Arm or leg muscles are composed of two different types of muscle fibers. These are called slow twitch and fast twitch. In general terms, slow twitch muscles contract slowly and are relatively weak but will keep on contracting over and over again for a long time. They are used for endurance events such as hiking all day. The fast twitch muscles contract quickly and are very strong and but tire out quickly. They are good for lifting a cooler full of 200 # of elk meat into the back of your pickup. Each of us is born with a genetic tendency toward a different percentage of slow twitch and fast twitch. In other words, if you are born with a tendency toward a high percentage of fast twitch muscles you will not be a good endurance athlete and if you were born the other way around you will never be Mr. Universe. However, all is not lost and gone. By training you can change the percentages of each type by increasing the one you prefer for the event that is coming up. So if you were born like me you will never bench press 300 pounds but you can get up to some impressive poundages. Similarly, if you were born to be a sprinter you can do endurance training and emphasis the growth of the slow twitch muscles so that you can run a marathon but you won’t win. So what kind of training does what. The easiest way to explain it is by using the things I know a little about (very little perhaps). If you take a 20 pound dumbbell and do 20 curls and presses every day then you build up slow twitch fibers. If you take a 40 pound dumbbell and do 5 curls and presses a day you build up fast twitch. The key is what percentage of your strength do you use and how long. Light weight but lots of repetitions are endurance while heavy weight that really strain your muscles build fast twitch fibers. A different example is running or biking where if you go slow but for a long time you will increase your slow twitch fibers but if you sprint a lot you will build fast twitch fibers. A good example of extreme fast twitch muscles are the linemen on a football team. For about 15 seconds they exert all of their strength and then they huff and puff for about a minute and the ball is snapped again and they use all of their strength again and then they recover for the next play and this goes on for about five or six minutes and then the ball changes sides and they rest for five or six minutes. So, if you are planning on getting in shape for hunting in mountainous terrain then you want a lot of slow twitch muscles but if you are actually planning on killing something you will need some fast twitch to put it in the truck. Peculiarly enough, upper body strength is very important to hunting because of the weight we carry as long range hunters. Backpacks really create a lot of tiredness in your shoulders and neck. Even when you legs are not tired your shoulders and neck can be very tired and the result is you are tired. A proper fitting backpack rests on the hips but we all get sloppy with that and then we get tired. Lack of a training program leaves you twitchless. I know there are a number of people who know as much or more than I about this stuff and they are welcome to make any comments or corrections.