Positive Versus Negative Strength
Over the past year there have been several arguments in high intensity training circles over whether there is a difference between positive (concentric) and negative (eccentric) strength. Some of these have been semantic arguments about the definition of strength, some attempted to provide alternative explanations for the observed difference during test results, some are still talking about the debunked theory of intramuscular friction. Some of these arguments were part of criticisms of the practice of hyperloading the negative portion of an exercise and equipment designed for this purpose, which will be addressed in a second article.
Negative strength is greater than positive strength.
Strength is the ability of your muscles to produce force. A muscle must produce force to lift a weight. A muscle must produce force to hold a weight motionless. A muscle must produce force to lower a weight more slowly than the acceleration due to gravity (if it didn’t the weight would simply drop). Strength can be positive (concentric contraction, lifting), static (isometric contraction, holding), or negative (eccentric contraction, lowering).
When a muscle contracts concentrically heads on the myosin filaments attach to the actin filaments forming cross-bridges which bend and pull, then release and repeat, causing muscle fibers to shorten. When a muscle contracts isometrically or eccentrically it forms more of these attachments. If the force against the muscle exceeds the force of contraction it begins to lengthen, and as the cross-bridges are stretched forcing detatchment they immediately reattach (approximately two hundred times faster than during concentric contractions). This difference in cross-bridging mechanics makes the motor units significantly stronger when contracting isometrically or eccentrically, so to stop lifting or begin lowering a weight your body recruits fewer motor units in the working muscles to reduce the force produced. Because of this it is less metabolically demanding to hold or lower a certain amount of weight than to lift it.
Another protein in muscle fibers called titin also contributes to the increase in eccentric strength. It is “wound” by the action of the myosin and actin during concentric contractions, then stiffens to resist lengthening during eccentric contractions.
This difference in positive and negative strength is easy to demonstrate. Perform a few strict test repetitions on a good biceps machine or barbell curls until you find a weight that is just slightly too heavy for you to lift. Rest for several minutes (to satisfy those who suspect congestion due to pump and the resulting friction is a contributing factor). Increase this weight by approximately twenty five percent and have someone help you lift it, then hold the movement arm or barbell motionless while they gradually transfer it to you. Although it is too heavy for you to lift you will find you are able to hold it, and lower it slowly under strict control.
While the implications for training will be discussed in more detail in another article there is one important consequence of this I want to mention now. Occasionally a novice trainee will stop exercises short of momentary muscular failure (MMF) because they are afraid they will drop the weight or movement arm and possibly injure themselves. While this concern may be reasonable when performing exercises where grip strength can be a limiting factor it is usually unfounded because of the difference in positive and negative strength (if this difference did not exist you would drop the weight whenever you reached MMF during an exercise).
Even after you have achieved MMF (the inability to continue positive movement in the prescribed form) you will be strong enough to hold the weight motionless for a period of time afterwards and lower it slowly. It is important to teach this to novice trainees to improve their confidence in their safety and willingness to continue to contract intensely as they approach failure when learning high intensity training.