Really it's not a deep dark mystery. Yet most golfers go along thinking that they just can not play well on the back nine. There is always an explanation for why we do the things that we do on a golf course. There is a reason why we slice and a reason why we can not make a putt. Targeting the reason for the fizzle on the back nine can seriously result in your ability to improve golf score and lowering your handicap.
Almost always the reason for poor play on the back nine is due to two factors: lack of hydration and being out of shape. The reason for the lack of hydration is naturally the intake of not enough water. This can be for a couple of reasons. Not wanting to go to the bathroom because of the inconvenience or just getting so wrapped up in the game that they forget to drink enough water and by the time the back nine is reached, they are feeling fatigue caused by the lack of hydration.
You do not need to drink water by the gallon, but you should carry a bottle of water along and take a small sip before and after you play a shot. This will keep the body supplied with the fluids it needs to function properly.
One of the first signs of dehydration is the loss of concentration. This in turn causes you to miss putts, mis-hit irons, slice and lose driving distance. Much of this can be avoided by just taking a sip of water here and there.
The other reason for feeling fatigue and playing poorly on the back nine is physical condition. To play 18 holes of golf takes a lot out of the average person physically. It even effects the professional golfers. Look at Tiger Woods, he constantly trains to stay in shape and this results to a large extent to his performance on the golf course.
I'm not saying that you need to have a fitness freak in order to play golf, but a little attention to our physical condition can go a long way to maintaining our strength from the start of the golf round to the finish.
You can get a lot of help in the area of golf fitness training from the many videos and books that are available on the market.