The ratio of positive to negative strength varies

The Varying Ratio of Positive to Negative Strength

A few years ago I wrote an article titled The Ratio Of Positive To Negative Strength And Implications For Training in which I explained that rather than a fixed ratio the difference between positive and negative strength varies with different concentric contraction velocities due to the force-velocity curve. Although your negative strength does not vary at different contraction velocities, your positive strength varies inversely to contraction velocity, so the ratio changes if you lift a weight more quickly or slowly. The ratio also changes as a muscle fatigues, with positive strength decreasing more slowly than negative strength, probably due to eccentric contractions being more metabolically efficient.

This has been known since the 1950’s and it’s implications for the ratio of positive to negative strength should be obvious, but for a long time many people have believed there is a specific ratio of positive to negative strength. The most popular being the claim by Nautilus inventor Arthur Jones that negative strength is approximately forty percent higher than positive strength. Recently, exercise physiologist and HIT instructor Ryan Hall made an interesting discovery about how Arthur Jones came up with these numbers. He wrote,

Drew,

I have been working on updating my “Exercise and Genetic Variability” presentation, and correlating genes with some of the testing Jones performed on the pre-MedX Nautilus servo-powered isokinetic devices. Some of the information I ran across while perusing the earlier research reminded me of this conversation [referring to the discussion quoted in The Ratio Of Positive To Negative Strength And Implications For Training – DB].

Much of the testing that Jones observed concerning the ratio between concentric, isometric, and eccentric strength was performed on the pre-MedX isokinetic machines. I had to look closely and study each graph, which revealed something to me. Almost all of the testing was performed using a leg extension machine, which was set to 110 degrees ROM, with the speed of movement set to 25 degrees per second, for both the concentric and eccentric phases of the movement. Each repetition would be completed in a little over 4 seconds for each phase (8 seconds for the entire rep). This time interval doesn’t include the turnarounds, which were totally unloaded due to the nature of a servo-powered motor. However, having the concentric phase standardized to a predetermined speed would account for Jones’ observation of a consistent ratio between the different phases in a fresh muscle.

Jones’ testing also demonstrated that this ratio doesn’t remain the same for a fatigued muscle, which is consistent with the research literature. I just thought you would find this interesting.

Ryan

The ratio of positive to negative strength varies

Jones also believed the difference in positive and negative strength were due to intramuscular friction, which we now know is wrong. In The Future of Exercise he wrote,

Everything in the known universe that has both mass and motion also has friction, and muscles are no exception. Whether it is an automobile, an airplane, a snake or a human muscle, friction acts the same way: inhibits positive function while enhancing negative function, thus reduces your positive strength while increasing your negative strength.

He reasoned that if negative strength is approximately forty percent greater than positive strength, muscular friction must reduce positive strength by twenty percent while increasing negative strength by the same amount, and isometric strength is exactly halfway between the two (During a phone conversation in the late 1990’s when I asked Jones why he thought the ratio of positive to negative strength changed with fatigue he said it might be because friction increased as the muscles became more ‘pumped’). In The Metabolic Cost of Negative Work he wrote,

While it is certainly true that an exerciser can lower more weight than he can lift, it does not necessarily follow that his muscles are actually stronger during negative work than they are during positive work.

The muscles may be stronger; but even if so, they are not as much stronger as they appear to be. The apparent gross difference in strength is, I think, primarily a result of friction… internal muscular friction.

While lifting a weight, the muscles must contract with sufficient force to move the imposed resistance… but they also have to overcome their own internal friction. Thus, doing positive work, friction is working against the muscles.

Whereas, during negative work, friction is working for the muscles instead of against them.

Jones was wrong about isometric strength levels also. Isometric strength is relatively constant like eccentric strength so while the ratio of isometric to negative strength would be relatively constant the ratio of positive to isometric strength would vary with lifting speed. Isometric strength would only be halfway between positive and negative strength at a specific, slow lifting speed.

It is important to understand these claims about the ratio of positive to negative strength because they have influenced resistance selection recommendations for various exercise protocols like negative-only, negative-accentuated, and isometric training, as well as exercise equipment designs. For example, the typical recommendations for negative-only training and static holds are to use loads forty and twenty percent heavier than you use for normal repetitions.  This would be too much if you normally lift very slowly or too little if you normally lift more quickly than the speed of movement used in Jones’ tests. A safer approach when starting to perform exercise protocols limited to negative and/or static contractions is to err conservatively and only increase the load by around five to ten percent. If it isn’t heavy enough you can always increase the load on subsequent workouts.

 

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