Welcome to power 47 of Strength & Conditioning for Therapists. I hope you’re all still keeping well, sane and enjoying a sensible and gradual return to activity. I’ve been asked several times over the last few weeks about rehabilitating or training muscle power; how to do it, how does it differ from strength training, the reps and sets etc…. So that’s what we’re going to talk about today and indeed next next time. This is the first in a 2-part series on training muscle power.

Muscle Strength vs Muscle Power

Muscle power and muscle strength are different indices of function and they require a different approach to optimal rehabilitation and conditioning.  Both parameters are important for performance and mitigating injury across a spectrum of patient profiles, from the the high-functioning athlete to the lower-functioning older-adult. 

What is Muscle Power?

the optimal amount of work performed in a given time period (1)

the rate of performing work (2)

the product of the force and velocity of muscle contraction (3)

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1. American College of Sports Medicine (2013); 2. Ratamess (2012); 3.Reid & Fielding (2012)

Definitions vary a little, but very basically, muscle power is the speed of muscle force production, which is governed by the equations of work over time, or if we’re discussing specific muscle contractions it’s force times velocity – see below. Power can be measured during things like a 10 minute cycling task, to a single explosive effort. We’re going to focus more on the latter.

Measuring Muscle Power

In laboratory settings where we’re afforded the luxury of dynamometers, we can measure the rapidity of force output during dynamic, isokinetic muscle actions, or indeed from static contractions. You may have heard of the index rate of face development (RFD). This is essentially the slope of the first part of the force-time curve during a maximal voluntary contraction. See below. The steeper the gradient, the faster the force production. This is really interesting to measure and it can give us detailed information about the contractile capabilities of an isolated muscle group. Further, it’s a contributing factor to muscle power.

However, if we’re talking about the ability to suddenly right posture in response to a perturbation, or throw a javelin for maximal distance, we need to take into account other things, and our laboratory measurements might need to be supplemented with more functional ones to judge ‘power’ performance. [In the interests of precision, whilst I’ve illustrated the concept of power being similar to RFD above, it’s not absolutely the same and I’ve simplified the concept to make the discussion and application easier]

These more functional and gross measures of power include the vertical jump test, counter-movement jump, 10m sprint and so on.

The Building of Blocks Power

Okay, so if we understand some of the physiologic determinants of power, i.e. what determines a fast force production, then we can start to focus the specificity of training and rehab interventions to optimally target this index of function and bring about the best gains.

If you remember, RFD is measured from a maximal voluntary contraction and it plays a role in muscle power. I love the figure by Maffiuletti et al, which I’ve adapted below that simply lays out the factors that contribute to that steepness or gradient of the force-time curve.

As you may well appreciate, there are many things that can influence the speed of muscle force production, some of which are easier to change than others. And when we’re talking about power and movement of limb segments and projectiles, we can add in a few more. However, to simplify, the following things are pretty important:

  • Fast twitch motor unit recruitment
  • Rapid cross-bridge cycling
  • Muscle and connective tissue stiffness

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You may notice that some of these are similar to the determinants of strength, and you’re right! So what’s different and why are we bothered about specificity? Well, muscle strength tasks, assessments and training don’t typically place an emphasis on speed. The aim of power training, however, is to develop an individual’s ability to be ‘explosive’ in their actions / contractions / movements and the end task, such as jumping, is often sub-maximal.

Training Muscle Power

Right, what do you need to do to optimise muscle power adaptations? I would first argue that you need enough strength. What!? Yes, I know I’ve just said that we need specificity. And we do…but remember back to when I talked abut the importance of muscle strength and that it underpins the development of a multitude of other elements of function and performance (see here for post in case you missed it ;-)? Well if we want our patient or client to be able to move fast, jump high, avoid a fall, we need them not only to produce force quickly, but we need them to have sufficient force to produce – i.e. enough fuel in the tank. More force = the ability to potentially jump higher, or at al., Weak patient = poor power development.

Once we’re satisfied the fuel tank of strength is full enough, then what? Well fear not, because heavy-resistance strength training can also have a strong stimulatory effect on RFD and power (Suchomel et al 2018). Yup. Because of the overlap in physiologic determinants of strength and power, whist you’re doing your heavy strength training you should be developing some power capabilities too 🙂

After that…

The key difference between power training and strength training is the explosive intent.

What we’re trying to achieve, or get our patients/clients to achieve is expression of high levels of force quickly. It is for that reason weightlifting movements may produce equal or greater power adaptations and improvements to vertical jump performance compared to plyometric training (Berton et al. 2018). And by weightlifting movements, I mean whole or parts of movements you see in Olympic lifting, not the typical dumbbell training in the gym. These weightlifting moments are executed with heavy load and with explosive intent.

Contractile vs Non-Contractile Tissue Properties

You may have noticed that I haven’t spoken about tissue stiffness yet. Indeed stiffer tissues (like the muscle-tendon unit), enable a quicker transmission of muscle force to bone … and thus better power performance. There are a couple of reasons why I’m not spending a great deal of time on this.

  1. The time course of adaptation of non-contractile tissues,
  2. The preparatory strength training phase

Heavy resistance training in itself can bring about a positive adaptation to the connective tissues and this is why heavy loading protocols are used to treat many tendinopathies (particularly of the lower limb). So your heavy strength work is good for tendons. I’m sure you’re aware that the time course of tendon adaptation is longer than for muscle strength. Typically 3-months or so to record a measurable difference in tendon stiffness in response to heavy loading, whereas you may see significant strength gains in just 4 weeks.

So, bottom line, if you do your strength work first, you can not only top up the fuel tank of strength, a happy by-product is tendon adaptation … as long as the programme is long enough and at a true strength loading level.

Summary

So where does this leave us? As I’ve said before, we need an individualised approach to rehabilitation and training. Not all individuals will require devoted and repeated blocks of periodised power work, but some will. Understand the performance demands and injury risks of the situations that the individual is returning to; this may be as simple of being able to attenuate the threat of injury caused by slipping off a curb … and this will still require a fast production of muscle force! Exercise design and selection and the periodised approach to rehabilitation can then be designed to deliver that appropriate training stimuli commensurate with what’s required.

Next time we’ll look in more detail at prescription of power-based exercises, including reps, intensity and exercise progressions.

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References

  • Ratamess (2012). ACSM’s Foundations of Strength Training & Conditioning
  • Reid & Fielding (2012). Exerc Sport Sci Rev. 40(1): 4–12 
  • Suchomel et al. (2018). Sports Medicine 48: 765–785
  • Berton et al (2018). J Sports Sci 36(18):2038-2044