The Theory of Exercise (yinyoga.com)

All forms of exercise share two features in common: first we must stress the tissues, then we must let the tissues rest. Yang tissues do better when stressed in a yang manner and yin tissues do better when stressed in a yin way. Stress has many negative connotations in our culture because we forget the "rest" part of this equation. But to have no, or little, stress in our life is just as damaging as having too much stress. We need to stress the body, and we need to rest it. There is a yin/yang balance here that leads to health. Too much of anything is not healthy. Yang exercise targets the yang tissues: the muscles. Muscles love to be rhythmically and repetitively moved. Any static holds are brief. The muscles are elastic and can take this type of exercise. However, to apply yang exercise to yin tissues could damage them. Yin tissues, being more plastic, require gentle but long-held stresses. Imagine bending a credit card back and forth one hundred and eight times every morning. It wouldn't take many mornings of this for it to snap in half. The credit card is plastic, just as our ligaments are. To rhythmically bend ligaments over and over again, as some students do when doing drop back from standing into the wheel, can, over time, damage the ligaments, just like the credit card was damaged The warning here is … do not apply yang exercise techniques to yin tissues! Applying a yin exercise to yang tissues could also be damaging! Holding a muscle in a contracted state for a long period of time is called "tetany" [1] and may damage the muscle.

Is it better to tighten muscles (yang) or relax them (yin)? That depends on your intention. We tighten our muscles to protect our joints. We relax our muscles so we can exercise our joints. What is your intention in the pose you are doing?

Many health care professionals shudder at the thought of exercising joints; they have the mistaken view that all exercise is yang exercise. Despite this concern it is possible to exercise ligaments, bones, and joints. In fact, it is necessary. However, being yin tissues means we must exercise them in a yin way. And then, please remember the important second part of this equation - we must let them rest! [2]

There is a lot of research proving the importance of stress and rest beyond just developing strength physically, but it is beyond the scope of this journey to go into it further. [3] 

Link: http://www.yinyoga.com/ys1_0.3_theory_of_exercise.php

Alzheimer’s & dementia prevention pillar #1: Regular exercise

According to the Alzheimer’s Research & Prevention Foundation, physical exercise reduces your risk of developing Alzheimer’s disease by 50 percent.

Regular exercise can also slow further deterioration in those who have already started to develop cognitive problems.

If you’ve been inactive for a while, starting an exercise program can be intimidating. But you don’t have to take up jogging or sign up for a gym membership. Look for small ways to add more movement into your day. Park at the far end of the parking lot, take the stairs, carry your own groceries, or walk around the block or pace while talking on your cell phone.

Tips for getting started and sticking with your exercise plan:

• Aim for at least 30 minutes of aerobic exercise five times per week. Try walking, swimming, or any other activity that gets your heart rate up. Even routine activities such as gardening, cleaning, or doing laundry count as exercise.
• Build muscle to pump up your brain. Moderate levels of weight and resistance training not only increase muscle mass, they help you maintain brain health. Combining aerobics and strength training is better than either activity alone. For those over 65, adding 2-3 strength sessions to your weekly routine may cut your risk of Alzheimer’s in half.
• Include balance and coordination exercises. Head injuries from falls are an increasing risk as you grow older, which in turn increase your risk for Alzheimer’s disease and dementia. Balance and coordination exercises can help you stay agile and avoid spills. Try yoga, Tai Chi, or exercises using balance discs or balance balls.
• Stick with it for a month. It takes approximately 28 days for a new routine to become habit. Once you’re over this hump, keeping up your exercise routine will feel natural. In the meantime, write realistic goals on a workout calendar and post it on the fridge. Build in frequent rewards, and within no time, the feel-good endorphins from regular exercise will help you forget the remote…and head out the door.
• Protect your head. Studies suggest that head trauma at any point in life significantly increases your risk of Alzheimer’s disease. This includes repeated hits in sports activities such as football, soccer, and boxing, or one-time injuries from a bicycle, skating, or motorcycle accident. Protect your brain by wearing properly fitting sports helmets, buckling your seatbelt, and trip-proofing your environment. Avoid activities that compete for your attention—like talking on your cell while driving. A moment’s distraction can lead to a brain-injuring thud!

The benefits of exercise

In addition to protecting against Alzheimer’s and dementia, regular exercise:

• Reduces stress
• Boosts mood
• Improves memory
• Increases energy

 Link: http://www.helpguide.org/elder/alzheimers_prevention_slowing_down_treatment.htm