Why You Need Variety

Each of your muscles is composed of three types of muscle fibre:

• Slow twitch
• Intermediate fast twitch
• Fast twitch

When exercising, these fibres are recruited progressively — a process governed by what is known as the size principle. Smaller, more economical slow twitch fibres are recruited first, with intermediate and fast twitch fibres called upon as effort increases:

• Walking or jogging uses almost exclusively slow twitch fibres.
• As pace increases, intermediate fibres are recruited.
• Fast twitch fibres are engaged as you speed up towards 5K pace, run hills or sprint.

The following gives a rough indication of fibre recruitment at different intensities. These are approximations — the exact percentages vary between individuals depending on fitness and genetics:

To train each muscle fibre type for maximum performance, you need to schedule a variety of weekly workouts, including a distance run, a long run, and one or two interval or repetition sessions per week.

Continuous Runs

Distance Runs

“Running your distance runs slowly won’t make you a slow runner. Training incorrectly — not doing the full range of workouts required to build speed — will make you a slow runner.”

The correct pace for distance runs is the pace that delivers the full range of aerobic benefits:

• Strengthening slow twitch muscle fibres
• Strengthening connective tissue such as bones and tendons
• Increasing the number and size of mitochondria (the aerobic energy-producing power plants) within each slow twitch fibre
• Increasing the number of capillaries surrounding each slow twitch fibre, which carry oxygen and nutrients to your muscles
• Increasing the amount of carbohydrate fuel (glycogen) stored within each slow twitch fibre
• Strengthening your heart so that it pumps more blood with each beat
• Improving your nervous system’s ability to recruit slow twitch fibres efficiently, creating a more economical stride

To trigger all these adaptations you need to run at a conversational pace — approximately 2 minutes per mile slower than 5K pace for faster runners and up to 3 minutes per mile slower for slower runners. This equates to roughly 65–75% of VO2max.

Running too hard delays recovery for your next session. VO2max demand at various paces is approximately:

Tempo Runs

Tempo runs can be completed at three different paces:

• Marathon pace: roughly 40–60 seconds per mile slower than 5K pace
• Half marathon pace: roughly 25–40 seconds per mile slower than 5K pace
• 60-minute race pace (lactate threshold): roughly 15–25 seconds per mile slower than 5K pace

Avoid the temptation to run all-out — that is the wrong approach for a tempo run.

Benefits of tempo running include:

• Increased mitochondria in recruited slow and intermediate fibres
• Increased capillaries for recruited fibres, improving supply of nutrients and oxygen
• Improved ability to utilise and clear lactate (a carbohydrate energy source) from within recruited muscle fibres
• Improved ability to buffer and export acidic hydrogen ions from recruited fibres, and manage other fatigue-inducing by-products of anaerobic energy production
• Improved ability to burn fat as a fuel source

Distance and tempo workouts are particularly effective for increasing the number of capillaries and mitochondria in slow twitch and some intermediate fibres.

Hills

Hills can be incorporated into your longer runs and offer a range of specific benefits:

• Hills increase the proportion of muscle fibres recruited while running, engaging intermediate and some fast twitch fibres that flat terrain does not
• By working more muscle fibres, hill running improves nervous system coordination between fibre types and the muscle groups they represent — such as hamstrings and quadriceps
• Long hills challenge the energy-producing capacity of all fibre types
• Running uphill promotes ankle flexibility, improving your stride
• Hill running increases Achilles tendon stiffness, which improves elastic recoil — the spring-like energy return in each stride
• Hill training stimulates adaptations similar to resistance training, improving your ability to produce force

While distance runs on flat terrain recruit mostly slow twitch fibres, hill runs draw upon a greater proportion of intermediate fibres and some fast twitch to provide the force required to battle gravity. Working all fibre types demands increased input from your nervous system, which adapts by developing more efficient neural pathways for communicating with muscles. This produces a more economical stride that benefits all your running — uphill, downhill, slow and fast.

Lengthening the Long Run

Approximately 90 minutes into a distance run, your slow twitch fibres begin to exhaust their individual glycogen stores. This forces other previously unrecruited slow twitch fibres to take over — and run long enough, intermediate fibres are activated too.

This progressive depletion triggers several beneficial adaptations:

• New capillaries are created for all recruited fibres as your body struggles to maintain supply lines over a prolonged period
• Glycogen storage capacity increases in all affected fibres — the carbohydrate fuel tank within each fibre can roughly double in size
• Improved aerobic energy production in all recruited fibres
• Improved ability to burn fat as an energy source
• An increase in aerobic enzymes, which facilitate aerobic energy production
• Improved nervous system control of recruited fibres, leading to a more efficient stride
• Strengthened recruited muscle fibres and connective tissue

Intervals and Repetitions

VO2max Workouts

A 5K is approximately 90–95% fuelled by aerobic energy — energy created within muscle fibres using carbohydrates, fats and oxygen. You likely already have adequate fuel stores. It is oxygen delivery that is the limiting factor.

You don’t increase your oxygen supply simply by breathing harder. You do it by improving the transportation of oxygen to your muscle fibres. While your heart and major blood vessels handle the bulk transport of oxygen, it is your capillaries that deliver it to the muscle fibres. Creating more capillaries, and larger and more numerous mitochondria to use that oxygen, is therefore the key adaptation to pursue.

VO2max is defined as the maximum volume of oxygen your body can consume per minute — not how much you can breathe in, but how much your mitochondria can consume to create energy.

For a VO2max workout to be effective — stimulating meaningful capillary and mitochondria development — you need to run repetitions at a minimum of approximately 90% of your current VO2max. Less intensity will not stimulate the required adaptation; more will increase fatigue without additional benefit.

Effective training paces are:

VO2max repetitions are best measured in time rather than distance, because it is the duration spent near VO2max that drives adaptation, not the distance covered at that effort.

It takes approximately 2 minutes of running for your aerobic system to fully ramp up to VO2max. This means repetitions should be a minimum of 2 minutes long, with a maximum of around 6 minutes (or 4–5 minutes at 3K pace). The recovery interval between reps should be 2–4 minutes.

Examples showing how total time at VO2max accumulates:

An exception to the 2-minute minimum applies when shorter repetitions (200–400m) are combined with very short recoveries of 30–40 seconds. VO2max remains elevated for 30–40 seconds after a repetition ends, so closely spaced short reps can accumulate effective time at VO2max — but the reps must be kept short to avoid excessive fatigue.

Short Repetitions and Intervals

Short repetitions strengthen your heart, sharpen your anaerobic system and improve leg speed.

Heart strength is improved by increasing stroke volume — the amount of blood your heart pumps with each beat. The greater the stroke volume, the more oxygen is delivered via your bloodstream to your muscles.

Anaerobic system development: When you accelerate off the start line in a 5K, your energy requirements increase immediately but your aerobic system cannot respond quickly enough — it takes 30–40 seconds for your lungs, heart and bloodstream to deliver a larger oxygen supply. During this period you rely on anaerobic energy (produced without oxygen). Short repetitions with longer recovery intervals allow you to repeatedly practise this 30–40 second anaerobic energy demand. You get better at producing anaerobic energy and better at buffering the fatigue-inducing by-products that accompany it. Note: short recovery intervals are counterproductive here, as they keep the aerobic system powered up and remove the stimulus for anaerobic development.

Leg speed: Short reps at 1500m to 3K pace recruit approximately 100% of your intermediate fibres and around 80% of fast twitch fibres — fibre types that distance runs and longer intervals do not adequately train, but which are recruited in a 5K. The result is improved leg speed.

Hill Repeats

Hill repeats involve reps lasting 30 seconds to 2 minutes, with rest intervals (jogging or walking) of double or triple the rep duration.

While workouts such as distance runs, tempo runs, VO2max reps and long hills increase the number and size of mitochondria in your muscle fibres, intense hill repeats significantly boost those mitochondria’s output of aerobic energy — maximising the gains from the foundation laid by other training.

Because hill repeats require extra force generation against gravity and shorten the distance your foot travels downward, they improve your ability to produce muscular force on the ground. The result is a quicker cadence and increased stride length — in short, you get faster.

Hill repeats also:

• Significantly strengthen all muscle fibre types
• Rewire your nervous system to recruit all fibre types more efficiently
• Increase your heart’s stroke volume

Use a hill that is challenging but not so steep it disrupts your stride. Do not run hill repeats by pace — instead target a slightly greater effort than 5K race pace (approximately 1500m–3K effort). Avoid running continuously; the rest intervals are essential.

Downhill Running

Nothing strengthens your quadriceps for running more effectively than downhill strides or downhill tempo running.

This workout involves running downhill at an accelerated pace, which increases the intensity of eccentric muscle contractions. An eccentric contraction occurs when a muscle both contracts and stretches simultaneously — this happens every time your foot strikes the ground during a stride, as your quadriceps contract to brake and support you while simultaneously lengthening to allow your knee to bend. (This contrasts with a concentric contraction, where a muscle contracts and shortens.)

There are two key reasons to include eccentric training:

1. Eccentric contractions produce substantially more force than concentric contractions, increasing the training stimulus for activated muscle fibres.
2. Eccentric contractions are associated with post-workout muscle soreness — regular exposure effectively inoculates your legs against it.

Your leg muscles, especially the quadriceps, respond by becoming significantly stronger than flat-terrain running alone would produce. Downhill running also strengthens connective tissue and improves your nervous system’s control of muscle contractions — enhancing leg stiffness, which is a measure of your leg’s resistance to bending at the ankles, knees and hips on foot strike. Less bending means less energy loss, less ground contact time and a more efficient stride. You will also develop improved knee lift, stride length and cadence.

Note that the incidental downhill running during normal distance runs is insufficient to produce these adaptations — dedicated downhill sessions are required.

Strides

Strides are short bouts of faster running with recovery in between — typically 50–150m (or 15–30 seconds) at speeds ranging from 5K to mile pace. To perform them correctly:

• Take 10–15 seconds to gradually accelerate
• Hold your top-end speed for at least 10 seconds
• Decelerate smoothly, then slow to a jog, walk or stand
• Repeat 1–5 times

Strides can be done at the end of a warm-up or tagged on at the end of a distance run (typically 2–4 strides).