The following is from an email from Owen Anderson . . .

exercise also has many abrupt transition points. For humans, the
movement speed of 2 meters per second represents one such precipitous
passage. At all velocities of 2 meters per second or less (e. g., at
all tempos of 13:24 per mile or slower), walking requires less energy
than running, and so we almost always walk at such speeds.

If we want to move faster than 2 meters per second, we invariably jog
or run, because running is more economical than walking at such
tempos. We automatically adjust gait to minimize the energy cost of
locomotion, and so we rarely see individuals jogging at 15-minute per
mile pace - or walking at 10 minutes per mile.

Another important transition occurs at a walking speed of about 5
kilometers per hour (a tempo of around 19 minutes per mile). Exercise
scientists have known for years that if one plucks an average person
"off the street" and asks him/her to walk "normally", he/she will
usually settle in at a pace of about 4.8 to 5 kilometers per hour.
This is nothing new: Human footprints left in Kenyan/Tanzanian mud two
million years ago suggest that these first imprint-leavers were
strolling through East Africa with an alacrity of 5 kilometers per
hour, too!

Not surprisingly, the velocity of 5 kilometers per hour is an
important threshold point. Above 5 kilometers per hour, the oxidation
of carbohydrate by leg muscles increases dramatically, and as a result
perceived effort rises significantly. Below 5 kilometers per hour,
carb-burning falls off, fat breakdown ascends, and perceived effort
moderates considerably. The human brain monitors carbohydrate
oxidation during exercise quite carefully - and rather perversely
cranks up perceived effort when carb-burning is on the upswing. In
effect, the brain tries to keep exercisers from burning up their
carbohydrate (glycogen) stores by making the process of doing so feel
too difficult. This is a key reason why sustained runs at a high
intensity such as vVO2max feel so incredibly hard. Nothing bad is
really happening to your muscles at vVO2max - it's just that the brain
doesn't like such red-hot exertions, given its constant worries over
the glycogen depots in the mu! scles. The brain is content at a pace
of ~5 kilometers per hour because carb-burning is minimal, and thus 5
km/hour is a universal walking speed.

An extremely important transition point during running is called the
lactate threshold, or lactate-threshold speed, which happens to be an
excellent predictor of running peformance. Lactate-threshold speed is
simply the running velocity above which lactate begins to accumulate
rapidly in the blood.

Historically, the lactate threshold was thought to be caused by a lack
of oxygen in the muscles, which thus forced the sinews to do more
anaerobic work, subsequently leading to a release of lactate into the
blood. The remedy for a lackadaisical lactate threshold was usually
thought to be high-mileage training, which was supposed to enhance the
functioning of the cardiovascular system and improve the delivery of
oxygen to the muscles (and the utilization of oxygen once it got
there). In theory, this expansion of aerobic capacity would cure a
languid lactate threshold.

However, such conceptions ignored the simple and unavoidable facts
that lactate threshold occurs at just 50 percent of max aerobic
capacity in many untrained individuals and at 85 percent of max
aerobic capacity in many elite runners - in other words in situations
in which oxygen is quite plentiful and the
oxygen-delivery-and-utilization system has not been taxed to its limit.

Recent research suggests that the real "problem" which produces the
lactate threshold actually is unrelated to oxygen delivery and in fact
resides in the "shuttle systems" which exist in the walls of
muscle-cells' mitochondria. To understand how this works, it is
important to know that within muscle cells molecules of an important
chemical called NAD work as "carriers". NAD's job is to pick up
high-energy hydrogens (which have been stripped off carbohydrate
molecules, for example) and then carry them to the "shuttle
mechanisms" in the walls of the mitochondria. The hydrogens can then
leave NAD, shuttle through the mitochondrial walls, and move inside
the mitochondria; in the presence of oxygen, the energy contained in
the hydrogens is then transformed into ATP, the actual energy muscles
need to perform the work of running. If the shuttle mechanisms are
operating too slowly during exercise, NAD takes some of its hydrogens
which should have been dropped off at the mitochondr! ial walls and
shuttled inward and instead donates them to a chemical called
pyruvate, thus forming lactic acid. As lactic acid accumulates, it can
begin pouring out of the muscles into the blood, thus creating a
lactate threshold.

As you can see, the formation of lactic acid can occur independently
of whether a muscle cell has adequate supplies of oxygen, and
lactate-threshold velocity is probably much more a reflection of
mitochondrial shuttling ability, rather than oxygen supply. If the
shuttles are working slowly and a runner is attempting to move
swiftly, lots of lactic acid will be formed (and the runner's lactate
threshold will be lousy).

To improve lactate threshold, then, your task as a runner is to
upgrade your mitochondrial shuttles. Running lots of miles at a
moderate pace just won't do the trick, because your shuttles can
handle such exertion quite easily (inward hydrogen movement through
the shuttles is moderate during moderate running, and so there is no
stimulus for the shuttles to undergo a make-over). What you need
instead is training which really taxes your shuttles.

One such workout involves going to your favorite place for a workout,
warming up thoroughly, and then alternating one-minute intervals at
close-to-max speed with two-minute, easy-jog recoveries. You don't
have to worry about your actual speed during the one-minute bursts
(just shoot for a pace which feels faster than your vVO2max), and you
shouldn't kill yourself with the session - the idea is to just keep
working until you feel satisfactorily tired. Your shuttles will be
extremely stressed by the repeated one-minute fly-outs - and the
resulting shuttle make-over will have you flying along in your races
as your high-energy hydrogens pour into your mitochondria.