Resistance training is the single most important thing you can do for your body if you want to live an active life and slow the aging process. Muscle loss related to aging is real. If you aren’t working your muscles against resistance, you’ll begin to lose muscle mass after the third decade of life and the loss will speed up after the fifth decade of life. It’s not just muscle tissue that you lose. You’ll also see a decline in bone density. The lack of strength training increases the risk of falling and also boosts the odds of breaking a bone from bone density.
But resistance training is even more than muscle and bone health. Working your muscles against resistance is beneficial for your metabolic health. Building muscle size and strength through resistance training helps with blood sugar control. More muscle tissue provides more room to store glucose. Plus, resistance training improves insulin sensitivity. A study in the Journal of Cardiology found that strength training may be more beneficial for people with type 2 diabetes than aerobic exercise for blood glucose control. Resistance training matters for your health!
Anytime you work your muscles against resistance, it’s a boon for your fitness and health as a whole. You can work your muscles against resistance in three different ways. Let’s look at what they are and how they differ.
Isotonic Exercise
Isotonic exercise is the most common type of movement that people use to train their muscles against resistance. For example, when you do a biceps curl or a leg curl, you’re doing an isotonic movement, one where your muscle shortens or contracts against a resistance. You hold a dumbbell in your hand and shorten the muscle as you draw the weight toward your shoulders through the range of motion of the exercise.
The upward phase of a biceps curls is called a concentric contraction because you’re shortening the muscle. The downward phase is called the eccentric phase of contraction. During this phase, the muscle lengthens against resistance. (the weight in your hand), but the force on the muscle is not equal throughout the exercise. For example, the force on your biceps falls off at the top and bottom of the movement.
Another example of an isotonic exercise is doing a bench press where you push the weight up and bring it back down in a controlled manner. Your muscles are contracting and lengthening, and the muscle is actively shortening and lengthening while the resistance remains constant. Since you’re pushing against a fixed resistance, the speed at which your muscles move changes as you go through each phase of the movement. To effectively do an isotonic exercise, you need some form of resistance to work against such as dumbbells, kettlebells, or a barbell. It can even be your own bodyweight, as with a push-up. Think of isotonic exercise as being “same tension” since you’re using the same resistance throughout the exercise.
Isometric Exercise
In contrast to the shortening of the muscle with isotonic exercises, muscles and joints don’t move or change in length with isometric exercises. Your muscles generate force, but they do so at a fixed length and against an immovable object like the floor. An example is a plank where you hold your body in a fixed position and your muscles contract to resist gravity. Other examples of isometric movements are walls sits, static lunges, and glute bridges. Your muscles generate tension at one angle with these exercises since you’re in a static position.
These exercises build strength, but they have limitations. Isometric movements only build strength at the angle you’re holding your muscle. In contrast, isotonic contractions build strength throughout the muscle’s full range of motion. An advantage of isometric exercises is they don’t place as much wear-and-tear on your joints and are safer for people who have had a recent injury. Isometric exercises are popular in rehab settings because they help prevent muscle atrophy while not carrying a high risk of injury.
Isokinetic Exercise
With isokinetic contractions, muscles contract and lengthen through their full range of motion at a constant speed. No matter how much force you apply, the speed of the movement stays constant. The best example of isokinetic exercises is machine-based exercises where the machine keeps the speed of the movement constant and controlled. They do this by just countering the force you generate so that the speed stays constant.
This approach helps develop consistent strength through the full range of motion of the exercise. It’s sometimes referred to as accommodating resistance exercise since it provides muscle overload at a preset constant speed. Isokinetic exercises are safer for people just starting and carry a lower risk of injury than isotonic exercises. Like isometric exercises, they are often used in rehab and physical therapy settings.
One characteristic of isokinetic exercise is it places constant tension on the muscle throughout the exercise, unlike isotonic exercise where the tension on the muscle varies during different phases of the movement. It improves muscle strength, endurance, and flexibility in a controlled manner. According to some research, isokinetic exercise builds strength faster than isotonic movements.
Progressive Overload for Muscle Gains
One of the principles behind muscle and strength gains is progressive overload, increasing the challenge of an exercise over time. Unless the challenge increases over time, muscles adapt to the current force placed on them and fail to grow any further.
There are many ways to increase progressive overload – increase the resistance, add more reps, increase total training volume, change the tempo, alter rest periods, and more, but the most common is to increase the resistance.
Increasing the resistance works well for isotonic exercises since you’re using a weight and can switch to a heavier one, but this doesn’t work for isometric and isokinetic exercises. With isometric movements, you’re using your body weight and that’s fixed. To add more overload, you could hold the isometric contraction longer. You could also increase your body weight by wearing a weighted vest but you are limited in the ways you can apply overload.
What about isokinetic exercise? Isokinetic machines use dynamometers set to maintain a specific speed. The dynamometer responds to the force you apply on it with enough counterforce to keep the velocity of the movement constant. If you increase the resistance or the force you apply over time while the machine maintains the same velocity, you’re making progress based on the principle of progressive overload.
The Bottom Line
Most people do isotonic contractions when they train, but there are ways to build strength without moving the muscle. (isometric) and using machines that keep the velocity consistent. The latter is most useful in a rehab setting while isotonic exercises are more practical for serious muscle hypertrophy and strength gains. However, they all have their place.
References:
- Otto Buczkowska E, Dworzecki T. Mieśnie szkieletowe i ich rola w utrzymaniu homeostazy glukozy [The role of skeletal muscle in the regulation of glucose homeostasis]. Endokrynol Diabetol Chor Przemiany Materii Wieku Rozw. 2003;9(2):93-7. Polish. PMID: 14575619.
- International Journal of Cardiology. Volume 168, Issue 4, 9 October 2013, Pages 3634-3642.
- Lee SEK, Lira CAB, Nouailhetas VLA, Vancini RL, Andrade MS. Do isometric, isotonic, and/or isokinetic strength training produce different strength outcomes?. J Bodyw Mov Ther. 2018;22(2):430-437. doi:10.1016/j.jbmt.2017.08.001.
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Isometric vs. Isotonic Exercises: What’s the Difference?
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Hate Planks? Here’s Why You Should Do Them Anyway
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