When last newsletter the Ranking Spotlight flickered over world record holder Helen Mangan, Helen revealed how, after her victory at the world championship in Boston, she had herself physiologically tested to check her anaerobic threshold (AT) and discovered that she had been training at the wrong intensity. Since changing her new training regime she's now knocked a good couple of seconds off her previous best. So, to quote Barry Manilow, what exactly is the Anaerobic Threshold and how do you work it out?First the biology: the anaerobic threshold is said to be when the accumulation of lactic acid in the blood reaches 4 milli mols. Above this point it cannot be metabolised at the same rate that it is being produced and remains in the muscles. Top athletes are tested regularly to determine at what heart rate lactic acid accumulation occurs. This is done by taking blood during exercise and analysing it. Now, unless you have a blood analyser, a small portable one of which will set you back about 600 pounds, this is a bit tricky, but there is another, simpler, way to approximate your AT.Apart from the accumulation of lactic acid as a result of exercise, there is also a build-up of carbon dioxide. This is carried back to the lungs in the blood stream and leaves the body when you exhale. As carbon dioxide is a toxic gas, its accumulation causes a change in the breathing pattern. If you're rowing along in an aerobic state there is no carbon dioxide build-up and you can carry out the exercise and talk comfortably. By gradually stepping up the output of effort you will reach a point where you will feel the need to take great gulps of air. This is not, as most people assume, to get more oxygen into the lungs but to get the carbon dioxide out. If you monitor your heart rate and note at what beats per minute this occurs, and then take that number and add 5, that will be your approximate AT and the most effective level to train at.Two things to remember, firstly when you step up the output you need to take small steps starting from a very low level and holding each step for at least 3 minutes; if you have a copy of the Indoor Rowing Training Guide then you can follow the step test protocol. Secondly, as you improve your heart and lung function then your AT will change so you will need to repeat the process roughly every 6 weeks.