As far as ergonomic design is concerned, a laptop computer is a disaster waiting to happen. If you place the screen far enough away from your eyes to minimize eyestrain, reaching the keyboard requires an uncomfortable stretch. If you place the keyboard directly under your hands, the screen is too close for comfort.
The best you can do with a laptop is a compromise between comfort, function, and attention to the positions of your hands, arms, shoulders, and head. If you can use a separate keyboard or screen, keep them far enough apart to position them correctly. If not, do the best you can to place the screen at an angle where you can look directly at the image, and keep your arms loose and your hands level. Take more frequent breaks to stretch your arms and hands, and shift position whenever you think about it.
The other potential physical problem with a laptop computer occurs when you're not using it: when it's in its case and hanging from your shoulder. Try to eliminate any unnecessary weight from the computer bag, and move the bag from one shoulder to the other often. If you have the option of using a bag with wheels instead of a shoulder bag or backpack, that may be a much better choice.
See Chapter 25 to read about accessories that can make using your laptop more comfortable.