Hyperextension is something that occurs in exercise when you extend your limb fully and too violently and that way end up jarring the joint or stretching the ligament. This can occur particularly in training for example when you might be practising punching by punching 'the air' or practising kicking in the same way. This means that your fist or foot don't make impact with anything and thus the impact is instead made on your joint. Of course over time this can cause your tendon, ligament or muscle to stretch and eventually tear causing injury, or it can cause your bone to wear away or damage cartilage. This can then cause short term pain in immediate response to the movement, or it can cause you to cause more serious damage over time – for instance this is listed as one of the most common causes of 'tennis elbow' which is commonly caused by tennis players from regularly repeating the same movement and not always connecting.
When Are You at Risk?
Whenever you train a movement that requires power but do not connect with a target at the end of that movement, such as in karate when you might practise punching against the air, or when learning football technique, you are in danger of hyperextension. As well as during training though, you can also cause hyperextension when you miss your target. For instance if you go to kick a football hard and miss this can cause your leg to become completely straight at a high speed and thus crack the knee. Another recent scenario in which hyperextension can be a threat is when playing motion sensor games such as the Xbox Kinect or the Nintendo Wii. These involve your practising movements in front of a laser or a camera and kicking imaginary balls or punching imaginary opponents in order to control virtual characters and items on the screen. As there is no actual ball to kick this can again lead to players becoming 'over excited' and swinging hard at the imaginary items and jarring their joints in reality.
Some movements are of course more of a risk than others. In particular the moves that are a risk are the moves that involve power in a straight line – such as punching, or movements that move the lower end of the joint such as swinging. Movements that involve the whole body and that involve twisting are often okay. For example a golf swing is relatively safe as most of the power comes from spinning your body around and the movement tends to 'slow down' rather than be stopped by your limbs reaching their full extension.
Hyperextension can be avoided then by using correct technique in many cases and by following through movements in a fluid fashion. In golf you are taught to continue the movement after you have hit the ball so that you 'swing through' and this is better for you as well as resulting in a much longer hit. In some activities however such as martial arts, the correct technique actually involve striking forward in a straight line – for instance in karate the idea is to punch directly forward. Normally this technique is fine because you will be connecting with the opponent or with a pad – it is only when you practice on your own such as when repeating 'forms' or 'kata' that you then end up causing hyperextension.
As such then, when practising such movements it is important to instead practice just short of fully extending the arm to instead practising just short of doing so with a slightly bent arm/leg. This way there is no impact on the joint and you can prevent yourself from damaging joints and ligaments. You should then also carry this same behaviour across to other activities such as using the Kinect or Wii.
If you find that your joints are hurting then you should take a break from the exercise and even use it to massage yourself. If the pain persists over a few days and you notice swelling then you should seek medical aid.