Just how hot should your warm-up be?
There is considerable disagreement among athletes and coaches about how intense a warm-up should be. Current thinking, however, suggests that a significant portion of the warm-up should be very intense if the ensuing workout or competition is to be carried out at a high intensity. One theory is that ‘firing up’ the brain and spinal cord with very high-level activity will prepare the nervous system to coordinate the muscles more efficiently during the top-quality work which is to follow. Another reasonable hypothesis is that high-intensity effort during warm-up does a better job of warming up the muscles, and thus of promoting flexibility, which may enhance movement and decrease the risk of muscle and tendon strains.
Little is known about the effect of warm-up intensity on range of motion at key joints in the body (‘flexibility’), and nothing is known about the link between warm-up intensity and the risk of injury. The research examining the relationship between warm-up and range of motion has produced very equivocal results. In a pair of studies, 15 minutes of warm-up on a cycle ergometer at a very modest intensity of 50 Watts and 15 minutes of similar effort combined with stretching exercises produced increased range of motion in the hip area, compared with no warm-up. By contrast, another investigation found that 20 minutes of exercise at a higher intensity of 86% of maximal heart rate had no significantly positive effect on range of motion at the knee.
In order to improve understanding of the effects of warm-up intensity on range of motion and athletic performance, particularly during high-intensity exertions, scientists at the University of British Columbia in Canada and the University of Otago in New Zealand recently studied nine male senior rugby-union players, with an average age of 22, mean weight of 176lb and average VO2max of 60.4 ml/kg-min.
Opening the hydrogen floodgates
The athletes completed four separate trials in the laboratory on four separate days. Each trial started with a range-of-motion evaluation of the hip, knee and ankle, followed by 15 minutes of warm-up treadmill running at 60, 70, or 80% of VO2max (three trials) or no warm-up at all (the fourth trial). In all four cases, a series of stretches for the leg muscles were then carried out for three minutes, followed by a second range-of-motion evaluation and then an all-out test on the treadmill. For the no-warm-up (control) trial, the athletes simply sat in chairs for 15 minutes before completing their three minutes of stretching. The popular proprioceptive neuromuscular technique of ‘contract-relax’ stretching was used in all cases, and special efforts were made to unkink the hamstrings, hip flexors, quads and calf muscles. For the all-out treadmill test which followed the stretches, the athletes simply ran for as long as possible at a velocity of just 13 km/hour – but on a very punishing 20% incline.
As it turned out, the active warm-ups really did warm up the athletes’ bodies compared with the control situation. Specifically, the 60 and 70% VO2max warm-ups elevated body temperature (measured via rectal probes) by almost a full degree centigrade, while the 80% VO2max effort upped temperature by another half-degree. Heart rate followed a similar trend, with average heart rate highest during the 80% treadmill effort, significantly lower in the 70% run, lower still in the 60% run and lowest of all in the control situation. However, because three minutes of stretching followed the warm-up runs and preceded the all-out tests, heart rates were about the same in all the active groups when the all-out tests began.
Stretching alone had no effect at all on range of motion, since there was no increase in range of motion in the control situation. However, ankle dorsiflexion (a movement which stretches the Achilles tendon and calf muscles) and hip extension were strongly promoted by all three warm-up intensities. By contrast, knee flexion (a measure of quadriceps flexibility) was not enhanced by any warm-up condition, and hip flexion (an indication of hamstring flexibility) was augmented only by the 80%-VO2max warm-up.
Why was the highest-intensity warm-up less effective than those of lower intensity? One possible explanation is that the more strenuous warm-up was more fatiguing for the athletes, but this would be a very tenuous conclusion to reach. Because stretching and range-of-motion measurements followed the warm-ups, the athletes did not start their all-out tests until at least five minutes after warm-up. And don’t forget that, when the all-out tests started, heart rates were exactly the same in all the active groups, so feelings of fatigue should also have been similar.
A more likely explanation is that the 80% VO2max warm-up was above the lactate thresholds of many of the athletes and, as a result, lactic acid might have accumulated in the athletes muscles’ to a greater extent than during the lesser efforts. The extra hydrogen ions dropped off in the muscle cells’ internal milieus by the lactic acid might not have been ‘cleared’ in the five-minute period before the all-out test, and these hydrogen ions could have slowed down the breakdown of glucose for energy during the inclined treadmill running, in effect leaving the athletes with less energy to complete their all-out tests. The extra hydrogen ions might also have made it more difficult for the muscle cells to actually contract, potentially leading to premature fatigue in the high-intensity group.
Does this mean you should not exercise intensely during your warm-up if you intend to perform intensely during your workout or race? Absolutely not! Bear in mind that the warm-ups used by the Canadian and New Zealand researchers were continuous in nature, and there is no reason for you to emulate this continuity. If the trouble with the 80% VO2max warm-up really was that it dripped too many hydrogen ions into the muscle cells, this effect could be circumvented merely by breaking the warm-up up into discrete components, with intense activity alternating with easy effort. A period of 15 continuous minutes at 80% VO2max is certainly not necessary in order to fire up the nervous system prior to exercise, nor is it needed to elevate heart rate appropriately. For the close-to-one-minute all-out effort used in this study, a 15-minute warm-up including 3-4 30-40-second high-intensity segments interspersed by easy exercise for the remaining time would have been entirely appropriate – and less likely to open the hydrogen floodgates.
Intensity is not the prime determinant of flexibility
The highest-intensity (80% VO2max) warm-up seemed to be best for promoting range of motion during hip flexion, which is not surprising since faster running does a better job of stretching out the hamstrings than slower efforts (forward swing of the leg occuring to a greater extent and at a higher velocity). Otherwise, the various intensities were about equally effective at loosening up the legs, indicating that intensity per se is not the prime determinant of flexibility.
What does this research mean to you as an athlete? Bear in mind that a warm-up should prepare you specifically for what you need to do in your race or workout. A 15-minute continuous warm-up at 80% VO2max, followed by five minutes of quiescence, bears little resemblance (either in time or intensity) to an above-VO2max effort like the one used in this study, and thus would not be the optimal warm-up for such an exertion. We shouldn’t reject warm-ups with high-intensity pieces simply because a continuous high-intensity warm-up didn’t work out so well in this research. As mentioned, a better match would have been a warm-up which included 3-4 short (30- to 40-second) segments at the goal intensity, which was well above VO2max; these short efforts would not have piled up the hydrogen ions and would have forced the nervous system to get ready for a super-high-intensity situation.
For races or workouts lasting for longer than one minute, it would be entirely appropriate to include longer warm-up segments at the lower intensities used for such prolonged efforts. If enhanced range of motion is truly desired, one could also add special dynamic drills to the warm-up which would take the key joints of the legs through broader motions than those associated with the basic acts of running, cycling and or walking; these more expansive movements should do a better job of ‘loosening up the legs’.