Runners love numbers. Whether it's pace, finishing times or resting heart rate, we view figures as a sign of our progress, accomplishments and dedication to the sport. So it's no surprise that many of us struggle with one number that increases each year, regardless of how hard we train: our age.
But thanks to all the health benefits that running brings, you don't need to sweat each time a candle is added to your cake. "There's a big difference between biological age, or how old your body thinks you are, and chronological age - how old the calendar says you are," says Steven Hawkins, professor of exercise science at California Lutheran University in the US. "The biological ages of runners are at least 10 years younger than their chronological ones, and this gap widens with time."
To help you keep that biological clock fooled, Runner's World takes a look at the life span of a runner, decade by decade, with advice from medical experts, trainers, nutritionists and amazing runners who define peak performance.
Teens: Decade of innocence
Runners in the throes of puberty have superpowers. A British study compared 12 boys and 13 men doing 10 sets of 10-second sprints. The boys sustained their power output better than the men, partly because teens regenerate creatine (a compound that supplies muscles with energy) quicker than older runners. Also, levels of lactate, the by-product of intense efforts, are naturally lower at this age.
Girls share the same ability to crank up the power - and can sustain it even better than boys. Japanese researchers found that in a series of sprints, teenage girls lost 10 per cent less power than boys the same age did. That said, as muscle mass piles on, boys have the distinct upper hand - or in this case, leg. "Proportionately, boys develop more muscles than girls do and have a natural power advantage," says Professor Cameron Blimkie of McMaster University in Canada.
Bones are still developing, too, and running helps make them as dense as possible. Blimkie's study on the bone density of female runners, triathletes, cyclists and swimmers found that runners had the highest bone-mineral density and strength.
You may be fit, but your growing body still needs to be handled with care. Bones develop faster than their supporting ligaments and tendons. As a result, joints and muscles can be prone to injury. "Some teenage female athletes get a condition called amenorrhea, which is when their regular periods stop. This can have significant implications," says Dr Jo Wallace, a lecturer in exercise physiology at Aberystwyth University. "It happens when they're training hard but aren't consuming enough calories. This causes their oestrogen levels to drop, leading their periods to stop. This can be serious, as oestrogen is needed to build healthy bones. But once girls with amenorrhea start to have a sufficient food intake, the periods will return as normal."
Wallace advises both girls and boys to not take on too much mileage, and points out that if they do get an injury, should give themselves enough recovery time: "When bones are still growing at this age, the tiny microfractures that occur naturally are increased through exercise. For most people, this is a good thing as it increases bone density in preparation for later life, when they start to deteriorate. But at this young age, there's the risk that microfractures can develop into hairline fractures, which can then turn into more serious injuries."
Work on building your form and endurance so you can become a balanced, injury-free runner further down the road. Practise sprint-specific drills such as high knees and skips to build a strong foundation, ideally under a coach's supervision. "If an athlete has good form during the early years, that helps so much with performance and injury prevention later," says coach Greg McMillan (mcmillanrunning.com). Boost aerobic capacity by upping long runs by five minutes weekly. Go by minutes, not miles - and when in doubt, take it slowly. "Not every training run should end with your hands on your knees," McMillan says.
"For healthy bone development, calcium-rich foods such as milk, yoghurt and cheese, together with exercise such as running, will build up the peak bone density for later in life," says Wallace. "But you also need to maintain sufficient energy levels for the activity you are doing by eating your protein and carbs too. Remember that your metabolism will increase as you do more exercise.
"Calories are important for maintaining fat levels, and fats help release the fat-soluble vitamins A, D, E and K, which are essential for the repair of body cells."
18, from Coventry
"I've been running since I was seven years old. When I was 11 I joined Coventry Godiva Harriers, where coaching made me faster. I went on to win the English Schools' intermediate 800m title twice, and in 2008 I was invited to join the Aviva-sponsored On Camp With Kelly mentoring and education initiative, led by Kelly Holmes.
"In my GB vest at last year's IAAF World Youth Championships I won a bronze medal and set my 800m PB (2:03.83).
"I had a setback this season when I damaged my ankle by twisting it badly while training - but it's been my first injury, and I'm getting back into it now.
"I normally train six times a week with more mileage work in the winter and more quality speed sessions in the summer. Even after a tough session, I feel fine the next day - as long as I've had a good night's sleep."
20s: Decade of invincibility
"From a physiological viewpoint, you really are at your peak in your 20s," says David Jones, professor of sport and exercise sciences at Birmingham University. "Although women reach their natural shape and size in their mid teens, men are still growing and building muscle mass in their early 20s, and will reach their optimum before they're 30.
"As muscles bulk up, the tendons enlarge to cope with the extra load. If you're training hard, your muscles will be slightly stronger than the average person's, and you will certainly have a better blood supply to the muscles. Your VO2 max - the amount of oxygen your body can consume while exercising - will be at its full potential too.
"Bone density also reaches its peak in the early 20s for men - a few years after it has peaked in women - and it remains constant during this decade."
You may start to feel twinges in your knees towards the end of your 20s. Cartilage, the gel-like, shock-absorbing substance that lines the ends of your bones, can become frayed as your 30th birthday looms.
Also, "young, unsupervised athletes usually don't get enough sleep, hydration or adequate nutrition," says Bradley Young, a sports psychologist at the University of Ottawa, Canada. "At some point, often seven or eight years after leaving school, you realise that you can't stay up until two o'clock in the morning and belt out an eight-mile tempo run the next day. You eventually learn to self-regulate to save yourself from becoming a post-education running casualty."
Your most important training tool this decade? Self-control. Cardiovascularly, you're a rock star, but your musculoskeletal system can't always keep up with your heart and your lungs. "The demands and impact of running are too intense on your joints and muscles to complete tough workout after tough workout without getting injured," says McMillan. He recommends taking at least one easy day between hard runs and incorporating impact-free cross-training activities into your routine.
"Runners in their 20s tend to either eat poorly or eat just to get by; they don't make the connection between food and performance," says running nutritionist Lisa Dorfman, author of The Vegetarian Sports Nutrition Guide (£12.99, John Wiley & Sons).
When you're running, you want your body to tap into easily accessible carbs for fuel, not drain your protein stores. "Protein aids muscle repair, boosts your immune system, maintains healthy hair and skin, and manages hormones and water balance," says Dorfman.
Before any run that exceeds an hour, eat about 40g of carbs (240ml of sports drink and half a banana). If you're going longer than 90 minutes, restock your carbs every hour with 470ml of sports drink or with a gel and some water. Within an hour of finishing your run, jump-start your recovery with a carb and protein snack (chocolate milkshake and toast, or a smoothie).
28, from Leeds
"I ran cross-country and middle distance at school, but in my late teens I stepped up to 5K and 10K. I’m sure all the running helped change my body shape from skinny to more bulky. In 2003, I did the Great North Run in 1:06:28, which was a real breakthrough for me, and I’ve now won the Midlands Cross Country title four times.
“I run about 90 miles a week and my training is very structured, but I’ve suffered with stress fractures to my foot, which has been frustrating. I eat loads of carbs, but quickly burn them off – and although I don’t have the perfect diet, I eat more good food than bad. I want to improve my PB at 5K (13:56) and 10K (29:04) while youth’s on my side.”