You don't need to do a large amount of exercises to gain great results from training. When addressing upper-body strength and muscle definition, it's pretty hard to beat the classic push-up and pull-up. These two exercises combined do a fantastic job of stimulating the entire upper-body musculature.
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For increased muscle definition, there are a number of ways to make push-ups significantly more challenging. A 2011 study from the "Journal of Strength and Conditioning Research" found that feet-elevated push-ups result in higher resistance when compared to standard push-ups. This is because these is a 10 percent increase of body weight being lifted. You can also add external load to the exercise with a weighted vest or through band resistance for an extra challenge. Performing multiple push-up variations in a variety of different repetition ranges can optimize the resulting muscle definition.
Pull-ups performed with a full range of motion are also a great upper-body muscle builder. Each repetition should be completed with the chest touching the bar to get the most out of the exercise. A 2010 study published in the "Journal of Strength and Conditioning Research" concluded that pull-ups were superior to chin-ups in terms of lower trapezius and latissimus dorsi activation, making them the best option for back development. For best results, hold a one-second pause at the top of each rep to increase tension in the back and arm muscles.