Okay, now let's use those same two ramps but this time stand at the top of the ramp and lower the 100-pound block. Which ramp would you use for this? In this case it would be easier on your muscles to use the ramp with the greater friction. Because friction opposes movement, when you lower the block, the friction works with you -- you're actually lowering the block minus the friction that is involved. Staying with the previous example, you'd be lowering 90 pounds down the smooth ramp and 85 pounds down the rough ramp.

Essentially, this is also what happens within your muscles when they contract. When you raise a weight, the friction within your muscles works against you -- you are lifting the weight plus internal muscular friction. When you lower a weight, the friction works for you -- you are lowering the weight minus internal muscular friction. Suppose there's five pounds of friction produced within a particular muscle when it contracts and you're lifting a 50-pound barbell. You're actually raising 55 pounds and lowering 45 pounds. In other words, your muscles don't have to work as hard when you lower a weight. So, the friction within your muscles decreases your concentric strength and increases your negative strength. In fact, for any given exercise, a muscle can always lower more weight than it can raise.

[It should be noted that the aforementioned illustration is greatly simplified. An in-depth analysis would include the effects of static and kinetic friction, and involve trigonometric calculations, because the 100-pound block is on an inclined surface. However, it is not within the scope of this article for such a detailed explanation. Nevertheless, this simplified version has been enough to prove the intended points in a factual manner.]

Mechanical friction

If a machine has an articulation between two or more surfaces and movement occurs around these surfaces, the machine will have some degree of mechanical friction. Mechanical friction is produced by a weight stack traveling over guide rods, a cable winding around a pulley, a chain moving around a re-directional sprocket or a bolt that pivots with a bushing. In an effort to design machines that feel smooth, many companies have reduced the amount of mechanical friction within their equipment. This has given rise to such practices as using belts or cables instead of linked chains, installing bushings in every plate (not just the top plate) and incorporating sealed bearings at pivot points. (It should be noted that free-weight exercises do not have any mechanical friction, since there are essentially no moving parts.)

The ramp analogy used earlier to explain the effects of internal muscular friction also applies to mechanical friction. In other words, when you raise a weight, you are actually lifting the weight plus internal muscular friction, plus mechanical friction (from the machine). When you lower a weight, you are actually lowering the weight minus internal muscular friction, minus mechanical friction. As an example, let's suppose you are doing leg extensions and the following is true: 1) the resistance is 100 pounds; 2) the mechanical friction inherent in the machine is 10 pounds; and 3) the friction within your quadriceps muscles is five pounds. In this case, the 100 pounds of resistance that you selected actually feels like 115 pounds when you raise it (100 + 10 + 5) and 85 pounds when you lower it. As the amount of friction increases, whether it be muscular or mechanical, the harder it is to raise a weight and the easier it is to lower it (100 10 5). As the amount of friction increases -- whether it be muscular or mechanical -- the harder it is to raise a weight and the easier it is to lower it. (When a muscle contracts, additional force is also necessary to stretch its antagonist. For example, when you contract your quadriceps concentrically, you receive extra resistance from your hamstrings.)

Muscular soreness

At one time or another, most of us have experienced some degree of muscular soreness. The two types of muscle soreness are acute and delayed-onset.

Acute soreness occurs during and immediately following exercise. It is believed that this soreness is associated with an occlusion of blood flow to the muscles (ischemia). As a result, metabolic waste products (e.g., lactic acid) cannot be removed and accumulate to the point of stimulating pain receptors in the muscles.

Delayed-onset muscular soreness (DOMS) refers to the pain and soreness that occurs 24 to 48 hours after exercise. It has been found that eccentric muscle contractions produce greater DOMS than either concentric or static contractions. The exact cause of DOMS is unknown. The most popular theory is that cellular damage occurs to the muscle fibers and/or connective tissue (e.g., tendons).

Perhaps muscular friction is at least partly responsible for muscular soreness. Any friction that involves movement of biological tissue across another surface will cause irritation. Too much irritation will injure the tissue. For example, if you were to use a hammer on a regular basis for short periods of time, you'd begin to develop callouses on your palm. Basically, these callouses are a protective adaptation to frictional heat. However, if you were to use the hammer for a long enough period, you'd develop blisters instead. In this instance, the excessive friction surpassed the tissue's adaptive ability because it was too much and too frequent. I suspect that this "blistering" effect may also occur within muscle tissue whenever there is excessive internal muscular friction from either concentric, static or eccentric contractions.

Perhaps future research studies will explore this as a possible cause of muscular soreness. At any rate, friction plays a large role in exercise -- and possibly afterward.

REFERENCES

Fox, E.L, & D.K. Mathews. The Physiological Basis of Physical Education and Athletics, 3rd ed. Philadelphia: Saunders College Publishing, 1981.

Jones, A. The Metabolic Cost of Negative Work. Athletic Journal. January, 1976, pp 40-41, 80.

Jones, A., et al. Safe, Specific Testing and Rehabilitative Exercise of the Muscles of the Lumbar Spine. Santa Barbara, CA: Sequoia Communications, 1988.

Joseph, A., et al. Physics for Engineering Technology, 2nd ed. New York: John Wiley & Sons, 1978.