Background: In exercises such as weight lifting, skeletal muscle contracts against a resisting force (see Figure below ). Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it.

Situation: Michael has become bored with his passive life style, and has decided to take up weight lifting as a hobby. He begins to exercise with weights several times a week. Michael's brother, Robert, isn't as motivated as Michael though. Robert decides to continue being mostly inactive and only exercises with weights every couple of months.

Question: Who will increase the size of their muscles more?
Answer: Michael

Situation: Michael has become bored with his passive life style, and has decided to take up weight lifting as a hobby. He begins to exercise with weights several times a week. Michael's brother, Robert, isn't as motivated as Michael though. Robert decides to continue being mostly inactive and only exercises with weights every couple of months.

Question: Who will increase the size of their muscles less?
Answer: Robert

Situation: Michael has become bored with his passive life style, and has decided to take up weight lifting as a hobby. He begins to exercise with weights several times a week. Michael's brother, Robert, isn't as motivated as Michael though. Robert decides to continue being mostly inactive and only exercises with weights every couple of months.

Question: Who will increase the strength of their muscles less?
Answer: Robert

Situation: Michael has become bored with his passive life style, and has decided to take up weight lifting as a hobby. He begins to exercise with weights several times a week. Michael's brother, Robert, isn't as motivated as Michael though. Robert decides to continue being mostly inactive and only exercises with weights every couple of months.

Question: Who will increase the strength of their muscles more?
Answer: Michael

Situation: Justin and Derek are accomplished competitive weight lifters. One day, Derek suffers a severe injury to his leg. Because of this injury, Derek is forced to stop exercising for several months. Justin remains injury free and continues to work out regularly on the other hand.

Question: Who will lose more size on their muscles?
Answer: Derek

Situation: Justin and Derek are accomplished competitive weight lifters. One day, Derek suffers a severe injury to his leg. Because of this injury, Derek is forced to stop exercising for several months. Justin remains injury free and continues to work out regularly on the other hand.

Question: Who will lose less size on their muscles?
Answer: Justin

Situation: Justin and Derek are accomplished competitive weight lifters. One day, Derek suffers a severe injury to his leg. Because of this injury, Derek is forced to stop exercising for several months. Justin remains injury free and continues to work out regularly on the other hand.

Question: Who will lose less strength in their muscles?
Answer: Justin

Situation: Justin and Derek are accomplished competitive weight lifters. One day, Derek suffers a severe injury to his leg. Because of this injury, Derek is forced to stop exercising for several months. Justin remains injury free and continues to work out regularly on the other hand.

Question: Who will lose more strength in their muscles?
Answer: Derek

Situation: Usain was once a competitive swimmer, but he became bored with the sport and decided to start running very frequently instead. Usain tried to convince his brother, Asafa, to take up running as well. Asafa didn't really like running, though, so he hardly ran much at all.

Question: Who will increase the strength of their cardiac muscles more?
Answer: Usain

Situation: Usain was once a competitive swimmer, but he became bored with the sport and decided to start running very frequently instead. Usain tried to convince his brother, Asafa, to take up running as well. Asafa didn't really like running, though, so he hardly ran much at all.

Question: Who will increase the strength of their cardiac muscles less?
Answer: Asafa