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When to Use Heavy Weights vs. Light Weights

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So you're standing at the weight rack, eyeing everything from 1 lb handweights to barbells, kettlebells, and more. Don't just grab something at random—what you lift has a significant effect of what kind of results you'll see long after you leave the gym.

One of the most critical components to an effective resistance training routine is determining the right amount of weight to lift. And the specific goal for a resistance training workout has an important bearing on how much load will be used and how many reps will be performed. For example, someone who wants to increase the size of her muscles—known as muscular hypertrophy—will generally focus on performing a lower number of repetitions (for example, 4-6) using heavier weight; someone who's aiming to improve their general muscular fitness will perform a higher number of repetitions (for example, 12-15) using lighter weight. (See more Ask the Celebrity Trainer: High Reps and Light Weights vs. Low Reps and Heavy Weights?.)

However, despite individual training goals, there are some general resistance training concepts that apply to all exercisers. The first is the principle of progressive overload, a fundamental training principle which essentially states that in order for the body to continue to make gains—especially in terms of muscular strength and endurance—the body must be continually challenged in ways that forces it to adapt.

When it comes to resistance training, one of the most common mistakes many people make is to complete the same workout—made up of the same exact exercises using the same amount of weight—over and over again (sometimes even for years!), with the hopes of seeing increased muscular strength and endurance changes in the body. While this can be a great approach to maintaining fitness, if your goal is to make new gains, you have to apply the principle of progressive overload in order to keep the body sufficiently challenged. One way to accomplished this is to gradually increase the stress that's placed on the body. (But be careful of The 5 Exercises Most Likely to Cause Injury.)

So how do you know if you’re lifting the right amount of weight? As a general rule of thumb, if you’re using adequate load during an exercise, ideally, the last one or two repetitions of a set should feel challenging to complete with proper form. This goal of fatiguing the muscles during each set of an exercise should take place within 90 seconds—regardless of your personal fitness goal—in order to keep the body sufficiently challenged and to reap the maximum training benefits.

As you probably notice during your resistance training workouts, not all movements and muscle groups utilize the same amount of weight in order to reach muscular fatigue. For example, compound movements which involve several muscle groups across multiple joints (like squats) typically warrant a heavier load than when you're performing more of an isolation-based exercise (such as leg extensions), which predominantly target a single muscle group at a specific joint. This helps to explain why, when you’re choosing which weight to use when performing some of your favorite shoulder sculpting exercises, you opt for the heavier pair of dumbbells during a shoulder press compared to the lighter set you might use when performing lateral raises.

Developing a better understanding for what muscles and joints are involved in each exercise can prove helpful in determining the right amount of weight to use, in addition to following the general rule of fatiguing the muscles by the last one to two reps. Below is a breakdown of a few compound exercises as well as isolation-based exercises that can help get you started on the path to choosing the appropriate amount of weight for you. Do bear in mind, however, that your personal fitness goals as well as your current level of conditioning also have a great influence on the appropriate amount of weight to use as well as on the overall structure of your resistance training regimen. (New to strength training? See these 8 Reasons Why You Should Lift Heavier Weights.)

Examples of exercises to utilize heavier weight (compound exercises):
Dumbbell front squats
Barbell lunges
Bench press
Barbell rows

Examples of exercises to utilize lighter weight (isolation-based exercises):
Dumbbell lateral raises
Concentration curls
Triceps kickbacks
Dumbbell flyes
Dumbbell front raises


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