The rationale for coordinating muscle contractions with heart rate is as follows: when your muscles contract (shorten), they temporarily restrict the flow of blood into the muscles. This means that if your heart were to contract at exactly the same time that your muscles were shortening, the heart would be trying to push blood into 'closed' muscles. It would be seem to be better to alternate muscle contractions with heart beats (i.e., let the heart beat in between muscle contractions, when muscles are relaxed and open to tidal flows of blood).

This pattern could help the heart propel a maximal amount of blood to the muscles because of another effect: as muscles contract, they force blood through the veins toward the heart. This wave of blood would increase the filling of the heart just before each beat, allowing the heart to send more energy-giving blood out to the muscles.

In theory, this alternation of heart and muscle contractions should work extremely well for cyclists, who would want to push down on a pedal just before each heart beat. However, things get complicated when you consider activities like running and walking. In those sports, the impact of footstrike creates a jarring effect on the heart which may actually help it expel more blood. An analogous situation would be banging your hand against a full bowl of water; the force of the impact sloshes water out of the bowl, just as the impact of a foot with the ground jostles blood out of the heart. Scientists call this mechanism the 'visceral-piston' effect.

For runners and walkers, then, it might be advantageous to coordinate footstrike and heart rate - letting a foot hit the ground each time the heart contracted. Anecdotal evidence supports this idea: elite athletes almost always use a footstrike rhythm of 180-190 steps per minute and a heart rate of 180-190 beats per minute during competition (although that doesn't mean that footstrike and heartbeat occur at precisely the same time). One problem with the theory is that muscular contractions tend to be fairly powerful during footstrike, which would tend to impede blood flow rather than assist

So far, the synching theory hasn't been extensively researched, although a team of investigators at Dalhousie University in Canada has just completed a useful initial effort. At Dalhousie, 12 experienced cyclists rode exercise bicycles at a constant power output of approximately 165 watts (this produced a heart rate of about 65 per cent of maximal). At that intensity, the cyclists tried out different pedalling frequencies, including a frequency which was almost exactly in synch with heart rate.

The cyclists also pedalled with whatever pedalling frequency they naturally preferred, and with a frequency which was 15 per cent higher than heart rate, 30 per cent higher than heart rate, 15 per cent below heart rate, and 30 per cent below heart rate. To give an example of how this worked, they used about 172 leg thrusts (86 revolutions) per minute while heart rate was 150 in order to pedal with a frequency which was 15-per cent higher than heart rate. In all cases, however, the cyclists' power output remained the same (as pedalling rate rose, pedal resistance decreased).

As it turned out, there was no special advantage to 'synching' heart rate with pedalling frequency; oxygen consumption remained about the same regardless of ped alling frequency. There was a trend, however, for the higher pedalling rates of 15 and 30 per cent above heart rate to be less efficient. The other pedalling rates (preferred, synched, 15, and 30 per cent below) were equally efficient.

One interesting aspect of the research was that the subjects' freely chosen pedalling rates were extremely close to their exercise heart rates. However, no physiological advantages were associated with such choices. One problem with the research is that the intensity utilized by the cyclists - about 50% V02max - was an extremely moderate, non-competitive one; bonuses in efficiency might occur if athletes tried 'synching' at higher effort levels. Also, the picture for runners remains unclear. More research will have to be conducted to determine whether runners should try to coordinate their footstrike rhythm with the thumping of their hearts. ('Effect of Cardiac-Locomotor Coupling on the

Metabolic Effciency of Pedalling, '' Canadian Joumal of Applied Physiology, vol. 18(4), pp. 379-391,
br> 1993 and Winning Rhythm, ' The Lancet, vol. 343, p.310, 1994)