In 2014 I wrote about the force–velocity curve and why controlled repetition speed matters. Since then I’ve seen the same misunderstanding come up repeatedly: people conflate the muscle force–velocity relationship (a capacity limit) with Newton’s second law (a requirement to accelerate a mass). They are different ideas that answer different questions—and when you combine them properly they support my recommendation to perform deliberately slow, low-acceleration reps, not ballistic ones. This fully updated article clarifies that distinction, adds a simple impulse/momentum example to show why force spikes don’t raise average tension, refines the eccentric discussion, and sets practical guidelines for choosing a cadence between 5/5 and 15/15 with brief holds only at loaded endpoints—all focused on maximizing stimulus while minimizing risk of acute and overuse injuries.
TL;DR: The force–velocity curve describes how much force muscle can produce at different contraction velocities (a physiological capacity limit), whereas Newton’s second law describes how much force you must produce to accelerate a mass at some rate (a mechanics requirement). They are different ideas—but together they explain why controlled, deliberately slow reps maximize usable tension, minimize force spikes, and are safer and easier to standardize than fast, ballistic reps.
Don’t Confuse the Force–Velocity Curve with Newton’s Second Law (2025 Revision)
I’ve recently fielded a barrage of messages from someone insisting I “don’t understand basic physics.” His mistake is a common one: he confuses the force–velocity relationship of muscle—a physiological capacity limit—with Newton’s second law—a mechanical requirement for accelerating a mass. They are not the same thing. Once you stop conflating them, it becomes obvious why controlled, deliberately slow repetitions are stimulate increases in muscular strength and size more effectively and safely.
Two Different Things People Keep Mashing Together
The force–velocity curve (physiology). When a muscle shortens faster, it can produce less force; when it lengthens under load, it can typically resist more force than it can produce statically and this capacity varies less with lengthening speed. These properties arise from cross?bridge mechanics (and, as research over the past two decades has shown, the behavior of titin during active lengthening). This curve describes what your muscles can produce at a given contraction velocity; it says nothing about external objects.
Newton’s second law (mechanics). To accelerate a mass, the net force applied to it must exceed opposing forces in proportion to the acceleration you want (F = ma). If you try to move a given weight more quickly, you must momentarily apply more force than merely holding its weight; if you change direction abruptly, you also have to overcome inertia to reverse its motion.
These answer different questions. The first tells you how strong you are at a given speed; the second tells you how much force you must apply to make a given load move at the speed you choose.
What This Means During Exercise
Now combine them. Because you are stronger at slower concentric speeds, you can choose a deliberate cadence and use more load for a given time under tension while keeping forces predictable. If, instead, you explode at the start of each repetition, you create a large force spike to accelerate the weight, then the kinetic energy you gave it reduces the force you need to apply for a moment—so the time?average force over that up?and?down movement, which begins and ends at rest, tends to settle near the weight of the load. Fast reps redistribute force into risky spikes; they don’t magically raise the average tension your muscle sees across the set.
A concrete example
Press a 50 kg bar (~490 N). If you surge with ~50% extra force (~735 N) for 0.3 s to get it moving, you’ve created a positive impulse. Immediately after, that momentum lets you coast with ~50% less force (~245 N) for roughly the same time as you guide the bar. The positive and negative impulses cancel if you start and finish the phase at rest. Net result: the average applied force over the concentric is close to the bar’s weight. The spike didn’t increase average tension; it just made the load profile spikier and your joints’ job harder.
Fast Reps: The Common Claims and Why They Don’t Hold Up
“Higher acceleration requires higher force, so fast reps build more muscle.” Momentarily, yes. But the rest of the rep is aided by momentum. Over matched time?under?tension and similar effort (e.g., to momentary muscle failure), slow, controlled reps let you use more load because your force capacity is higher at slow concentric speeds. That supports equal or greater average tension with far fewer force spikes.
“Faster reps let me use a heavier weight because I do more reps in less time.” That compares rep counts, not time. Match sets by TUL, and slower reps typically permit a heavier load due to the force–velocity relationship. The meaningful training variables are effort and time under load, not how many times you pass a sticking point per minute.
“More mechanical work per minute means more growth.” Mechanical work (force × distance) is not the same as the metabolic stress and tension exposure that drive hypertrophy. When effort is equated, a wide range of loads and tempos can build muscle; slow, controlled repetitions simply standardize the stimulus and reduce risk of injury.
Negatives Are Not a Loophole
Because you can lower more weight under control than you can lift, people sometimes infer that adding speed to negatives must be better. In practice, what matters is what you can smoothly control through the full range without abrupt reversals. Extremely slow negatives aren’t necessary, but abrupt negatives and ballistic turnarounds are exactly where tissues are most vulnerable to force spikes. Control the negative and don’t bounce.
How to Train This Way—Without Turning the Page Into Math
Choose a cadence anywhere in the range from 5/5 to 15/15 and stick with it. At the lower end, 5/5 is just slow enough for most people to achieve low?acceleration turnarounds over typical exercise ranges of motion. At the upper end, 15/15 is about as slow as most people can move continuously and uniformly rather than in segmented starts and stops.
Perform a brief 2–3?second hold at the end point if the target muscles are meaningfully loaded in that position. Otherwise, reverse direction smoothly with low acceleration. On compound pushing exercises take 2-3 seconds to perform the lower turnaround to counter the tendency to bounce or fire out of the start.
Measure sets by time under load (TUL) and perform them to momentary muscle failure. As a rule of thumb, aim for ~60–90 seconds of TUL. With the holds included, that works out roughly to:
- 5/5 ? ~14–16 s per rep ? typically 4–6 reps for ~60–90 s.
- 10/10 ? ~24–26 s per rep ? typically 3–4 reps.
- 15/15 ? ~34–36 s per rep ? typically 2–3 reps.
Progress by increasing resistance only when you can complete your upper target rep count or TUL in strict, correct form. If you are unable to perform at least the lower target rep count or TUL in strict, correct form, reduce the load until you can.
Safety and Longevity
Fast or sudden starts and stops produce large, poorly controlled peaks in joint and tendon loading—exactly the sort of thing that accumulates into overuse problems or sudden injuries. Slow, controlled movement with deliberate turnarounds and end-point holds keeps forces inside a predictable range and reduces repetition?to?repetition variability.
Arthur Jones explained this decades ago:
“…fast or sudden movement during exercise does not produce fast muscles, or stronger muscles, or bigger muscles, it produces only one thing…injuries. If in doubt about the best speed of movement during exercise, try doing it slower rather than faster; faster is never better, is usually worse, and is frequently dangerous.”
Bottom Line
- The force–velocity curve tells you how much force muscle can produce at a given contraction speed.
- Newton’s second law tells you how much force you must produce to accelerate a given mass at a given rate.
- Put together, they explain why heavy weights don’t move fast—and why chasing speed creates force spikes without improving average tension. If you want maximum stimulus with minimum risk, use controlled 5/5 or 10/10 repetitions with 2–3 second holds at both turnarounds, match sets by time under tension, and train to a consistent, high effort.
References:
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Hutchins K. SuperSlow: The Ultimate Exercise Protocol. 2nd ed. 1992.
When explaining exercise principles and criticizing beliefs and practices which violate them, it is common to be accused of dogmatism by one holding erroneous beliefs. This is often an attempt by the accuser to dismiss novel assertions or criticisms of his beliefs without providing sound counter-arguments. Ironically, this is frequently the result of his own dogmatism.