Posted: November 26, 2004

Science of Sport: One-Leg Training

By Owen Anderson, Ph. D. - Copyright © 2002-2004

Most experienced athletes would never deliberately cycle for 20 kilometers using one leg only, sprint 100 meters as fast as possible on only one appendage, swim 1000 meters as quickly as possible using only a single arm for propulsion, or carry out a rowing workout with just one upper limb. Such training seems a bit ludicrous and unnatural, and it certainly does not appear to be specific to the demands of competition. Nonetheless, excellent exercise physiologists have taken a close look at such unusual efforts, and their research findings contain some very valuable and practical information for serious athletes.

For example, in a fascinating study carried out several years ago, subjects trained only one leg (by pedaling on an exercise bicycle) for 13 total sessions, 15 minutes per session, at an exercise intensity which elicited a heart rate of about 170 to 180 beats per minute (1). At the end of each week of training, the subjects were tested at the same submaximal work rates utilized at the start of the training period. Of course, this weekly testing allowed the investigator to calculate how fast the "training effects" (decreases in heart rate, drops in ventilation rate, and declines in lactate levels) were occurring.

The results obtained in this Tennessee study were extremely interesting. For example, average heart rate at the initial submaximal intensity dropped steadily over the training period, starting at about 180 beats per minute and sliding to less than 160 beats per minute after the 13th training session. Ventilation rate (in liters of air per minute) also headed south, starting initially at 75 liters per minute at the submax intensity but falling more than 20% to less than 60 liters per minute after 13 training sessions. Likewise, blood-lactate levels, which initially crested at 10.5 mmoles/liter, plummeted to less than 7 mmoles after the 13th workout.

If you are thinking that the subjects' cardiovascular and pulmonary systems had truly adapted to the training, you are not alone. After all, don't the lower heart and ventilation rates observed after the 13th training session imply that the cardiovascular and pulmonary systems had undergone major changes during the training - changes which allowed them to handle the submaximal cycling intensity with greater ease?

If that is in fact what truly happened, then the subjects should have been able to carry out the submaximal-intensity workout with their other, untrained legs and enjoy similarly low heart and ventilation rates. After all, if the heart and respiratory muscles are better, they are better, right? They should be able to handle a fixed intensity of exercise with equal aplomb, whether that exercise is carried out by a trained or an untrained leg.

Well, it didn't work out that way. When the subjects, after their 13th workout, switched over and begin pedaling away on their bikes using only their untrained legs, they had to go right back to physiological "square one." Heart rates shot up to 180 again, ventilation soared to 75 liters per minute, and lactate once more climbed to 10.5! It was as though the heart and respiratory systems were completely untrained, as though the 13 hard sessions had produced no positive effects whatsoever.

To put it another way, there was no transfer of the training effect from one leg to the other. Experiments like this one and others reveal that the heart-rate and ventilatory responses to extended submaximal exercise are determined not by specific, isolated adaptations of the cardiovascular and respiratory systems - but by the training state of the muscles involved in the exercise.

The lesson here is not that you should never engage in sustained, one-limb exercise. In fact, as you will see in a moment, there are times when strenuous one-limb exertion is exactly the right thing to do. The lesson is rather that the cardiovascular and respiratory systems are rather tightly controlled by input from the cardiovascular and respiratory "control centers" in the nervous system and also as a result of feedback from the muscles involved in carrying out a certain level of work. Basically, decreases in the number of muscle cells required to carry out a task or a reduction in the output from special neural receptors in muscles can signal the nervous system to stimulate the respiratory system and heart to lesser extents. This explains why heart and respiratory rates gradually dropped as the cyclists' trained legs responded to the 13 workouts. As the leg muscles became stronger and more efficient, there were fewer muscle cells needed for the task of cycling at a particular intensity, and the "firing rate" of the intramuscular neural receptors also dropped (fewer of these were being stimulated, since there was less total muscle action). The nervous system interpreted the activity as being lighter than before and thus decreased its stimulation of the heart and respiratory muscles.

When the untrained legs took the stage, however, the signals coming from the muscles were quite different; they were in fact just like the signals emanating from the muscles of the trained legs when they first embarked on their 13-session training program. The nervous systems of the subjects became alarmed and fired up the cardiovascular and respiratory systems - to the tune of 180-beat per minute cardiac action and 75 liter per minute ventilation.

Of course, if the heart and respiratory muscles had really been "better" after the 13 training sessions, they would not have had to be fired up to such a great extent. Since they responded as though nothing positive had happened to them during the 13 sessions, we can conclude that they did not improve at all. Our original hypothesis that the cardiovascular and respiratory systems upgraded themselves in response to the training falls apart, and we see that the fitness elevation was really a response associated with the muscles (and the nerves controlling them). The declines in heart and ventilation rates at the fixed submaximal intensity were red herrings suggesting cardiorespiratory improvement, when the real star of the show was in the neuromuscular system!

Without belaboring the point too much, we can remind ourselves that these observations point out how important it is for training to be specific to the demands of one's ultimate competitions. It does one little good to be generally fit - to have lower-than-average heart and breathing rates during prolonged and strenuous exertions which are not closely related to one's specific athletic endeavor, for example. Such phenomena are not necessarily signs of a strong heart or great respiratory action, since both heart and ventilation rates could shoot up dramatically during one's precise sporting activity if one has not carried out the exactly correct kind of training. As an example of this, a marathon runner might impress all who know him with his ability to run 20 or more miles at sub-marathon tempos with very low heart and ventilation rates, and seemingly little fatigue. This in fact should not be impressive at all, since running at his goal marathon pace might suddenly produce very high cardiorespiratory responses and high levels of fatigue, if the proper training has not been carried out. A "good-looking heart" is only good-looking in the specific circumstances in which it is displaying its wares (remember that in the Tennessee study the heart looked good with the trained leg but looked terrible when it hooked up with the other leg). Thus, it does little good for endurance athletes to work on their "cardiovascular development" in a general way; rather, they should get their heart used to providing enough red fluid for the muscles during the specific moments of competitive effort.

Since there is so little carry-over of fitness from trained to untrained legs when the action of interest is sustained, submaximal, endurance-type activity, one would expect the same to be true during less-prolonged movements which call for the displaying of strength rather than endurance. That is, if one extensively trains the muscles of one arm, there should be strength improvement in that arm only, not in the opposing appendage.

However, the reverse is actually true, and - even more amazingly - this helps to prove our principle regarding the paramount importance of the neuromuscular system and the essentiality of the specificity of training. For example, when just one arm is strength-trained using exercises such as biceps curls, a portion of the gain in strength is "transferred" directly over to the other arm, even though the other appendage has carried out no training whatsoever (2). Similarly, when athletes extensively strength-train one leg, some of the consequent increase in strength is also displayed by the other leg, even though it is completely untrained (3). Why are such transfers possible, and why do they take place with strength training - but not with endurance work?

To understand what is going on, we once again have to focus on the neuromuscular system. As it turns out, when a trained arm (or leg) gains strength, some of the augmentation of strength can be attributed to hypertrophy of the involved arm muscles; to put it simply, as the arm muscles get bigger, they can exert more force. However, some of the strength aggrandizement is the result of a change in nervous-system function. In effect, the nervous system develops an increased ability to activate and synchronize the most crucial motor units within the involved arm muscles - and also develops the capacity to more optimally inhibit those motor units and muscles which might interfere with the desired movements.

The hypertrophied muscles, of course, are able to do nothing for their friendly "twins" over in the other arm. Nervous-system activity is a completely different story, however. The nervous system is a flexible and talented collection of nerve tissue, and the activation-synchronization-inhibition patterns which are learned by the central nervous system (brain and spinal cord) in order to enhance the strength of the trained arm can be applied directly to the other arm. The nervous system is simply smart enough to say, "Hey, I learned to do this with one arm. Why not try the same scheme with the other?" Thus, the nervous system can make an untrained arm stronger; the untrained arm will never be as puissant as the trained appendage, but it will be better, even though it has not expended even one calorie on resistance exercise.

Why can't this happen during endurance activity, too? To some extent, it can, but the trouble is that during endurance activity adaptations must occur in muscles which will allow movements to be sustained for more than a fleeting moment. We're talking more mitochondria in the muscle cells, more aerobic enzymes, and even more capillaries wrapped around the muscle fibers. Without those changes taking place (and they can't take place unless a muscle group is endurance-trained), endurance activity falls flat on its face, even though the nervous system is doing its very best to strengthen and coordinate things.

What do these revelations about the transfer of strength mean to you as an athlete? As mentioned, the discoveries reinforce the strategy of specific training, because while the nervous system is a powerful and adaptable thing, it will not transfer general strength from arm to arm or from leg to leg - it will only transfer strength during specific motions from one appendage to the other. Practicing a tennis serve with your right arm will make you stronger while serving with your left arm, too, even if you complete no practice with your left limb. However, tennis-serve practice with your right appendage will not make you more forceful while bowling with your left arm, or while rowing a boat or throwing a ball.

And here are the key points: When you go the gym to make yourself "stronger," you are absolutely fooling yourself - and wasting your time - if you utilize movements (against resistance) in the gym which are dissimilar to the motions you use in your sport. The nervous system can learn and adapt, but it will only learn what you teach it; you can't expect it to learn other patterns of control to which it is only lightly exposed. Thus, strengthening movements for cycling must duplicate the actions of the legs during cycling, strengthening activities for swimming must replicate swimming strokes, force-enhancing drills for running must mimic the biomechanics of running, and so on.

Some other points should not be missed. If an injury prevents you from using an arm or leg for awhile, you don't have to stop training: You can retain strength in the furloughed appendage by training the non-injured limb intensively in a sport-specific way (remember, try to avoid a reliance on general movements which are unrelated to your sport; you want the transfer of strength from trained to untrained limb to be useful). Similarly, if you ever have the misfortune to suffer a stroke which leaves one side of your body temporarily unmovable, don't wait: Began strenuous strengthening work with the unaffected side of your body, completing the movements you most want to regain in the affected side, because some of the newfound strength in the unaffected side should transfer over and spur the recovery of your hard-hit body regions.

We mentioned earlier that there are times when you will want to train one limb at a time, instead of engaging in the tedium of synchronized, two-appendage exercise. This is true, in spite of our realizations that endurance-related fitness gains do not transfer well from one limb to the other and that strength improvements only partially move from one leg to the matching stem, or from one arm to the corresponding upper appendage.

To understand why one-leg training is sometimes desirable, consider a very recent investigation carried out at the Swimming Performance Laboratory at the National Institute of Fitness and Sports in Kagoshima, Japan (4). In this research, nine healthy, well-trained male subjects had their maximal aerobic capacities measured during both one-leg and two-leg cycling.

If this seems a little strange to you, bear in mind that any sustained activity, from playing tiddlywinks to boxing to playing squash to running, swimming, or cycling at as high an intensity as possible, is associated with a maximal rate of oxygen consumption (VO2max). You do not have to use all of your appendages when testing for VO2max - nor do you have to be engaged in a traditional endurance sport like marathon running or 40-K cycling. Whatever you do will have a max-possible rate of oxygen consumption which is specific to what you are actually doing, even if a relatively small muscle mass is involved in the activity.

When the Japanese-subjects' VO2max values were recorded, the athletes started out on their cycle ergometers at a constant 80 rpm and a work-output level of 80 Watts. The exercise intensity was then increased by 40 Watts every three minutes until the subjects were too exhausted to continue; the highest oxygen-consumption rate achieved during the test was judged to be VO2max, and the exam was completed both while pedaling with one leg and while working with the standard two legs on the pedals.

The average VO2max achieved by the nine athletes during the one-leg cycling turned out to be 3.84 Liters of oxygen per minute (or approximately 51.8 ml of oxygen per kilogram of body weight per minute). So, there should be no problem guessing what VO2max would be during two-leg exercise, right? If one leg by itself could grab 3.84 Liters of oxygen per minute (51.8 ml/kg-min) from the blood, then two legs should be able to handle double that, or 7.68 Liters per minute (103.6 ml/kg-min).

Oops! In reality, the two legs didn't do quite that well. In fact, two legs working furiously together reached a VO2max of only 4.73 Liters per minute (63.8 ml/kg-min), about 1.23 times - not double - the rate of oxygen consumption attained by one measly leg. What was going on? How could the max rate of oxygen consumption for one leg be roughly 81% as great as the 02 consumption of two legs working as furiously as possible in simultaneous fashion? And what does this tell us about how we should train?

Fortunately, these mysteries can be solved fairly readily. As it turns out, other studies have also shown that when one-leg exercise is carried out, VO2max measured during one-leg exertion is usually equal to about 75 to 80% of the VO2max measured during a regular, two-leg test (1 & 5). While this may seem strange, the mechanism is actually straightforward: It is known that there is a greater dilation of the arterioles (small arteries) leading into working muscles and thus a higher total blood flow to the working muscles during one-leg exercise, compared with two-leg effort.

Why are the arterioles so stingy when athletes attempt two-leg exercise? The problem is that if the same degree of dilation of the arterioles occurred in each leg when both legs were exercising close to maximally, the muscles' capacity for blood flow would exceed the heart's ability to supply blood, and blood pressure could fall precipitously (6). There would be too much blood in the leg muscles - and not enough in other key parts of the body.

To prevent this from happening, the arterioles become more parsimonious with the blood they are letting slip through to the leg muscles when the two legs are working synchronously. This maintains blood pressure, but it also reduces the oxygen-consumption rate for each leg, compared to the situation which prevails when a leg is working all by itself. Of course, this is why the two-leg VO2max value is not simply the sum of the one-leg max aerobic capacities.

Note that this means that one-leg exercise is not limited by the central circulation (the cardiovascular system); it is limited by what the muscles of the leg can do. No matter how intensely a single leg works, it does not exceed the capacity of the cardiovascular system to supply oxygen to sustain the work. In the Japanese study, for example, the most-intense-possible one-leg cycling still evoked just 81% of two-leg VO2max, indicating that the heart could have pumped more oxygen to the single-leg muscles, if they had been able to receive and utilize it. Similarly, the average max heart rate achieved during passionate one-leg cycling was just 174, well below the true max of about 190. In contrast, two-leg exercise was limited by the central circulation, which had to clamp down on arterioles in the legs in order to preserve whole-body blood pressure.

So, even though maximal one-leg exercise did not maximally stress the cardiovascular system, oxygen consumption took place at a rather ferocious pace during one-leg exertion. In addition, even though maximal two-leg exercise did maximally stress the cardiovascular system, oxygen consumption took place at a more subdued pace in each leg during two-leg effort.

If this seems confusing to you, putting some numbers on the situation should help. Remember that during maximal two-leg exercise with a heart rate of 190, the VO2max for the Japanese cyclists was 4.73 Liters/minute, or about 2.36 Liters per minute per leg. During maximal one-leg exercise with a comfortable heart rate of only 174, VO2max was a lofty 3.84 Liters/minute per leg. Thus, we see that during maximal two-leg cycling, each leg attained only 2.36/3.84 = 61% of its potential oxygen-consumption rate, even though the cyclists were working as hard as possible and heart rate had maxed at 190.

Now, exercise physiologists agree that one of the key ways to improve competitive fitness is to work for as long as possible at intensities close to VO2max, without overtraining. The heightened rates of oxygen consumption seem to spur key adaptive developments within muscles, including the synthesis of new mitochondria and aerobic enzymes, as well as the creation of new capillary beds. Yet, we see from our Japanese example that the athlete who is working maximally with two legs can not expect to rise above about 61% of VO2max for a single leg - and thus is not producing the maximal-possible stimulus for adaptation in the leg muscles.

This is why we mentioned earlier that there are occasions when single-leg (and sometimes single-arm) training is exactly the right thing to do. Such training is not limited by the cardiovascular system, and thus extraordinarily high rates of oxygen consumption can be reached. The heart is willing to pile on the blood and oxygen to a single limb, because blood pressure can still be maintained when a single appendage is so favored. Such colossal oxygen consumptions should produce heightened adaptations in the limb working maximally by itself, compared with the adaptations associated with two-leg exercise. Of course, after one limb is trained by itself, one should not forget to give the other limb its opportunity to train maximally, too.

What does this mean in practical terms? Every couple of weeks, a serious runner would want to - on one leg at a time - "sprint-hop" up a challenging hill. It should take at least two minutes to reach the top of the hill (otherwise, single-leg VO2max may not be reached), and the overall workout can be carried out as a series of "reps;" the athlete can sprint-hop up the hill on the right foot, jog down, sprint-hop up the incline on the left leg, jog down, go up on the right again, and so on - until tired. Oxygen-consumption rates in each leg will attain levels which would be unapproachable during two-leg workouts.

Cyclists could perform a variety of one-leg workouts, but one of the best would involve warming up and then completing three-minute, one-leg work intervals at close to max intensity, with three-minute, easy-pedal recoveries, alternating back and forth between the right and left leg for the work intervals until fatigue requires an end to the efforts. Once again, O2-consumption rates in each leg will surmount those attained during plain-old-fashioned two-leg cycling.

While such training is attractive, you can't train in one-leg fashion all the time, of course, because you need to develop the coordination and efficiency associated with the two-leg efforts you will need in your sport. Nonetheless, one-limb training gives you a tremendous "bang for your buck;" extraordinary adaptations can take place in your muscles, even though the one-leg workouts proceed with modest average heart rates. In short, we have to thank all those exercise physiologists who have carried out seemingly unusual one-limb research. Their efforts should help athletes achieve unusually high two-limb performances. ©

References

(1) Claytor, R. P., 1985. Ph. D. dissertation, The University of Tennessee, Knoxville.
(2) Medicine and Science in Sports and Exercise, Vol. 20 (5 Supplement), pp. S135-S145, 1988
(3) European Journal of Applied Physiology, Vol. 64, pp. 117-126, 1992
(4) Medicine and Science in Sports and Exercise, Vol. 32(10), pp. 1737-1742, 2000
(5) Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology, Vol. 52, pp. 976-983, 1982
(6) Exercise Physiology: Theory and Application to Fitness and Performance (Fourth Edition). (2001). Boston: McGraw Hill.

By Owen Anderson, Ph. D.

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