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Live Long and Prosper: Be Fit to Dive
By Fred Leal, MSc, PhD, CSCS
I know, right? One more article about fitness. What the heck! You may be asking yourself “Doesn’t this guy have anything better to write about? Actually… No, I do not. Nothing is more important to me than human lives, so… here it goes.
Since I know you guys have read several articles about “scuba diving and fitness” I am going to be as straight forward as possible (without being too “sciency”) and present important physiology/medicine concepts with their “translations” followed by how you can improve your fitness levels. However, I am intentionally not going to mention any medical conditions or medications that may impair one’s ability to perform physical activity, since they are numerous and have different impacts on people. Moreover, if you know you have a medical condition or if you are on medications ask your physician if you are “okay to dive.”
Whether you are a vacation diver or an avid diver who braves cold and murky waters during the winter you are performing an activity that demands a certain level of physical fitness. The Diver’s Alert Network (DAN) is a great resource for those who want to know more details about the relationship between scuba diving and cardiovascular fitness; however, I will mention briefly that DAN states that “nearly one third of all diving fatalities are associated with an acute cardiac event” . There is consensus in the medical field that cardiovascular fitness is a significant predictor of coronary heart disease events (angina and myocardial infarction, for example) and that people with moderate cardiovascular fitness have lower risk of coronary heart disease . It is definitely possible to reduce the number of fatalities in scuba diving by addressing cardiovascular fitness.
What is fitness?
The Oxford dictionary  defines fitness as “quality or state of being fit; the quality of being suitable to fulfil a particular role or task” while Corbin & Le Masurier  define it as “the ability of your body systems to work together efficiently to allow you to be healthy and perform activities.” What we commonly know as fitness has several components but for this article I will mention two: aerobic and muscular fitness.
Aerobic fitness is your body’s ability to deliver oxygen to all cells of your body and is usually measured using the units VO2 or METs. VO2 is the amount of oxygen your heart pumps to all cells of your body (“V” stands for volume and “O2” stands for oxygen) and is measured in milliliters of oxygen per kilogram of body weight per minute (mLO2/kg/min) . To illustrate different metabolic requirements in VO2, I will mention that lying down in bed quietly requires around 3.5 mLO2/kg/min, running at 5mph (8km/h) requires about 25 mLO2/kg/min and the highest VO2 ever recorded was 96 mLO2/kg/min. Moreover, I will mention that a person’s highest VO2 is their “VO2max”, a term that will be used later on in this article.
MET means “metabolic equivalent” and is another unit used to estimate how much energy is expended during activities. For example, 1 MET is the amount of energy you need to stay alive while lying in bed and a person walking on a treadmill at 3mph (4.8km/h) at 15% incline uses 9.5 METs. There are several calculators online that allow you to calculate METs for your preferred activity but those that use treadmill and stationary bicycles are more accurate.
Muscular Fitness can be quickly defined as the muscles’ ability to perform the work demanded from them and this idea can be explored eternally by those who wish to discuss the intricacies of physiology. However, I promised I was not going to be “sciency”, so I will go straight to the point and state that divers should be worried about muscular endurance and muscular strength. Muscular endurance is being able to perform muscular contractions repeatedly over extended periods of time prior to reaching fatigue whereas muscular strength is the ability to generate forceful contractions.
Fitness and scuba diving
We all have heard or read that divers should respect the limits of training, experience, and equipment but divers should also dive within the limits of their fitness level. There is plenty of scientific data showing that scuba diving is an activity that requires very little energy expenditure when the dive is “beautiful and perfect”; however, more experienced divers (recreational or technical) know that sometimes things do not go 100% as planned and we may need to dig deeper to end the dive safely. Moreover, a diver must be physically fit not only because of the energy expenditure underwater but also to be able to cope with all activities associated with diving, such as carrying tanks and equipment or being able to re-enter a boat or kick back to shore after the dive .
Another very relevant concept in this discussion is maximal steady state, which is the highest workload that allows for continuous exercise without increase in ventilation and without a gradual increase in blood lactate . Blood lactate is a formed constantly but when its concentration in blood and tissues start to increase gradually, it is just a matter of time until a person reaches absolute fatigue and is unable to perform work. Maximal steady state can usually be maintained for 50-60 minutes  and it is around 50% of VO2max.
When it comes to the physical effort performed by divers, there is scientific data stating that divers dealing with weak currents (about 0.5 knots), wind or small waves require VO2 of 20mLO2/kg/min (about 6 METs)  and that being able to sustain VO2 of 25.2mLO2/kg/min (7 METs) is adequate for most recreational divers . However, because energy expenditure can increase by 50% suddenly during a dive , to make a long story as short as possible, it is generally recommended that divers have a VO2max around 42-45.5mLO2/kg/min (12-13 METs)  to be considered “fit to dive”.
You can assess your VO2max on a treadmill quickly and easily. Using a treadmill, if you are able to maintain the speed of 3mph (4.8km/h) at 20% incline you are working at 11.6 METs and if the incline is 25% you are working at 13.6 METs. However, once again, repeating myself and being intentionally annoying to you, I will state clearly that you must consult your family physician and be cleared to perform this extremely demanding test.
How to change aerobic fitness
When it comes to improving aerobic fitness there is no big secret behind it: you need to walk, run, cycle and/or swim. The most important thing is to remember your physiological limits and that begins with understanding your maximal heart rate (HRmax), which is calculated by “220 – age”. For example, a 30-year-old person’s HRmax is 190bpm (beats per minute) and a 60-year-old person’s HRmax is 160bpm. Given this limit, calculate your training zone using percentages: a 30-year-old person can train at 60% of their HRmax by maintaining HR at 114bpm (60% of 160) during their physical activity. The fitness industry commonly refers to the aerobic zone as the zone between 70% and 80% of HRmax; however, if you feel you are not quite there yet, stay at a lower intensity and improve gradually week by week.
There are several approaches to training, such as High Intensity Interval Training (HIIT) and Tabata, that have been shown to make positive differences and detailed explanation on how they work can be found easily online but they should be performed by people who already have good overall physical fitness levels since they place different (more dangerous) demands in the cardiovascular system. If you are a beginner, you will reach 60% – 80% HRmax very quickly and you should remain training in that zone for a while. I know “for a while” is not specific but only after a personal assessment can a trained professional determine how long is long enough; therefore, keep training until you notice that the exercise that once was strenuous now is a “piece of cake”, then increase the intensity.
How to change muscular fitness
Again, there is no secret here: work out! Beginners should start with the good old 3 sets of 10 using weights that will not make them feel exhausted at the end of the third set. Advanced trainees can follow a “periodization of training”, which means cycling through 4 phases of training: endurance, hypertrophy, power, and strength. Each phase can be as short as 1 day (for extremely advanced athletes) or can be the commonly used 1 week. During each phase, you need to change the resistance (weights), the number of sets, the number of repetitions in each set as well as the rest period between sets. However, I will state clearly that the factor that will make a true positive impact on the average person’s life is muscular endurance. In order to increase muscular endurance, one should perform 20+ repetitions of an exercise per set, about 5 sets, and about 3 minutes of rest between sets.
I will end by mentioning that discussions regarding “the best training method” will never end because we are all different and respond and adapt differently to stimuli, so don’t feel discouraged by the number and variety of “experts” and “gurus” who claim everyone else is wrong. Always use mature reasoning, train within your limits and continue on your own path towards improvement.
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