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This isn't necessary, but it will be very helpful, especially if you're new to weight training. A personal trainer can talk with you about your goals and design workouts to help you reach them. Most gyms have personal trainers on staff, so ask about these services at your gym. Make sure you inform a personal trainer about your diabetes. This will help him design workouts that will work best for you. It is also important for safety, because your trainer can monitor you for signs of hypoglycemia or other complications. When working with weights, aim to do two or three sets of 8-12 repetitions within each set. Let your body rest for about a minute in between sets. When lifting, breathe out when you perform the lift or exertion, and breathe in when returning to your start position. You can apply these general guidelines to any weight-based workout. This focus will give you faster results over your whole body. The major areas you should concentrate on are the back, chest, arms, and legs. There are several workouts you can do to train each of these areas and increase your overall muscle mass. While beginners to weight training often overlook the back, it is important for balanced upper body strength. There are large muscles here that can be strengthened and toned with several good workouts.  Pull-ups. This simple exercise can be performed with only a bar and no other equipment. It trains the muscles of your upper and mid back. Grasp the bar with your hands roughly shoulder-width apart, then pull your chin over the bar. Make sure you keep your shoulder blades pinched together to ensure that you are keeping the workout focused on your back muscles. As an added bonus, pull-ups will strengthen your biceps as well. See Do Pullups for a more detailed description of performing pull-ups. Lat Pull-down. This workout is similar to a pull-up, except it is done sitting down with a machine. Grab the bar with your hands a bit wider than shoulder-width apart. Then pull the bar down to your chest and smoothly raise it again. Cable row. With this workout, you'll sit down on a bench and pull a weighted handle towards you. This workout trains the middle back, as well as the biceps. Click here for a video description of this workout. Shrugs. This workout will train your trapezius, the muscle between your neck and shoulders. To perform a shrug, stand with your feet shoulder-width apart. Hold a barbell, or a dumbbell in each hand. Then raise your shoulders to your ears. The chest contains the pectoralis major and minor muscles. These are large muscles that can add a lot of mass if trained properly. Try the following workouts to increase the size and strength of your chest.  Pushups. You don't need any equipment for this workout, just the floor. It will train your chest, as well as your triceps. Read Do a Push Up for a detailed description of proper technique and variations of this workout. Pushups are also a good way to warm up before lifting heavier weights. Dumbbell press. This is similar to a pushup, except it is done on a bench with dumbbells. Lay back on a bench with a dumbbell in each hand. For your starting position, hold the dumbbells just above your shoulders. Then press your hands up and have the dumbbells touch above your chest before returning to the start position. Pectoral Flyes. This workout is done with a machine. You sit upright on a bench and place your hands on the pads or handles that will be on each side of you. Then you push forward and have your hands meet in front of you. This exercise isolates the chest more than pushups or the dumbbell press, so use it if you want to focus on the chest. The two major muscle groups in your arms are the biceps and triceps. You should train both groups to strengthen your arms and build muscle mass. Several of the exercises already mentioned will also train your arms: pullups and pull downs will train your biceps, and pushups and dumbbell presses will train your triceps. In addition, there are other exercises that will isolate the arm muscles and train them specifically.  Bicep curls. With this exercise, you will isolate the biceps. You can perform this exercise standing or sitting. Take a dumbbell in each hand and start with your arms at your sides. Then bend your arms at the elbows, bringing your hands up to your shoulders. Click here for an instructional video demonstrating a proper curl. Tricep push downs. This exercise isolates the triceps and is done with a machine. Grab either a bar or rope attached to a weight. Your starting position should be with your hands in front of you about chest-height. Then, using only your elbows, push the bar down to your hips before returning to the start position. Some of the biggest muscles in your body are in your legs, so training this area will give you a big increase in muscle mass. There are numerous workouts you can choose from, all of which will train your legs in different ways.  Squats. This workout primarily trains the quads and glutes, but it also works the entire leg and lower back. Read Do a Squat and watch this video to learn the proper technique before trying this. It would also be a good idea to consult a trainer at the gym to instruct you- it is a great workout, but you can seriously hurt your knees, back, and neck if you don't use proper form. Leg extensions. This workout trains the quads. It involves placing your feet into a machine. Then you extend your legs forward, which will lift a weight attached to a cable.  Lunges. This workout will train your hamstrings, on the back of your leg. To perform the workout, hold a dumbbell in each hand. Then stand with your feet shoulder-width apart with your arms at your sides. Take a big step forward with one leg, then bring your other leg forward so you are standing with your feet shoulder-width again. Read Do a Front Lunge Exercise for more details on this workout.  Calf raises. This workout will train your calves, which are on the lower part of your leg. They can be done either with a machine or by just holding a weight. To perform the workout, place your feet halfway onto a ledge so your heels dangle off the back. Then, either holding a weight or using a machine, shift your weight to the balls of your feet and lift your heels up. Then return to the starting position with your heels dangling off the side of the ledge. When you workout with the same exercises for several weeks, your muscles start to adjust, and the exercises won't give you great results anymore. This is called a plateau. To avoid this, don't let your muscles get used to the workout. Perform only one or two of these suggested workouts per muscle group at a time. Then in a few weeks, replace the exercises with a new one. By keeping your muscles guessing, you'll ensure that you see maximum results from your workout regimen.
Consider using a personal trainer. Learn the general rules for weight training. Focus on the largest muscle groups. Train your back. Train your chest. Train your arms. Train your legs. Vary your workouts.