Explaining the difference between eccentric and concentric muscular contractions can be difficult but necessary when devising programs for improved health and fitness. Both are types of isotonic muscular contractions as opposed to static, held, or isometric muscular contractions.
Concentric contractions of the biceps brachii muscle in the bicep curl occurs when moving the dumbbell from the beginning position close to the thigh to the elbow flexed position with the dumbbell closest to the shoulder/chest area. In this case, the length of the biceps muscle is shortening, the joint angle (between the arm and forearm) is decreasing. As a kid in science class, I was taught that there is a "pull" from the center of the earth. This pull of gravity makes objects on Earth (as opposed to the moon), want to stay stationary. As we pick up a dumbbell in a bicep curl and fully flex the elbow, we are generating muscular force from the biceps that is able to overcome the weight of the dumbbell. Thus, we have taken the once stationary dumbbell and moved it away from the center of the Earth (moved it against the pull of gravity). This illustrates a concentric contraction of the biceps.
From this fully flexed elbow position (weight near chest/shoulder), we will likely lower the weight in a controlled fashion back to the original starting position against the thigh. This allows the biceps brachii muscle to lengthen gradually and with conscious control to protect us. The joint angle (between the arm and forearm) increases as we lower the weight toward the original starting position. This demonstrates an eccentric contraction of the biceps, sometimes referred to as performing "negatives".
When training someone, especially a novice to weight training, it is best to stress concentric as opposed to eccentric contractions in the exercises that you ask the client to perform. For instance, if you ask a client to perform a biceps curl, rather than lowering the weight on a prolonged count, it is best to make this the shorter duration part of the exercise. Less muscle tissue damage occurs in a concentric than eccentric muscular contraction. As a result, the client should experience less soreness although still benefiting from performing the exercise.
Isometric or static contractions of a muscle occur when the joint position is "held" or tension is developed in the muscle but there is no resulting change in the length of the tensed muscle. If, during the bicep curl, one was to hesitate for several moments, say at 90 degrees of elbow flexion, this "held" position would demonstrate an isometric contraction of the biceps brachii. Chances are, you have experienced a wall sit or an exercise where one actually, with back to the wall and knees and hips flexed to 90 degrees, "sits" against the wall. Likely, you will eventually stand up from the exercise because the tension and pain in the quadriceps femoris group (anterior thigh muscle) becomes intolerable.
When training a person, it is best to train them using all three types of contractions described above. Choosing an emphasis of concentric vs. eccentric muscular contractions or utilizing static contractions will depend upon a variety of factors including goal of the exercise, exercise familiarity of the client, training time available, to name a few.
Kim Fischer, Ph.D., ACSM cPT and owner of Empowered By Learning LLC, created the ACSM Certified Personal Trainer Exam Preparation workshop and accompanying Study Guide and Action Plan for Passing the ACSM Personal Trainer Certification Exam to support people in reaching their goal of becoming a certified personal trainer by the "gold standard" organization in the industry. Kim guarantees that workshop participants will pass the exam or their next workshop is free. Learn more about how to simplify and organize the process and the content for the ACSM Certified Personal Trainer Exam by getting a free excerpt of Kim's step-by-step Action Plan when you subscribe at http://www.empoweredbylearningllc.com/
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