Yes. Clearly.

I'd disagree w/you Lank, but I don't understand what you mean.

Amongst well-trained athletes additional improvements in aerobic fitness - above those brought about through aerobic base training - may only occur when training intensity is increased, in the form of high intensity interval training sessions at 90-100% of VO2max (Acevado and Goldfarb, 1989; Billat et al., 1999; Stepto et al., 1999). Training at this level leads to improvements in: both the aerobic and anaerobic energy pathways, improved oxygen transport, improved lactic acid clearance, Improved Lactate Threshold (LT), and improved Speed and neuromuscular co-ordination. Training at this level is particularly effective at improving the Lactate threshold, VO2max, and economy of motion.

Examples of HIT Sessions for runners:

5x2000m at 10k pace, 60 sec jog recovery

4x1600m at 5k pace, 90 sec jog recovery

5x1000m at 3k pace, 3min jog recovery

The amount of HIT sessions that can be undertaken is limited due to an increased risk of overtraining. This is because training to often at these high levels will lead to increased levels of stress hormones. Research has shown that when HIT sessions were increased to three times per week, there were increases in the level of norepinephrine (a stress hormone) amongst the athletes, indicating an increased risk of overtraining (Billat et al., 1999). It is generally recommended that HIT sessions should make up 5-10% of total training volume (Daniels, 1998; Bompa, 1999; Neumann et al., 2000). You should perform no more than two HIT sessions per week if you want to reduce your risk of overtraining.