The upper body doesn’t have the luxury of a built-in workout that your legs do in walking from place to place, so requires targeted exercise. Strengthening your upper body, especially the shoulders and back, will give you the posture to fight unnecessary injuries and maintain form in lengthy, grueling workouts. 

Strong and correct posture is important for alignment and to prevent back pain even when you’re not working out. Toning the back and shoulder muscles, strengthening them to support the top half of your body is a great way to achieve proper posture.

Strong arms will also help propel you in other exercises that rely heavily on your legs. Strong arms and shoulders can give runners an essential boost of speed and a solid upper body can help them maintain balance. Exercise your upper half to get that agility and final burst of swiftness as your approach the finish line.

Working the upper body also has its benefits in strengthening abs and core muscles, and helping you burn calories at a faster rate. At rest, muscle tissue burns the body’s calories faster than fat does, so toning your entire body will create a more efficient way to burn calories. Additionally, most upper body exercises require a tightened core to keep balance and form, so pumping iron for your uppermost muscles will also enhance your abs.

When people think toned arms, they mostly think biceps and triceps, but these other upper body muscles are just as important to work:

Important Muscle Groups that women tend to ignore:

Neck and Upper Back (Trapezius): This layer of muscle runs across your upper back, through your shoulders and up your neck, and is used during daily activities like supporting and turning your head. Lugging a heavy purse around all day on one shoulder can cause some major muscle imbalances within your trapezius.

Exercises to target these muscles: Rows and Lateral Raises

Spinal Extensors(Erector Spinae): This bundle of muscle and tendons line and support the spine, most notably at the lower back. These muscles are used in everything from maintaining proper spinal alignment to flexing and extending the spine during daily activities like picking things up off the floor.

Exercises to target these muscles: Cobra pose and Swimmers

Rotator Cuff: Women generally have limited upper-body strength, so once you’ve started working out, it’s important to strengthen the link between your arms and your trunk—the rotator cuff. Strengthening the rotator cuff will help limit shoulder injuries for those that are beginning or advancing their upper-body strength training.  

Exercises to target these muscles: external and internal rotations, moving up and down or in and out.

Shoulders and Mid Back(Deltoid): Most women work their shoulders by doing a pressing movement or a side raise and neglect the posterior portion. Strengthening your rear deltoid muscles helps to pull your shoulders back. 

Exercises to target these muscles: Bent over Delt Fly

http://www.fitday.com/fitness-articles/fitness/this-is-your-brain-on-video-games.html

http://www.shape.com/search/site/biceps%20and%20triceps