Resistance-training exercises are classified according to the number of joints at which movement takes place. Isolation, or single-joint, exercises involve motion at one joint only. Compound, or multiple-joint, exercises require movement at several. This means that compound exercises work multiple muscle groups. Compound exercises include bench presses, squats, shoulder presses, rows and chin-ups. Flys, lateral raises, front raises, leg extensions and leg curls are isolation movements. Include both types of exercises in your workout to receive benefits that are specific to each.
Isolation
It is easier to isolate a specific muscle when performing a single-joint movement. During compound movements, muscles at several joints are involved and only the weakest will be stressed. For example, if you perform chinups to work the muscles of your back that extend your upper arm, you might find that the muscles of your arm that flex your elbow give out first. In this case, it is impossible to present a significant stimulus overload to your back because you won't be able to perform enough repetitions. A single-joint movement that involves the same motion at the shoulder with elbows straight would circumvent this limitation. An example would be stiff-arm pulldowns.
Efficiency
When you perform compound exercises, you work a number of muscle groups with one movement. This saves time compared to having to work each one separately. For example, performing squats for lower body involves motion at your hips, knees and ankles. Your hip extensors, knee extensors and ankle plantarflexors will all be involved. To train these muscles with isolation exercises, you would have to do prone leg raises, seated knee extensions and calf raises. This would take longer and require special equipment like a seated leg-extension machine.
Mechanical Specificity
Physical activities of living rarely involve motion at one joint only. For example, walking, running, bicycling, taking out the trash and lifting your baby all require motion at a variety of joints and involvement from multiple muscle groups. Mechanical specificity refers to the degree of similarity between a training exercise and performance. The more similar a training activity is to actual physical performance, the greater the probability that strength will be transferred.
Rehabilitation
Single-joint exercises are useful when rehabilitating an injury. Under these circumstances, it is unlikely that an injured joint will be stressed adequately with multiple-joint movements because other muscles will compensate to protect the injured area. This is acceptable when the injury is healing, but habits form within your nervous system during this time that are hard to break. Once the injured area is able to contribute again, forcing it to do so in isolation will break these tendencies and allow you to relearn how to use the muscle synergistically when others are involved.
Technique
Multiple-joint exercises require more coordination and are difficult to master. For example, learning how to perform barbell squats properly requires time, effort and, in some cases, specialized coaching. Performing exercises improperly increases the risk of injury. If you are unsure of proper technique, perform isolation exercises that involve less complex movement patterns.
References
- "ACE Personal Trainer Manual"; American Council on Exercise; 2010
- "Strength and Conditioning: Biological Principles and Practical Applications"; M. Cardinale, et al.; 2011
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