Comeback Trail - Page 2
Matters of the Heart
During detraining, rapid and dramatic changes occur in the heart, which is the engine driving your endurance. After only four to seven days of no running, the heart's left-ventricle dimensions, wall thickness and mass begin to decrease, contributing to less blood being pushed through the heart.
Equally as dramatic are the associated reductions in maximal oxygen uptake. After two to three weeks of detraining, Sharon's VO2max, or maximal oxygen uptake (the body's ability to process oxygen), decreased four percent, causing her race times to slow by about two to five percent. For ultrarunners who take time off, the longer the intended race, the more noticeable the drop in endurance.
haron's V02max decline was less steep than Joe's because she had less fitness to lose and was still active. A study of female track athletes who took three months off after their racing seasons showed they lost 15 percent of the VO2max they had reached during the race season. This means that Sharon's aerobic fitness, after six months' vacation, has returned to its original level of pre-training fitness—she's lost all her cardiovascular conditioning.
Joe is even worse off. One study found that completely bedridden people lost their VO2max faster than if they were moderately active. This same study found that VO2max declines by a staggering 27 percent, a reduction of about one percent of VO2max per day over three weeks. Thus being bedridden encourages a much steeper decline than just being moderately active and not training. And the VO2max of very fit runners declines more and faster than those who are less fit.
Joe's sub-maximal heart rate began to decline after the first 20 days of bed rest, and his sub-maximal stroke volume (the amount of blood ejected from his left ventricle with each heart beat) declined 25 percent. His maximal cardiac output (the maximum amount of blood pumped from the heart in one minute) also dropped 25 percent. The result was a decreased ability to transfer oxygenated blood from the lungs to the working muscles and return carbon dioxide to the lungs.
If this isn't disheartening enough, after two to four weeks of detraining, both Joe and Sharon's blood volume—and therefore oxygen-carrying capacity—decreased nine percent.
Also, their muscles lost capillaries, the small arteries that deliver oxygen to the working muscles (contributing to lower VO2 levels).