Eccentric Contractions: The puzzles

Tension during eccentric contractions never decreases during the stretch.

At long muscle length, the maximal isometric tension decreases with increasing muscle length. This is the descending limb of the length-tension curve. Sliding filament crossbridge theories account for it as decreasing overlap of thick and thin filaments making fewer crossbridges able to form. However when muscle is actively stretched, the tension rises throughout the stretch. At short lengths, where tension rises with length, the tension rises as predicted.

Soleus muscle being stretched at long length. Length traces above, tension below, Isometric active tension is less at the longer length, but during the stretch, the tesion rises throughout. (Above) This is true even after passive traces are subtracted. At the end of the stretch, the length is the same as in the isometric at long length, yet the tension remains higher (See "Tension after a stretch does not fall to the expected value" below).

Stiffness does fall during a stretch.

Several workers have shown that stiffness does fall during the stretch. This makes the rise of tension even more puzzling, as stiffness is usually taken as an indication of the number of crossbridges attached.

Tension (upper) and stiffness (lower) measured by vibration, during stretch of a frog single fibres. From Sugi, H. & Tsuchiya, T. (1988). Stiffness changes during enhancement and deficit of isometric force by slow length changes in frog skeletal muscle fibres. Journal of Physiology 407, 215-229.

Tension after a stretch does not fall to the expected value.

After a stretch finishes, if stimulation continues, the tension gradually declines to a steady value. However, the value reached is not that appropriate to the final length, but is always above it, as shown in both the records above. When the experiment is carefully done with maximally activated single fibres, the final tension is always close to that appropriate to the initial length. This extra tension above that expected is known as "the permanent extra tension following stretch", and is proportional to the amount of stretch. It also is not parallelled by stiffness, ie stiffness does fall at least proportionately with length.

The force-velocity curve never falls at high stretching velocity.

During more rapid stretch, the tension generated is a balance between increasing tension per cross-bridge, due to more stretch between attachment and detachment of the average bridge, and fewer crossbridges, due to reduced opportunity for attachment when attachment sites move past more quickly. Simple models predict that the second effect should dominate at high enough speeds, as the first effect must be limited. That is, there must be a limit to how much force a bridge can generate. (Note that no such limit was included in the Huxley 1957 model, and it shows very extended bridges under high speed lengthening.) Experimental measurement never show the fall at high speed expected from this.

From Harry, J. D., Ward, A. W., Heglund, N. C., Morgan, D. L. & McMahon, T. A. (1990). Crossbridge cycling theories cannot explain high-speed lengthening behavior in frog muscle. Biophysical Journal 57, 201-208.

Repeated stretches shift the length tension curve.

At the end of his very famous paper on the force-velocity curve of muscle being stretched, Katz (1939) noted that a series of eccentric contractions lead to a shift in the length tension curve to longer muscle lengths. Furthermore this could occur with little tension loss, so that tension at long length could actually rise as a result of the eccentric contractions. The shift was accompanied by a number of other changes, all consistent with "a conversion af active contractile material into passive elastic material".

Frog single fibre. From Morgan, D. L., Claflin, D. R. & Julian, F. J. (1996). The effects of repeated active stretches on tension generation and myoplasmic calcium in frog single fibres. Journal of Physiology 497, 665-674.

Eccentric contractions damage muscle.

Either maximally activated stretches, or more physiological eccentric contractions, produce histologically observable damage, release of intracellular proteins and Delayed Onset Muscle Soreness, or DOMS. This is the familiar tenderness and soreness one to two days after unaccustomed eccentric exercise.