have protective effects on the brain.
There are psychological explanations, too. Exercise may
boost a depressed person’s outlook by helping him return to
meaningful activity and providing a sense of accomplishment.
Then there’s the fact that a person’s responsiveness to stress is
moderated by activity. “Exercise may be a way of biologically
toughening up the brain so stress has less of a central impact,”
It’s likely that multiple factors are at play. “Exercise has such
broad effects that my guess is that there are going to be multiple
mechanisms at multiple levels,” Smits says.
So far, little work has been done to unravel those
mechanisms. Michael Lehmann, PhD, a research fellow at
the National Institute of Mental Health, is taking a stab at the
problem by studying mice — animals that, like humans, are
vulnerable to social stress.
Lehmann and his colleagues subjected some of their
animals to “social defeat” by pairing small, submissive mice
with larger, more aggressive mice. The alpha mice regularly
tried to intimidate the submissive rodents through the clear
partition that separated them. And when the partition was
removed for a few minutes each day, the bully mice had to
be restrained from harming the submissive mice. After two
weeks of regular social defeat, the smaller mice explored less,
hid in the shadows, and otherwise exhibited symptoms of
depression and anxiety.
One group of mice, however, proved resilient to the stress.
For three weeks before the social defeat treatment, all of
the mice were subjected to two dramatically different living
conditions. Some were confined to spartan cages, while others
were treated to enriched environments with running wheels
and tubes to explore. Unlike the mice in the bare-bones cages,
bullied mice that had been housed in enriched environments
showed no signs of rodent depression or anxiety after social
defeat (Journal of Neuroscience, 2011). “Exercise and mental
enrichment are buffering how the brain is going to respond to
future stressors,” Lehmann says.
Lehmann can’t say how much of the effect was due to
exercise and how much stemmed from other aspects of the
stimulating environment. But the mice ran a lot — close to 10
kilometers a night. And other experiments hint that running
may be the most integral part of the enriched environment,
Looking deeper, Lehmann and his colleagues examined the
mice’s brains. In the stimulated mice, they found evidence of
increased activity in a region called the infralimbic cortex, part
of the brain’s emotional processing circuit. Bullied mice that
had been housed in spartan conditions had much less activity
in that region. The infralimbic cortex appears to be a crucial
component of the exercise effect. When Lehmann surgically cut
off the region from the rest of the brain, the protective effects of
exercise disappeared. Without a functioning infralimbic cortex,
the environmentally enriched mice showed brain patterns and
behavior similar to those of the mice who had been living in
Humans don’t have an infralimbic cortex, but we do
have a homologous region, known as cingulate area 25
or Brodmann area 25. And in fact, this region has been
previously implicated in depression. Helen Mayberg, MD, a
neurologist at Emory University, and colleagues successfully
alleviated depression in several treatment-resistant patients
by using deep-brain stimulation to send steady, low-voltage
current into their area 25 regions (Neuron, 2005). Lehmann’s
studies hint that exercise may ease depression by acting on
this same bit of brain.
Getting the payoff
Of all the questions that remain to be answered, perhaps the
most perplexing is this: If exercise makes us feel so good, why is
it so hard to do it? According to the Centers for Disease Control
and Prevention, in 2008 (the most recent year for which data
are available), some 25 percent of the U.S. population reported
zero leisure-time physical activity.
Starting out too hard in a new exercise program may be one of
the reasons people disdain physical activity. When people exercise
above their respiratory threshold — that is, above the point
when it gets hard to talk — they postpone exercise’s immediate
mood boost by about 30 minutes, Otto says. For novices, that
delay could turn them off of the treadmill for good. Given that,
he recommends that workout neophytes start slowly, with a
moderate exercise plan.
Otto also blames an emphasis on the physical effects
of exercise for our national apathy to activity. Physicians
frequently tell patients to work out to lose weight, lower
cholesterol or prevent diabetes. Unfortunately, it takes months
before any physical results of your hard work in the gym are
apparent. “Attending to the outcomes of fitness is a recipe for
failure,” he says.
The exercise mood boost, on the other hand, offers near-
instant gratification. Therapists would do well to encourage
their patients to tune into their mental state after exercise,
Otto says — especially when they’re feeling down.
“Many people skip the workout at the very time it has the
greatest payoff. That prevents you from noticing just how
much better you feel when you exercise,” he says. “Failing
to exercise when you feel bad is like explicitly not taking an
aspirin when your head hurts. That’s the time you get the
It may take a longer course of exercise to alleviate mood
disorders such as anxiety or depression, Smits adds. But the
immediate effects are tangible — and psychologists are in a
unique position to help people get moving. “We’re experts
in behavior change,” he says. “We can help people become
motivated to exercise.” n