This is the excruciating moment a woman appears to snap a bone in her leg while using gym equipment. The unidentified woman cries in pain as her bone apparently snaps, moments after she lifts a heavy weight using a leg press. A leg press is a machine used in strength or resistance training. It requires a person to push weight away therefore engaging their leg muscles. But when the woman she is halfway through her second repetition, having already lifted the weight, she relaxes her leg while it is extended, rather than keeping her knee bent to support the weight. The unidentified woman screams in agony as her leg bone snaps. She was using a leg press machine at the gym, in which she was pushing away weights with her leg muscles . But the weight is too heavy and bears down on her leg, forcing it to bend the wrong way. The force causes a painful injury to her leg. While the woman writhes in agony, a man standing nearby in a suit watching her workout rushes to help. While it is unclear what exactly injury she sustained, an expert told MailOnline she could have fractured the tibia (the bone between the knee and ankle) or the femur (bone that extends from the pelvis to the knee). It is also possible she sustained an injury to the posterior cruciate ligament, the strongest ligament in the knee. Sammy Margo, a chartered physiotherapist said: ‘On the second repetition, her leg went into hyperextension, which means the ligaments go beyond themselves and the knee has bent back on itself. ‘She could have torn her posterior cruciate ligament,' she said. 'The ligament prevents the knee joint from what physios call "posterior instability". 'This means it prevents the tibia from moving too much and going behind the femur - the knee buckling back on itself. ‘It’s also possible she pulled the ligament off the leg bones and they fractured. She would have dislocated her knee.’ Fitness experts today warned of the dangers of carrying out exercises such as a leg press without the proper training or supervision. Sit on the machine with the back and head against the padded support. Place feet on the foot plate about hip width apart, ensuring the heels are flat against the platform. The legs should form a 90 degree angle at the knee and the knees should be in line with the feet. Brace the abdominal muscles and push the platform away with heels. Don't lock the knees or relax when the leg is extended. Slowly allow the platform to lower back to its starting position by bending the knees in a controlled manner. Make sure heels are always touching the plate, the knees are not bowing outwards or inwards and ensure the head is steady. Start with a modest weight and progress to a higher weight if comfortable. Some said that resistance machines such as the leg press are dangerous because they isolate muscles in a way that does not mimic 'real life' movement, while others warned a person should always concentrate when carrying out such exercise. Will Longhurst, fitness manager at Reebok Sports Club, London, said: 'This video is an extreme example of what could happen during exercise when not performed correctly, however that is a underlying point that should be addressed. 'A number of people visit the gym with little or no experience of how to use the equipment effectively and more importantly safely. 'Often gym-goers will lift weights that are too heavy or use machines with poor technique as highlighted in the video. 'If it is your first time in the gym or you are starting out with a new routine you should try, where possible, to consult a fitness professional or coach who will be able to show you the how the machine or equipment should be used properly. 'Personal trainers are an invaluable source of expertise and to make a positive start to somebody’s experience of gyms it is worth using a good personal trainer to teach all the basics in the gym. 'There are countless exercises that can be performed in the gym but they do all generally revolve around similar movement patterns, such as the push, pull, hip hinge or squat. 'A good understanding of these moves basic moves with good technique, posture and body alignment will assist every gym-goer with most of the beginner and intermediate exercises. 'By having the understanding of what good technique looks and feels like, gym goers will be able to perform their exercises in a safe manner and make progress quickly to more advanced exercises.' Helle Hammonds, founder of GymClass, London, said machines such as a leg press can be dangerous as they isolate muscles in a way that does not mimic how a person would normally move. After lifting the weight for the second time, the woman relaxed her legs instead of keeping them bent. This meant the weight bore down on her legs, forcing them to bend the wrong way and snapping her bone . She said: ‘The problem with leg press machines and many other weight machines is that they create a way of moving weights whilst isolating certain muscles and joints. ‘The body is not supposed to move like this. Instead when the human body moves an object we tend to use a whole bunch of different muscles and joints to get the job done. ‘These weight machines create an unreal environment, using inappropriately heavy weights and incorrect form can, as the video shows, cause problems. ‘I have seen many injuries to people's lower backs from leg press machines in particular, as they lift weights that are too heavy. ‘Through research the fitness industry has now moved away from sending clients on this kind of machine and I would recommend people use free weights to do exercise such as a squat, spilt lunge or a waking lunge.’ Tom Eastham, a trainer for Push Mind and Body retreat, added that concentrating while lifting weights is key. He said: 'You should never relax your legs in a fully extended position due to the risk of hyperextension. 'If she is using a heavy load, she should be mentally focused at the task in hand, this would ensure her muscles are switched on and functioning properly. 'She could also be new to strength training and as a result her joints might not be used to being under heavy stress. 'If she is hypermobile she might be at even greater risk of injury as these peoples muscles do not know when to stop movements and can have serious injuries as a result. 'Golden rules for the leg press or any other strength based exercises would be: focus, brace then contract.'
Woman is lifting weights with her legs using a leg press machine . When she lifts the weight she keeps her leg extended rather than bending it . Weight bears down, pushing her leg the wrong way and injuring her leg . She writhes in agony as a man standing nearby rushes to help her . Expert today told MailOnline people should never use machines like leg presses without appropriate training or supervision .