Why Strength Training is a Dead End

Part 3: The force-velocity myth

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Credit: Kirby Lee for USA TODAY Sports, CC-By

In part 2 we already saw that there probably isn’t such a thing as pure strength: the strength of a muscle varies per task. Fortunately, most strength coaches nowadays barely seem interested in the strength of isolated muscles. After all, working in kinetic chains is said to be more functional.

Nevertheless, many coaches (and scientists) are guided by a property of isolated muscles: the force-velocity curve. Isn’t that peculiar? Grab a drink and take a seat, ’cause this is quite the story!

The force-velocity curve for dummies

The force-velocity curve in essence boils down to this: the faster a muscle contracts, the less force it is able to generate (see red curve in the figure below). This is pretty intuitive. If you perform a biceps curl with a small weight you can blast it upwards, whereas you’re barely able to set a big weight in motion.

Different types of training have different effects on the force-velocity curve (red curve = before training; adapted from (1)).

So according to the figure above, a muscle actually does have a certain maximal force capability or to be more precise, a certain force-velocity profile. This isn’t just nice to know for Jeopardy!, because we are supposedly able to apply it in practice. That is, with appropriate training we’re able to shift the curve upwards (1,2). That means more force at the same velocity (and perhaps even a greater maximal velocity).

It’s important to take into account here that you mainly get better in the range in which you perform your training. If you train with heavy weights, you’ll mainly get better at producing force at lower speeds (see blue curve), whereas training with relatively light weights will lead to more ‘speed-strength’ (see green curve). But regardless of how you train, there always is an important bonus: the curve remains smooth at all times. Hence, training with heavy weights will in time make you more explosive as well. So what’s my damn problem with strength training?!

A brief history

Let us start at the start: where does this force-velocity curve originate from? Welcome in 1938! Nobel Prize winner Archibald Vivian Hill — A.V. to friends — snapped a frog’s neck (poor frog!) and cut out one of its leg muscles in order to conduct a series of experiments on it (3).

He determined the muscle’s force-velocity curve by attaching different weights to it and hammering it with an electric jolt each time (see figure below). In addition, he made sure that the muscle was at maximal tension before allowing it to shorten. As soon as he released the muscle, it was able to shorten at maximal velocity — sort of like a catapult.

A.V. Hill’s quick-release experiment, but that’s none of my business (adapted from (4)).

In this way he gathered a series of data points, through which he was able to draw a handsome curve. And guess what… The heavier the weights were, the slower the muscle contracted. The force-velocity curve is born!

From frogs to humans

Apart from the observation that we aren’t frogs, you’ve probably noticed that this was a pretty unnatural situation. Fortunately, a number of scientists therefore decided to determine the curve in living people as well (5). Unfortunately, most people don’t respond particularly well when zapped with electricity, hence a different approach was required.

The solution? Participants that really try their best and isokinetic devices (see video below). These devices ensure that (nearly) the entire movement is performed with a predetermined velocity. As such, they enable you to determine the maximal force for different velocities. And as a result you are once again able to gather a series of data points and draw a curve through them. This must be our lucky day, because this curve is remarkably similar to that of Hill!

An isokinetic leg extension

A small footnote is that isokinetic experiments always involve multiple muscles, whereas Hill truly looked at isolated muscles. But given the fact the curves match very well, this actually makes the case for applying the force-velocity curve stronger!

FYI: Shortly after World War II an interesting study — to put it mildly — was conducted in which experiments were performed on truly(!) isolated muscles in living persons (6). This was made possible by the radical surgeries that were performed on them; presumably they were war victims. Here too our trusty force-velocity curve popped up.

And then things went wrong

The moment of truth: does this somewhat artificial muscle property also determine how more natural movements come about? For clarity’s sake, “natural movements” here refer to movements in which more than one joint is involved: running, jumping, pushing, pulling, etc. A.k.a. complex movements. If so, then we’re done here and we should head out to the gym to start working on our force-velocity profiles ASAP. If not, then (unfortunately) we have to dig a bit deeper still.

Spoiler alert: no. Why can’t live ever be simple… Admittedly, for many complex movement you’ll find — just like with the Hill curve — that an increase in speed is associated with a decrease in force (7–11). But in the figure below you can also see that the line is much more straight than the original one. Quasi-linear versus hyperbolic, to throw about some fancy words.

In complex movements the force-velocity relationship is much more linear than that of isolated muscles (adapted from (12)).

How is this possible? In movement there is a paradox: generally we displace our center of gravity or our hands/feet in a straight line, but in order to do so we have to rotate segments of the body (13). To see this for yourself just make a pushing motion and look at the movements of your hand, lower arm and upper arm. These transformations from rotations to translation — a.k.a. segmental dynamics — explain for an important part why the force-velocity relationship for complex movements differs from the Hill curve (12).

It’s true that individual muscles still adhere to the Hill-curve, but because of segmental dynamics their force contributes less and less to the force of the total movement as this movement becomes faster and faster. To put it more simply: if you perform an explosive squat your muscles generate a considerable amount of force, but quite a lot of it is ‘wasted’. This is the result of the segmental dynamics that are in play there.

Extrapolation extravaganza

Nevertheless, many coaches and researchers assume that the force-velocity relationship that they find in complex movements is mainly determined by the intrinsic force-velocity relationship or contractile capacity of muscles. For instance, the force-velocity relationship in jump squats is presumed to be largely dependent on the contractile capacity of leg muscles (14,15). But just by performing the exercise from a slightly deeper or less deep starting position, the exact force-velocity values can differ very noticeably (16,17). Nevermind comparing different forms of jumping, running, kicking, etc… These differences are at best partly explained by the contractile capacity of muscles; segmental dynamics probably play a (much) more important role (12,16).

Even though it’s tempting to extrapolate phenomena such as the force-velocity curve beyond their original context, in reality this is seldom justified. If we truly wish to understand complex movements, we’ll have to (among other things) figure out how the central nervous system takes segmental dynamics into account when coordinating these movements (18). This will be a monumental task, but it is certain that precise control of bi-articular muscles, such as the hamstrings, is crucial in this (13,19).

So?

My point: improving force-velocity values in an exercise/movement does not automatically lead to improvement of these values in a goal movement, even if it involves the same muscles. After all, the output (force, velocity, power, etc.) of a complex movement is strongly dependent on the specific control of involved muscles (see also part 2). The advice to ensure “stresses that allow for a more complete adaptation to occur across the entire force-velocity curve” (20) is thus indeed an empty advice.

FYI: Despite all this there is considerable evidence that an athlete’s force-velocity profile in a jump squat could be very valuable for programming jump training (21). This however doesn’t mean that such a profile has fundamental value; there are good reasons for its intended application being strictly limited to the context of two-legged vertical jumping. There it may very well be ‘the best we have’ for practical purposes, but we have to keep digging a lot deeper if we wish to truly understand performance.

A lifeline for strength training?

Still, stronger muscles could in theory lead to better performance of complex movements. The emphasis here is on “could”, because in order to do so adjusted control is essential; if the control remains the same as before, performance will even deteriorate. For instance, a 10% increase in force production by the knee extensors would lead to a 1 cm increase in jump height with a new, optimal control; with the previously optimal control, it would actually lead to a decrease in jump height of 7 cm (22)!

This however doesn’t mean that we (periodically) should try to increase ‘pure’ strength. That’s because even on an intramuscular level, thinking from just a force-velocity perspective is misleading. For example, it’s tempting to think that during the takeoff phase of a jump the knee extensors shorten very quickly and therefore generate relatively little force. In reality however, their muscle fibers barely shorten at all and hence produce a lot of force (23). By doing so they put their tendons under considerable tension, which they abruptly release during the very final part of the movement; a catapult effect if you will. And as mono-articular muscles, most knee extensors are still relatively simple; what happens in bi-articular muscles is even more difficult to estimate.

When trying to increase muscle force production with only the force-velocity curve in the back of our minds, we thus overlook all sorts of structural and coordinative aspects. Isokinetic training is the prime example of this: even though it causes velocity-specific improvements on isokinetic tasks (24,25), in practice we’ve noticed that this doesn’t contribute much if anything to sport performance. The adaptations are (once again) specific. More muscular force in a certain exercise/movement is therefore all but a guarantee for more force in a goal movement. Does the pattern become clear? Hint: you mainly get good at what you do. Sometimes life fortunately can be simple.

FYI: Nevertheless many still view isokinetic testing as a way to assess ‘pure muscle strength’. Exactly how problematic this notion is, is strikingly underlined by the following observation (26): “It is not only the muscular capacity to deliver power that determines the dynamics of a movement, but the movement dynamics also determine how much power can be delivered by muscles.”

No matter how bad we’d want it, human performance avoids being captured in bite-sized chunks. What we measure during isokinetic testing — or ideally during Hill-like experiments — only gives us an indication of the contractile capacity of muscle fibers; it does not tell us at all how this capacity is manifest in ‘real life’.

Conclusion

Is it peculiar that people heavily rely upon the force-velocity curve in strength training? No and yes. No, because people have always been looking for certainty and clarity; the force-velocity curve is a proven phenomenon and offers these things. Yes, because even though this curve tells us something about the contractile capacity of a muscle, this — especially in complex movements — doesn’t mean much without specific control from the central nervous system.

Those who still aren’t prepared to surrender the sovereignty of the force-velocity relationship, must realize the following: muscles are embedded in an information processing system with countless feedback loops and mechanical interactions. It thus would be an absolute miracle if this relationship would be enough to understand performance. The central nervous system plays a key role; hail the new king.

Practical pointers

• Emphasize improving the specific control of important movement patterns as much as possible; this is a precise and time-intensive process.
• Regard isolation exercises as accessory, but for these too ensure a serious degree of specificity; for this analyse the working of a muscle in goal movements.
• Force-velocity relationships may only be a valuable tool in very specific cases; improving two-legged vertical jumping is such a case (21).

References

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