Positive Versus Negative Strength – The Friction Theory is Wrong

Decades ago, Nautilus founder Arthur Jones theorized muscular friction was the source of differences observed between positive (lifting, concentric) and negative (lowering, eccentric) strength, reducing positive efficiency while increasing negative efficiency. On several occasions Arthur has stated the following,

“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”.

While the levels of friction in most exercise machines have this effect, research has shown muscular friction is practically non-existent. Although the exact mechanism isn’t yet fully understood, current scientific consensus is the differences in positive and negative strength are due to differences in cross-bridge mechanics. Dr. Michael Reedy of the Duke University Cell Biology department provided me with the following explanation,

“In a nutshell – more crossbridges attach and hang on tightly – due to to either or both causes of recruitment:

1) backbending distortion of one-headed crossbridges allows the second head of each myosin to attach, and they backwalk a few steps, smoothing the plateau of force that develops in phase 2 of ramp-stretch. (I love this idea, inspired from the Linari paper, but good evidence for it is not available yet.)

2) all weak-binding M*ADP*Pi heads of myosins that collide with backsliding actin hang on tightly to resist lengthening– and the more generous interface geometry for braking attachments by M*ADP*Pi allows more myosins to attach and evolve into brakes than are able to attach and evolve into purely isometric or shortening motors.

Much of my structural research over the next couple of years will focus on getting evidence for or against 1) and 2). We get snapshots of muscle structure by x-ray diffraction, and by 3D EM tomography of thin sections from fibers quick-frozen during mechanical actions and responses of interest.”

The Linari paper Dr. Reedy mentioned is A combined mechanical and X-ray diffraction study of stretch potentiation in single frog muscle fibres. M. Linari, L. Lucii, M. Reconditi, M. E. Vannicelli Casoni, H. Amenitsch, S. Bernstorff, G. Piazzesi and V. Lombardi, J. Physiol. 2000;526;589-596.

In addition to Arthur Jones’ friction theory being wrong, I believe his recommendation to emphasize the negative portion of the repetition by taking longer to perform it is also incorrect. Since negative work has been shown to be more metabolically efficient, if the negative phase of the repetition is too long relative to the positive, inroading may be less efficient and it may take longer to recruit and stimulate all the target muscles’  motor units. Spending more time performing the easier part of the repetition reduces the intensity of the exercise rather than increasing it.