I’ve been really enjoying of late teaching lots of courses on dynamometry and a question that often comes up is: how long do we hold the contraction for? So in this post we’re going to answer just that: How to assess strength – contraction length and rest explained.
Type of Contraction
In this post we’ll be referring to isometric contractions, where there’s no appreciable change in joint position or muscle length and as applied to all types of dynamometry where this happens, which includes hand-held.
How to cue the contraction? Following a warm-up and a few practice attempts, cue the person that you’re assessing to push “as hard and as fast” as they can. Make sure you give plenty of encouragement – remember, you want to record their maximal force output, else it’s a pointless test.
Here’s an example of me assessing the brilliant dance physio Liz Bailey’s plantar flexion strength on one of my courses – click the image to watch the vid. Note that this would require methodical improvements as Liz manages to lift up the bar at the end of the effort!
In some circumstances, patients will be hesitant, or clinically, you’re not confident to explosively load the previously injured tissue, in which case a more graded contraction would be more appropriate, but nevertheless still maximal. If efforts don’t represent a maximal voluntary contraction (MVC), then you’re not assessing strength.
Contraction Length
Okay, so once the person is producing an MVC, how long to hold it for? Well, if you reference some HHD software, it’ll tell you to hold for 5 seconds, until the software has ceased recording. I don’t agree. 3 seconds is usually sufficient. Why? Most people will be able to produce their maximum force in a well-conducted isometric test within 300ms – that’s one third of a second! See the image below constructed from data taken from a study we conducted in young healthy males and females from a knee-extensor test.
Ha! I hear you say, that’s not your average clinical population. And you’d be right. But let me reassure you that a rapid time to peak can still be present in older populations (in fact it should be something that you aim for) . Look a the data here from Suzuki et al. (link to paper). Peak force was achieved in 350ms in individuals with mild knee osteoarthritis.
In fact, deficit, or lengthy times to peak could be a marker of an impaired rate of force development and a key marker to improve in rehab. Below are some data from our study, which clearly shows that following 6-months post ACLR this individual’s time to peak, whilst still achieved within 3 seconds is clearly impaired, as is their RFD.
So, how long to hold a contraction?
- 3 seconds in habituated, practiced individuals
- 5 seconds where a graded contraction (more gradual production) is required
Oh, and by the way, the relaxation part of the curve can be interesting to inspect to see how the removal of force occurs. Individuals who are inhibited, or in pain, tend not to want to ‘let go’ … but that’s another topic.
Rest Explained – How Much?
This is an often-overlooked parameter in strength testing, but it’s super important. In a strength test we’re asking an individual to recruit maximally (or try to) from their fast twitch motor unit pool and to sustain that contraction for approx 3 seconds. This requires fuel – ATP in fact and following an MVC we need rest to replenish the fuel in order to be able to repeat the same intensity and duration of effort.
It’s beyond the scope of this post to cover the different energy systems, relative contribution and timings in detail, however, to summarise, 10 seconds’ rest should be sufficient to enable neuromuscular recovery between one or two strength efforts and for participants to set themselves up to go again.
When is 10 seconds too little?
It’s important to pay attention to the data as you record it. On some occasions, you may see that the person’s efforts show a pattern of decline, i.e. each is a little less than the other. Where you’re performing 3 or so MVCs and assuming that you haven’t fatigued them in the warm-up, 10s should be sufficient to enable neuromuscular recovery, but perhaps the person requires a little more time to muster the motivation to go again.
In this instance, don’t be afraid to give a little more inter-trial rest. For this reason, some researchers use a 30 s inter-trial rest.
Another situation where you need to consider rest is where your participant shows a clear learning effect, where efforts increase across trials. Sounds counterintuitive perhaps? However, when you observe this – often times with someone new to strength testing or following a long period of disuse – you should continue testing until you see the scores plateau. We want the scores to plateau because you’re seeing true maximal performance, not because the person has fatigued.
In this instance I’d recommend separating blocks of 5 ‘maximal’ contractions with a minute and a half or 2 minutes rest. This is to give every opportunity for the participant to perform to their best and to minimise the influence of fatigue associated with multiple consecutive maximal contractions.
Summary
Right, what’s the answer to: How To Assess Strength Contraction Length & Rest Explained.
Contraction length – keep it short and explosive where possible, 3-5 seconds maximum.
Rest – minimum of 10 seconds between MVCs and 1.5 – 2 minutes between sets of MVCs
ENROLLING SOON
References
Hannah R, Folland JP, Smith SL, Minshull C. Explosive hamstrings-to-quadriceps force ratio of males versus females. Eur J Appl Physiol. 2015 Apr;115(4):837-47.
Suzuki Y, Iijima H, Nakamura M, Aoyama T. Rate of force development in the quadriceps of individuals with severe knee osteoarthritis: A preliminary cross-sectional study. PLoS One. 2022 Jan 11;17(1):e0262508.
Minshull C, Gallacher P, Roberts S, Barnett A, Kuiper JH, Bailey A. Contralateral strength training attenuates muscle performance loss following anterior cruciate ligament (ACL) reconstruction: a randomised-controlled trial. Eur J Appl Physiol. 2021 Dec;121(12):3551-3559.
