Beating Exercise Fatigue

You know that heavy feeling in your legs that makes you slow down? It starts in your head. Here's how to teach your mind to let you run longer and stronger


Posted: 6 March 2007
by Matt Fitzgerald

As any runner who’s ever felt their legs turn into lead anvils at the end of a hard session or race knows, running further or faster all boils down to a battle against fatigue. So you train to increase either the distance or the pace – or both – you can sustain without tiring. But hold on just a second: fatigue might not be what you think it is. According to the latest exercise science, that dead-tired feeling could be all in your head. What’s more, the way you’ve been training to prevent fatigue in your legs may not be the best way to tackle it where it really starts: in your brain.

Throughout most of the 20th century, the prevailing theory of exercise fatigue was the so-called "catastrophe" model. According to this theory, fatigue is an involuntary drop in performance caused by the loss of homeostasis (balance) somewhere in the body. For example, owing to lactic acid build-up, the pH balance in the muscles is upset and becomes too acidic for them to function properly, causing you to slow down. Or the muscles become depleted of glycogen (their primary fuel source) so there’s no longer sufficient energy available to sustain performance.

However, in the 1980s a new generation of exercise scientists, led by Professor Tim Noakes of the University of Cape Town, South Africa, began to poke holes in the catastrophe model. First, they found that the common functional breakdowns – things like lactic acid build-up and glycogen depletion – don’t always occur as we tire. In fact, studies showed that fatigue often develops before the muscles reach a level of acidity that would cause direct muscle dysfunction, and that you can feel tired even when there is still muscle glycogen available in the working muscles.

What’s more, these researchers also argued that the old catastrophe model couldn’t account for the peculiar phenomenon known as the "end spurt" – the ability to pull a sprint finish out of the bag at the end of a marathon. If fatigue was always caused by direct physiological events within the muscles, runners who began to slow down during the latter stages of a race couldn’t possibly sprint the last 100 yards or so. "Athletes can often surge during the latter stages of a race, knowing that they won’t have to continue once they cross the finish line," says Chris Abbiss, a biomedical and health science researcher at Edith Cowan University in Western Australia.

In one 2001 experiment on end spurts, Australian researchers put a group of subjects through a 60-minute simulated time trial on stationary bikes. They interspersed six all-out sprints throughout the hour-long effort. The results showed that maximum power output began to decline in the second sprint, indicating a very early onset of fatigue. Maximum power continued to decrease through the fifth sprint, but then suddenly shot up in the sixth and final sprint. If the fatigue that began to set in as early as the second sprint had been caused by a catastrophic loss of homeostasis in the muscles, where did the cyclists find the energy for the final surge?

Are We Finished Yet?

As more research challenged the catastrophe theory of fatigue, an alternative emerged, suggesting that fatigue is a protective mechanism that the brain uses to prevent a catastrophic breakdown. Feedback signals from the body to the brain indicate the imminent likelihood of a physical crisis if exercise continues at the current intensity level. In response to these signals, the brain decreases muscle activation and produces feelings of discomfort and loss of motivation, resulting in reduced exercise performance. That is, your brain tells your body to stop working so hard.

The new, brain-centred model of exercise fatigue – called the central governor theory – also explains the end spurt. Proponents of this model believe that, throughout exercise, the brain continually reads feedback signals from the muscles, blood and elsewhere in order to answer the question "How much longer can my body go at the present work level before something terrible happens?" When the answer received is "not much longer", the brain reduces motor output to the muscles and generates those familiar feelings of suffering to reinforce the need to slow down. But when the finish line is within sight, your brain allows you to pick up the pace, knowing it will all be over soon. "The brain diminishes muscle activation in the middle of the race in anticipation of calling in a reserve towards the end," says Frank Marino, head of the School of Movement Studies at Charles Sturt University in Australia.

Studies involving simulated races (usually on stationary bikes) have shown that a decline in performance caused by fatigue almost always coincides with a drop in electrical activity in the muscles. "This suggests that the brain reduces the drive to the muscles as a means of protection," says Marino.

The Power of Thought

If fatigue really does start in your head, how do you train to delay its onset? By doing sessions that will convince your "central governor" that you’re capable of achieving your goal times without suffering bodily harm. Such sessions should be highly race specific – simulating both the pace and the distance of your goal event – so that your brain will be less likely to feel endangered during the race and won’t put the brakes on motor output to your muscles, causing you to hit the wall.

The perfect example of a highly race-specific session for a 5K runner is an interval session of 5 x 1,000m at 5K race pace, with three-minute jog recoveries between the efforts. Because the session entails running 5K at 5K goal pace, it serves as proof to the brain that your body can handle the workload. If you’re training for a longer race, such as a half-marathon or a marathon, you need to alter this approach by running only about half the race distance at goal pace, since not even the toughest elite runner could recover quickly enough from a session totalling 26 miles of race-pace intervals.

Most runners can’t jump right into demanding race-specific sessions without first developing the speed and distance components of race fitness individually. So start by doing one challenging speed session and one challenging distance session each week, along with however many easy runs you want.

Early on, most of your speed training should be much faster and your distance training much slower than goal race pace. As your body adapts, your speed efforts should become longer and slower, your distance efforts longer then faster, until your sessions closely simulate the upcoming race experience. Once you have proven to your brain that you can complete these race-specific sessions, come the big day it should respond to your efforts with a "been there, done that, no problem."

Trick Yourself Fit
Sessions to get your mind and body in shape and in sync

These core speed and distance sessions prepare your brain and body for peak race performance at key distances. For each race distance, training is divided into four phases, with the number of weeks to spend in each given (follow the peak phase with a standard taper). Do one speed session and at least one distance session each week throughout all four phases, along with however many easy runs you want. Always include a one-mile warm-up and one-mile cool-down in every session.

Start each phase with a manageable version of the suggested session (initial speed sessions should feature a challenging but not excessive number of intervals, and early distance sessions should test your endurance but not overwhelm you). Then gradually add intervals to the speed session and length to the distance session from week to week within each phase. To calculate the appropriate paces for the sessions, enter your goal race time into our online race-time predictor, to find out equivalent finishing times for other distances, then convert to a minutes-and-seconds-per-mile pace.

5K
TRAINING PHASE: BASE (4 WEEKS)
Speed sessions: 4 to 12 hill sprints
Distance sessions: 5- to 10-mile variable-pace run
TRAINING PHASE: BUILD 1 (3 TO 4 WEEKS)
Speed sessions: 4 to 12 x 400m @ 1-mile pace with 2 minutes recovery
Distance sessions: 4 to 12 miles @ marathon pace
TRAINING PHASE: BUILD 2 (3 TO 4 WEEKS)
Speed sessions: 3 to 6 x 600m @ 3,000m pace with 3 minutes recovery
Distance sessions: 3 to 8 miles @ half-marathon pace
TRAINING PHASE: PEAK (4 WEEKS)
Speed sessions: 2 to 5 x 1,000m @ 5K pace with 3 minutes recovery
Distance sessions: 2 to 5 miles @ 10K pace
10K
TRAINING PHASE: BASE (4 TO 6 WEEKS)
Speed sessions: 4 to 12 x 400m @ 1-mile pace with 2 minutes recovery
Distance sessions: 5- to 10-mile variable-pace run
TRAINING PHASE: BUILD 1 (4 WEEKS)
Speed sessions: 3 to 6 x 600m @ 3,000m pace with 3 minutes recovery
Distance sessions: 4 to 12 miles @ marathon pace
TRAINING PHASE: BUILD 2 (4 WEEKS)
Speed sessions: 2 to 5 x 1,000m @ 5K pace with 3 minutes recovery
Distance sessions: 3 to 8 miles @ marathon pace
TRAINING PHASE: PEAK (4 WEEKS)
Speed sessions: 2 to 5 x 2,000m @ 10K pace with 4 minutes recovery
Distance sessions: 2 to 5 miles @ 10K pace
Half-Marathon
TRAINING PHASE: BASE (6 TO 8 WEEKS)
Speed sessions: 4 to 12 x 400m @ 1-mile pace with 2 minutes recovery
Distance sessions: 5 to 9 miles @ a steady pace
TRAINING PHASE: BUILD 1 (4 WEEKS)
Speed sessions: 3 to 6 x 600m @ 3,000m pace with 3 minutes recovery
Distance sessions: 6- to 12-mile variable-pace run
TRAINING PHASE: BUILD 2 (4 WEEKS)
Speed sessions: 2 to 5 x 1,000m @ 5K pace with 3 minutes recovery
Distance sessions: 4 to 12 miles @ marathon pace
TRAINING PHASE: PEAK (4 WEEKS)
Speed sessions: 2 to 5 x 2,000m @ 10K pace with 4 minutes recovery
Distance sessions: 3 to 8 miles @ half-marathon pace
Marathon
TRAINING PHASE: BASE (6 TO 8 WEEKS)
Speed sessions: 3 to 6 x 600m @ 3,000m pace with 3 minutes recovery
Distance sessions: 6 to 12 miles @ a steady pace
TRAINING PHASE: BUILD 1 (4 WEEKS)
Speed sessions: 2 to 5 x 1,000m @ 5K pace with 3 minutes recovery
Distance sessions: 10 to 16 miles @ a steady pace
TRAINING PHASE: BUILD 2 (4 WEEKS)
Speed sessions: 2 to 5 miles @ 10K pace
Distance sessions: 6- to 12-mile variable-pace run* / 14 to 20 miles @ a steady pace
TRAINING PHASE: PEAK (6 WEEKS)
Speed sessions: 3 to 8 miles @ half-marathon pace
Distance sessions: 4 to 12 miles @ marathon pace* / 16 to 24 miles @ a steady race
* Beginner and intermediate runners: alternate between these two sessions every other week. Advanced runners: do both of these sessions every week.
Session Key

Hill sprints: Find a 100-400m hill that is steep enough to challenge you over this short distance (ideally it will have about a 5-7 per cent gradient). Run up the hill hard (roughly one-mile race pace intensity) and jog back to the bottom for recovery.

Variable-pace run: Start at an easy jog, then increase your pace slightly every one to two miles. Run the last one to two miles at half-marathon pace


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endurance, fatigue, motivation, raceday psychology, progress
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