What’s the perfect body composition for triathletes?

For running and cycling, the optimum body composition is relatively straightforward. Since the propulsive force required to overcome gravity and generate movement comes from muscular contraction, it follows that a high power-to-weight ratio (plenty of lean muscle tissue and a minimum of body fat) is beneficial. But in the water things aren’t so simple because, unlike most other body tissues, body fat is less dense than water.

The upshot of this is that the more body fat a swimmer carries, the more buoyant they’ll be in the water. Higher buoyancy means that less of the body will be under the waterline, which in turn means less drag to overcome during movement through the water. But it’s not quite as simple as that because piling on pounds of fat can result in slower movement through the water due to ‘form drag’.

Excess fat on the human body tends to be carried unevenly, largely in the abdomen, thigh and buttock areas. Movement through the water produces swirling eddy currents around these protruding areas and can slow swimming velocity. This is what’s known as form drag. Form drag is also increased by poor alignment when moving through the water. For example, if your legs begin to drop or your head rises in relation to your trunk, this presents an additional frontal area that increases form drag.

HOW MUCH FAT?

Increasing body fat in swimmers improves buoyancy (aiding performance) but also increases form drag (a hindrance). To see which effect dominates as body fat levels rise, scientists artificially increased body
fat levels by 2% or more in a group of swimmers who’d been swimming competitively for at least three years. This was achieved by fitting latex pads beneath a spandex tri-suit in the same areas where swimmers might be expected to gain body fat.

While the ‘artificial fat’ improved buoyancy, it also increased the 50-yard time by around 0.8secs or about 0.2secs per additional pound of fat added. In other words, the detrimental effects of increased form drag greatly outweighed
any benefits of extra fat increasing buoyancy.

Does this mean that lower body fat is always beneficial for triathletes? Not necessarily. Triathletes typically have very low body fat in the lower body, which can make it harder to keep the legs horizontal in order to maintain a streamlined position. A little extra fat (and extra buoyancy) could help aid a streamlined position, resulting in a net drop in overall drag.

Gender may also be important in this respect. Female fat tends to be disproportionately distributed in the lower half of the body, giving a bit more lift to the legs that, in turn, reduces form drag. By contrast, males tend to put on excess body fat in the abdominal region; an expanding waistline shifts their buoyancy forward, which tends to make the legs sink, increasing form drag.

How important is body fat percentage for athletes?

OPTIMUM BODY COMPOSITION

What constitutes the optimum body composition for triathletes is tricky to answer because it depends on so many other factors such as body distribution, body shape and the nature of the swimming event. Some researchers claim that optimum body fat levels range from 10% to 20% for male swimmers and from 15% to 25% for women, while others have suggested that body composition may not be particularly relevant in sprint performance and that – in
men especially – muscular power is what really counts.

TAKEAWAY TIPS

Excess body fat hinders cycling and running performance and increases form drag in the water. This may be less important for female triathletes or for very short swimming events.

Triathletes with lean and muscular legs should pay particular attention to optimising form in the water.

Don’t get too hung-up on optimum body composition. Monitor it using callipers or a body fat monitor; your optimum composition will be one that accompanies your best times.

Andrew Hamilton Sports performace science writer

About

Andrew Hamilton (BSc Hons, MRSC, ACSM) has been involved in the fitness and sports-performance industries for over 30 years. As a former cyclist, runner and triathlete, Andrew became interested in enhancing performance through structured training and nutrition. After training to become a fitness teacher, Andrew taught in the fitness industry for 10 years before taking a degree in chemistry at the University of Aberdeen with a view to underpinning his growing interest in nutrition and performance. With extensive practical experience of the fitness industry and an in-depth scientific grounding, Andrew began writing for Ultra-Fit Magazine (then the UK’s longest established fitness publication) back in 1997 and subsequently became its features editor. During the following years, he became accredited to teach and assess the national NVQ qualification in Exercise and Gym by gaining D32/33 Assessor’s Award and was also awarded full membership of the Royal Society of Chemistry (MRSC) as well as being nominated for Energise Nutrition’s ‘Fitness Professional of the Year’ award. In 2003, Andrew became a member of the American College of Sports Medicine, universally acknowledged as the world’s leading authority on fitness, sports training and sports medicine. Since 2005, Andrew has been the editor of and a writer for Peak Performance, the authoritative online sport performance publication that reports on the latest sports science findings, and how athletes and coaches can apply these findings to maximise performance. In addition, Andrew is also a contributor to and sub editor for Peak Performance’s sister publication Sports Injury Bulletin, which provides the latest sports-injury science and best practice for physiotherapists and clinicians with athletes in their care. In addition to his regular work, Andrew also writes for other sports-related publications. In recent years, these have included: ‘Cycling Weekly’, ‘The British Journal of Cycle Coaching’, ‘Athletics Weekly’, ‘Australian Ultra-Fit’ and ‘Workout Magazine’. Andrew has also advised as a scientific consultant to the fitness and sports nutrition industries, including High5, one of UK’s leading sports nutrition manufacturers. After 30 years in this industry, Andrew is as passionate as ever about fitness and sports performance; indeed, he can still be spotted cycling and running the hills of the Northumberland Pennines in the far north of England where he now lives.