If you’re a regular reader of PP, you know that we continually extol the merits of high-intensity – rather than high-volume – training. That recommendation makes sense, for the following reasons:
1) Scientific research consistently identifies intensity, rather than volume or frequency of training, as the most potent producer of fitness. What the people in white laboratory coats are telling us is that if you have a choice between covering a lot of miles, carrying out a lot of workouts, or cranking up the intensity of your sessions, you’re almost always better off with the latter strategy.
2) An unassailable, cardinal tenet of training – the specificity of training principle – states that you should make the physiological and neuromuscular demands of a significant portion of your training similar to the demands placed on your body during actual competition. Competition is almost always intense, not lethargic. Therefore, at least a couple of your weekly training sessions need to be of a searing intensity as well. You simply can’t swim 400 metres, cycle 10 miles, or run 10K at your best-possible speeds if your training is focused on covering long distances at caterpillar paces.
3) To perform as well as possible, every serious endurance athlete needs to optimise three key physiological variables – VO2max (maximal aerobic capacity), lactate threshold (the exercise intensity above which the bloodstream begins to be swamped with large pools of lactate), and exercise economy (the actual oxygen cost of running, cycling, or swimming at a particular speed). Scientific research has identified the optimal intensity for VO2max upgrades to be between 90 to 100% VO2max (that is between 93 to 100 per cent of maximal heart rate). Investigations have pinpointed the superior intensity for lactate-threshold lift-offs to be about 87 to 90% VO2max (90 to 93 per cent of max heart rate). And studies have shown that the best workouts for enhancing economy are hill sessions – running uphill, cycling uphill, or swimming up: oops! Well, swimming at high speeds or against resistance will do. Note that the very best training sessions to optimise the three key physiological variables all involve very high – not moderate or low – intensities of exercise.
4) Tough, high-intensity workouts, but not moderate or easy ones, spur the pituitary gland into action, causing it to release unusually large amounts of a special chemical compound called human growth hormone (HGH). Human growth hormone builds muscles, repairs bones, nurtures ligaments, and heals tattered tendons. HGH also tends to increase fat metabolism and spare lean tissue (muscle), improving body composition and overall muscular power. Those are the kinds of effects which athletes want! Research has identified the optimal training intensity for evoking this HGH response to be above lactate threshold (above about 90 per cent of maximal heart rate). Yes, intense sessions win again!
And now there’s yet another reason to ignite your workouts with a little more intensity. Researchers at the University of Connecticut have recently determined that high-intensity training has profound effects on the nervous system, in effect stimulating it to increase the number of connections it makes with the muscular system. This improved ‘connectivity’ should heighten coordination, muscular control, and power during athletic activity.
The Connecticut investigation is a welcome addition to the existing body of training research, because almost all past studies have focused on the heart and muscles and ignored the nervous system. In the Connecticut exploration, researchers asked a group of Sprague-Dawley laboratory rats to engage in either high-intensity or low-intensity exercise for a period of several weeks. A third collection of rats remained sedentary during the training period.
To understand what the Connecticut investigators were looking for, you need to know a little bit about nerve-muscle physiology. Basically, when a nerve cell wants to tell a muscle fibre something, it ‘communicates’ by both physical and chemical means The physical part is that a branching ‘arm’ of a nerve cell – called an axon – must actually grow toward a muscle cell until the two are in extremely close physical proximity. They don’t actually touch, though, and the small gap between them is called the synaptic cleft. The place where muscle and nerve meet is called a ‘neuromuscular junction’ (NJ).
Across this microscopically small chasm – from nerve cell to muscle fibre – can move tiny drops of a chemical called acetylcholine. When a nerve cell wants to get a muscle fibre going, it starts sending the acetylcholine across the chasm. Once enough acetylcholine collects on the muscle’s membrane, the muscle ‘becomes excited’ and contracts. Without chemical stimulation from a nerve cell, a muscle would remain dormant.
Now, what happens to the neuromuscular junctions when someone undergoes training? Or, more specific to our point of interest, what happens to NJs during high-intensity training? Is it different from what goes on during lower-intensity work?
To find out, the Connecticut researchers used a special technique called immunofluorescent staining. Basically, they were able to mark the nerve axons, the acetylcholine in the axons, and the acetylcholine attachment points on muscle cells with a special antibody stain which they could detect using a laser-scanning microscope. They then carefully studied the NJs of the rats which had engaged in high-intensity and low-intensity training.
What they found was that the mode of training (high vs. low intensity) had no effect at all on the number of acetylcholine vesicles in nerve axons (vesicles are little ‘pouches’ which hold the messenger chemical), nor did it have an impact on the number of receptors on muscle cells – places to which the acetylcholine can attach and produce muscle excitation. To put it another way, high- intensity training did not make it easier for nerve cells to stimulate muscle fibres by giving them more acetylcholine or by creating more docking points for acetylcholine on the muscle fibres.
However, high-intensity training did produce a very major advantage: It created LONGER axon branches (the nerve processes that reach toward muscle cells) and dramatically increased the number of secondary axon branches, which are small offshoots of axons which can go in a myriad of directions and create new NJs with previously unconnected muscle cells. In other words, the high- intensity training allowed nerve cells to ‘reach out and touch’ an increased number of muscle fibres.
Why is this good? With greater connection of nerves to muscles comes greater control of the muscular system. With greater control comes greater coordination, and with improved coordination comes enhanced economy. With enhanced economy comes a lower cost of exercise (e.g., an ability to exercise at a lower fraction of VO2max), and therefore a capacity to move more quickly for longer periods of time without fatigue.
And with increased connectivity also comes increased power. A single nerve axon carrying a message from the nervous system might initially only provoke eight muscle cells into action, but if it has more axon branches after high-intensity training, and therefore more NJS, it could swing 12 to 15 muscle cells into play simultaneously. More force would be created per nerve-cell message, leading to heightened muscular power and faster running, cycling, or swimming speeds.
As we always like to say, high-intensity training is more than okay: It’s the way to keep training doldrums at bay, to make your competitors pay, and to smooth the pathway to personal-best performances.