TRAINING
Steve Bird
Warm Up: more than just about feeling warm Introduction Warming up before an orienteering event is an important part of your race preparation and failing to do so can put unnecessary strain on your body, which may adversely affect your performance and perhaps increase the risk of injury. Warming up is not just about feeling warm, although this is one of the consequences and benefits. A warm-up is important because of the considerable physiological adjustments your body has to make when you go from being at rest to running. To illustrate this point I have put some indicative figures in Table 1. These are illustrative, as the exact values will vary from one person to another, as well as being affected by how fast you run, how fit you are and your body size, but nevertheless they do indicate the magnitude of the responses your body has to produce. As with previous articles the text of this piece is worded in the context of Foot Orienteering and running, but the same principles apply to Mountain Bike Orienteering, or indeed any strenuous exercise.
Your body at rest At rest your body needs relatively small amounts of oxygen, typically in the region of around 0.6 litres a minute, or if we represent this in terms of body weight, about 10 millilitres of oxygen per kilogram of body weight per minute. This is supplied to the muscles and other organs of the body by breathing gently to oxygenate the blood and by maintaining a steady heart rate that delivers the blood around the body. Typical values may be in the region of 12 breaths per minute, each of about 0.5 litres of air, giving a lung ventilation of about 6 litres per minute. At rest the heart will deliver this oxygenated blood with a heart rate of around 72 beats per minute, with each beat ejecting about 70ml of blood (a factor known as the stroke volume). This means that, at rest, the heart will eject around 72 x 70ml of blood a minute, approximately 5 litres. The amount of blood ejected each minute is known as the cardiac output. Of this cardiac output, only about 20% is directed towards the muscles, since they are at rest and do not require much oxygen. The remainder is sent to other organs of the body such as the brain and gut.
Your body during exercise When you exercise your muscles need more oxygen. When you walk the increased demand for oxygen is slight and can be met by small increases in the rate and depth of your breathing, accompanied by a slight increase in your heart rate, perhaps up to 90bpm. If you jog there is a greater demand for oxygen and hence your breathing will increase more dramatically and your heart rate may rise to 120 bpm. In addition to this, the stroke volume of your heart will also increase, causing more blood to be ejected with each beat of your heart and, furthermore, a greater percentage of your cardiac output will be sent to your exercising muscles. This is achieved by the selective dilation of the blood vessels supplying your exercising muscles and a constriction of those supplying other organs such as your gut. All these of changes are in proportion to the strenuousness of the exercise. So running at a steady pace requires a greater oxygen uptake and hence greater responses by the cardiorespiratory system than gentle jogging. Indeed, when working close to a maximum sustainable running speed, the sort of speed that can be maintained during an event, very large responses are required. Typical figures for a fit elite orienteer may be a heart rate of 165 - 180 bpm, a stroke volume of 120 – 140 ml per beat, a cardiac output of 18 - 25 litres per minute, 75-80% of the cardiac output being directed 22 THE AUSTRALIAN ORIENTEER JUNE 2006
to their exercising muscles and a ventilation of 130 - 160 litres of air per minute, with a resultant oxygen utilisation of 60 – 70 ml/ kg/min. (Note: the figures given here in the text are for a young, fit, male. The exact values will depend upon the size, age and gender of the individual, as well as their level of fitness.)
Why warm up? The reason why a warm up is a good idea, is because your body cannot instantaneously go from a resting state to that which is optimal for hard running. It takes time for it to reach a state whereby you can comfortably sustain a good running speed without becoming fatigued. So a warm-up involving a gradual increase in speed from jogging to running will elevate your heart’s cardiac output and your lungs’ ventilation, promote a greater blood flow to your muscles and increase the temperature of your muscles, which along with other biochemical changes will enhance their extraction of oxygen from the blood as it passes through them and promote its effective utilisation. It therefore prepares your body for the hard running you are about to do. Fortunately the benefits of a good warm-up are retained for several minutes after it’s been completed, so you won’t lose any benefits whilst you’re in the start boxes.
