Decades ago, Nautilus founder Arthur Jones theorized muscular friction was the source of differences observed between positive and negative strength – reducing concentric efficiency while increasing eccentric efficiency. On several occassions 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 significant levels of friction in exercise equipment can certainly have this effect, research has shown muscular friction is practically non-existant. 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 tructure 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. The full text is available at http://jp.physoc.org/cgi/content/full/526/3/589
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 inroading may be less efficient and it may take longer to recruit and stimulate all the target muscles’ available motor units. Because of this, I believe the negative phase of the repetition should be performed only slowly enough to maintain strict form and no slower. Spending more time performing the easier part of the repetition reduces the intensity of the exercise rather than increasing it.
Rather than a longer negative phase, the positive and negative should be of roughly equal duration. This duration should be at least long enough for an adequately slow movement to minimize acceleration during turnarounds. In my experience a four to five second positive and negative cadence is slow enough for most people to maintain reasonably good form and focus on intensely contracting the target muscles over typical exercise ranges of motion.