can contribute to changing the tendon's angle of pull.
This is especially true if children and adolescents wear shoes with little support underfoot, or high-heels, during their growth years. The tendon may also be subject to sudden injury. When the patellar tendon is strained, a few of its many fibres may be torn. This causes pain when you use the tendon, but it does not necessarily stop the tendon from working through its normal range of movement. As a result of a strain, or partial tear, the tendon may become thickened, and tight, because of scar tissue forming in the torn fibres.