The amount of force your muscles are capable of producing varies with both the type and velocity of contraction because of differences in cross-bridge mechanics and the rate at which cross-bridges can be formed. This is known as the force-velocity curve.
When you lift a weight your muscles contract concentrically, heads on the myosin filaments attach to actin filaments, bend and pull them, then release and reattach. When you hold or lower a weight your muscles contract eccentrically, the myosin heads work more like breaks, trying to hold on, then bending back, detaching and reattaching. These braking attachments are stronger, which is why you can hold or lower a much heavier weight than you can lift. A few years ago Dr. Michael Reedy of the Duke University Cell Biology department explained it to me as follows,
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.
When your muscles contract concentrically, the faster the contraction velocity the lower the force your muscles can produce because the limits of the rate of cross-bridge attachment and detatchment result in fewer attachments being formed. The graph below, adapted from Skeletal Muscle Structure, Function, & Plasticity: The Physiological Basis of Rehabilitation by Richard Lieber (page 62), shows how muscle force rapidly decreases with increases in concentric contraction velocity, and how much more force your muscles can produce contracting isometrically and eccentrically.
This is part of the reason you can’t lift a heavy weight as fast as you can lift a lighter weight, and why you lift more slowly as you fatigue. This is also why it is more effective to increase the resistance and the tension on your muscles by increasing weight than by increasing velocity. Although more force is required to accelerate a weight more rapidly to increase velocity, since the faster you lift the less force you can produce you will have to use a lighter weight. By moving more slowly you can use a heavier weight, especially if you increase the relative time spent performing the much stronger eccentric portion of the repetition.
Keep in mind while the force-velocity curve, load and tension are important they are only a few of many factors which must be considered and balanced against each other. The slower you lift the more weight you can use for a given time under load, but you do so at the expense of mechanical work which appears to contribute to microtrauma. Also, although you can only lower as much weight as you lift (unless you have a person or machine assist you during the positive) the duration of the positive and its length relative to the negative also affect the load you can use since concentric contractions are more fatiguing.
For example, whether you perform five positive emphasized reps using a ten second lifting and three second lowering cadence or five negative emphasized reps using a three second lifting and ten second lowering cadence, your reps and time under load will be the same, but the negative emphasized will be easier with the same weight (this is also a good example of how mechanical definitions of work and power and even average resistance over time are poor ways of measuring what’s happening during exercise; same mechanical work, same power, same resistance, same time under load, different levels of difficulty).
Shouldn’t we be trying to make exercise harder though? Absolutely, but there are many ways to do this, and one is by manipulating your speed of movement and the relative duration of the positive and negative to increase the load you can use and the tension on the target muscles for a given time under load which increases your average intensity. Negative emphasized reps are only easier than normal or positive emphasized reps if you use the same weight. If you use a heavier weight, however, you can increase the tension without reducing your time under load, metabolic stress, or mechanical work.
Continuing with the previous example, whether you perform the positive emphasized or negative emphasized protocol if you use a weight that would allow you to achieve momentary muscular failure after the same number of reps and time under load your intensity of effort — how hard you are working relative to your momentary ability — would be one hundred percent. However, since you can use a heavier weight for negative emphasized reps the intensity of effort at the start of the exercise would be higher as would your average intensity of effort.
As a general rule, also taking into consideration safety and efficient loading, you should move at least slowly enough during exercise to be able to reverse direction smoothly, without bouncing or jerking the weight, to be able to maintain correct body positioning and/or alignment over the full range of the exercise, and to be able to focus on contracting the target muscles. If you’re not sure how slowly you need to move to do this at first, it is better to move too slowly than too quickly. However, beyond some point, moving more slowly during the positive relative to the negative can decrease the load you are capable of using. If you perform the positive too fast have to reduce the load due to the force-velocity curve. If you perform the positive too slow relative to the negative, you will have to reduce the load due to a faster rate of fatigue. Somewhere in between there is an optimum range of positive and negative cadences that provide the best balance of being able to produce a higher level of force during the positive and having a high enough ratio of negative to positive duration to allow for heavier loads and more tension without the total rep being so long the mechanical work is significantly reduced. I don’t know what this cadence might be, but I am currently experimenting with negative emphasized protocols and will be proposing a study designed to answer this question to a few people I know in research.
It is important to keep in mind I’m only speculating about the greater mechanical work increasing microtrauma, that tension also increases microtrauma, and that either way, microtrauma is only one of many factors which stimulate improvements in strength and size. While all else being equal it is plausible, I am not aware of any good evidence that more mechanical work per time, which requires a faster contraction velocity and lower load, produces more microtrauma than less mechanical work per time, with a slower contraction velocity and heavier load. The results of Ellington Darden PhD’s recent experiments with a single extremely slow negative emphasized rep, of Mike Mentzer’s and John Little’s static holds, and of Ken Hutchins’ SuperSlow and timed static contraction show little or no mechanical work is required to stimulate impressive increases in strength and size if the tension is high enough.
It is also important to keep in mind that load is not as important as tension (and it is possible to significantly increase load while reducing tension on the target muscles by limiting exercise to the portion of the range of motion where the lever against them is small), and all of this assumes you are using strict form and either training on machines with properly designed cams or performing free weight and body weight exercises in a manner resulting in relatively congruent strength and resistance curves. How you perform each repetition is far more important than how many repetitions you perform or how much weight you lift.
While it is unknown whether there is an ideal speed of movement during exercise or an ideal ratio of positive to negative speed, the force-velocity curve makes it clear that repetitions should be performed slowly, rather than quickly. From the second edition of Skeletal Muscle Structure, Function, and Plasticity: The Physiological Basis of Rehabilitation by Richard Lieber page 60:
Muscles are strengthened based on the force placed across them during exercise. The force-velocity relationship of muscle indicates that high velocity movements correspond to low muscle force, and that low velocity movements correspond to high muscle force. Since strengthening requires high force-producing exercises, the velocities must, necessarily be relatively low. High velocity movements may have other beneficial effects (e.g. improve muscle activation by the nervous system), but not at the muscle tissue level. The take home message – keep velocity low for strengthening.
It has been experimentally determined from biochemical studies that the cross-bridge connections between actin and myosin attach at a certain rate and detach at a certain rate. These rates are referred to as rate constants. At any point, the force generated by a muscle depends on the total number of cross-bridges attached. Obviously, this number represents the net balance between the number of cross-bridges attached versus detached. Because it takes a finite amount of time for cross-bridges to attach (based on the rate constant of attachment), as filaments slide past one another faster and faster (i.e., as the muscle shortens with increasing velocity), force decreases because of the lower number of cross-bridges attached. Conversely, as the relative filament velocity decreases (i.e., as muscle velocity decreases), more cross-bridges have time to attach and to generate force, and thus force increases.
…and from the fourth edition of Exercise Physiology: Human Bioenergetics and Its Applications by George Brooks on page 390 it says,
As compared to lifting light loads, isotonic responses to given stimuli when lifting heavy loads results in a greater latent period, slower movement, and less movement. The effect of strength training is to make the load appear lighter. The force-velocity relationship is hyperbolic in nature. Greater loads produce slower speeds but greater tension.