Hi there and welcome to the Strength & Conditioning for Therapists blog series for 2020, post number 35. Happy New Year, I hope you’re looking forward to a exciting and prosperous 2020.

So I thought it fitting to kick off the series again by recapping some of the pertinent aspects of muscle function and to clarify important terminologies, especially since there are a lot of new followers of this series – WELCOME EVERYONE 🙂 . Thereafter we’ll be delving into topics such as:

  • how to train / rehab muscle power optimally
  • strength and conditioning for older adults
  • do strength gains translate into improved function?
  • is hypertrophy important?
  • injury specific rehab
  • and much more

And don’t forget, I’ve produced a FREE 14-page Strength & Conditioning Guide for Therapists to help you apply some of the pertinent concepts with your patients. If you haven’t got your copy yet, you can download it right here.

Right, let’s get started!

Muscle strength

Now whilst the understanding of muscle strength has improved over the last couple of years, unfortunately we’ve seen the emergence of a multitude of different definitions, some of which might be pertinent to describing specific aspects of athletic performance, some of which are downright silly. Regardless, a multitude of definitions is unhelpful for the rehabilitation professional who is trying to rehabilitate strength performance. If we don’t have a common understanding of what strength is, then clearly interventions to rehabilitate it will vary, as will their efficacy. So, what is muscle strength?

“The ability of muscle/muscle group to develop maximal contractile force against a resistance in a single contraction”

Kraemer & Ratamess (2004) [other references available!]

For me, this is the true definition of muscle strength. Some call it maximal strength, I call it strength and encourage that we use this as the reference point for rehabilitation and conditioning interventions. It’s representative of maximal capacity, relatively easy to measure and enables a standardisation of approaches to its rehabilitation. It’s also the definition used in a plethora of research publications.

So, here are some other ‘sub definitions’ that you might have heard of:

Agile strength: “The ability to decelerate, control and generate muscle force in a multiplanar environment. Examples: Picking up and carrying a young child, laundry basket or duffle bag”

Hmm, could we argue that this is just control, or function? It’s certainly not a maximal contraction, so perhaps power…? I’m not sure this term is helpful, and to assess this type of performance it’d probably involve a test of function…?

Strength Endurance: “The ability to maintain muscular contractions or a consistent level of muscle force for extended periods of time”

I can see why this term has come about, perhaps describing the ability of a 400m runner to keep pace, but physiologically it’s conflicting and really not helpful. This is just muscular endurance.

Explosive strength: “Produce a maximal amount of force in a minimal amount of time; muscle lengthening followed by rapid acceleration through the shortening phase. Focus is on the speed of movement through a range of motion (ROM)”

I’m afraid to say, this is actually a thing. It’s also called rate of force development, or in a performance situation, muscle power. I mentioned explosive force production in a post wrote last year in the discussion of male / female differences.

Relative strength: “Amount of force generated per unit of bodyweight.” Yes, this is correct. I referred to relative strength in a previous post when discussing male / female differences. But in order to get this figure, you need to the initial unit in the calculation, which is (maximal) strength.

There are various other terms, but these are some of the most common variations. Just remember though muscle strength is the maximal contractile force of a muscle/muscle group in a single contraction and your design of rehab interventions will be made much easier and more efficacious.

Muscle Endurance

“The ability of a muscle or a muscle group to repeatedly exert a sub-maximal force”

ACSM (2017) Guidelines for Exercise Testing and Prescription [other references available!]

This is a fairly simple one. We can sub-classify muscle endurance into high-intensity or low-intensity and further if we look at the muscle group level or whole body exercise.

Rate of Force Development (RFD)

The title really explains the index here. RFD is :

The scientific measurement of speed of force production, or muscle power during isolated muscle contractions.

As you can see, this index is typically measured in a laboratory during isolated muscle contractions. Have a look a the image below, here we’re measuring the slope of the force-time curve to quantify RFD. The steeper the gradient, the greater the force production in a set amount of time. This index has been purported to be important in mitigating injury risk.

rate of force development

Muscle Power

This could be viewed as the functional or performance measurement resulting from RFD capabilities i.e. we’re not measuring isolated muscle contractions, we’re often measuring the effects of the speed of muscle force production in accelerating a object/body. For example, the vertical jump is a test of lower limb power, higher jump performances require greater baseline levels of muscle strength and the ability to produce a proportion of it quickly to propel the mass of the body upwards against gravity. Likewise, throwing a projectile for distance can be a test of power (as long as there’s not a high level of skill and technique required).

Karate kick with jump

Power = Force × Velocity

Power production is enhanced when one or both components of the power equation (i.e., Power = Force × Velocity) are enhanced. We will look at this in depth in a future post.

Electromechanical Delay (EMD)

This may be a new term to you. It’s not that commonly spoken of but I wanted to highlight it as its something that can dramatically influence muscle contractile performance. Broadly speaking, EMD is

“the time delay between the onset of electrical activity (EMG) and the measurable onset of muscle force”

Minshull et al. (2009)

Basically, before we even start to produce force, there’s a series of physiologic events that occur, which means there’s a delay between the signal being received at the muscle site to contract (the EMG) and the initiation of muscle force production, or its transmission to bone to enable movement (See figure above). Under most circumstances, the majority of this delay is due to the taking up the slack in the system – reeling in the compliant tissue. Think of it like a car towing another car and the delay represented by the tow rope.

It’s very difficult to measure but worth bearing in mind during end-stage rehab. I’ve spoken a lot about this index before, so if you want to read more have a look here and here.

Muscle Hypertrophy

I thought I’d pop this in here because so often strength training is confused with body building. It’s not the same. To achieve optimal results (for strength OR size), each require a different conditioning strategy. To achieve morphological change, we need sufficient mechanical overload to enable

“…anabolic processes [to] prevail over catabolic processes to promote a net increase in muscle protein synthesis and corresponding enlargement of fibers”

Glass (2005)
Muscle strength versus body building

Strength is indeed related to hypertrophy, or muscle cross-sectional area, but there are important neural processes that also determine strength gain. Plus the latency of change of each is different. Therefore, especially in rehabilitation settings, it’s important to consider strength and hypertrophy separately and maintain the distinct specificity of each rehabilitation or conditioning intervention.

As noted above, we’ll also be covering hypertrophy as the S&C series progresses.


Right, so I think we’ve covered some fundamentals things here and set the scene for the upcoming posts.

If you have any special requests for topics for this series, please drop me a line and I’ll be glad to try incorporate it! Contact me

Have a great day! 🙂


KEEP YOUR EYES PEELED! Details of the March enrolment for the Strength & Conditioning For Therapists Online course are coming soon…!


  • Kraemer & Ratamess. (2004). Fundamentals of resistance training: progression and exercise prescription. Med Sci Sports Exerc. 36:674-88
  • ACSM (2017) ACSM’s Guidelines for Exercise Testing and Prescription 10th Ed. Wolters Kluwer, USA.
  • Minshull et al (2012). Single measurement reliability and reproducibility of volitional and magnetically-evoked indices of neuromuscular performance in adults. J Electromyo Kines. 19: 1013 1023
  • Glass (2005). Skeletal muscle hypertrophy and atrophy signaling pathways. Int. J. Biochem. Cell