Measuring Up: Do Hand-Held Dynamometers and Force Plates Add Value to Shoulder Rehab?

Dr Angela Cadogan, PhD, NZRPS
​Specialist Physiotherapist (MSK)

Force measuring technology has been around for decades but has only recently become more affordable and accessible to the mass physiotherapy market. The ability to objectively measure force and track changes over time provides the patient with a source of motivation, takes the guess work out of manual strength testing, provides us with deeper insights into force production and provides funders with objective measures of progress.

​With increasing amounts of time and money being spent on equipment and collecting force data, maybe it’s a good time to pause and consider what value we are getting from these devices in our shoulder rehabilitation?

Ahead of a workshop I am involved in with other Specialist Physiotherapists at the upcoming Sport and Exercise Physiotherapy NZ conference I thought I’d take a look at some key questions that we should all consider when using force technology. There are more questions than answers in here I’m afraid but hopefully some good food for thought.

I’ll focus on three main questions:
1. Why are we testing?
2. What is the quality of our data?
3. How are we using the data to inform our rehab?
 
1. Purpose of force testing
Force measures are widely used as proxies for strength and power in physiotherapy. Force measures and derived force data such as force ratios have application in injury prevention, rehabilitation and performance monitoring and optimisation.

In New Zealand, the reporting of force “outcome measures” at standardized time-points has recently become mandatory for clinicians as part of funded clinical pathways for patients following injury or surgery. This provides patients, therapists and funders with objective measures of progress, can improve patient motivation in rehab and creates a large database from which valuable insights may be gained in specific populations.

However, when the selection and timing of the test is not clinician-driven but externally mandated, there’s a risk that clinical reasoning processes are by-passed, and these measures become more about ticking boxes and passing tests than informing clinical decisions.

These tests should not replace our clinical reasoning but should add to it by providing more data from which we can decide the next steps in the rehabilitation priority list. Next time you use your HHD or force plate in clinic stop for a moment to consider: Why am I doing this test, what is it measuring and how will the test data inform my rehabilitation?

2. Data Quality
We’ve all heard the saying “rubbish in- rubbish out”. Force data is no different and our clinical decisions are only as good as the data we collect. Measurement reliability is one of the biggest challenges in hand-held dynamometry. Reliable measurements are essential to ensure that the data reflects a true change in strength or function. Without reliability, you risk either over-treating perceived deficits that aren’t real or missing genuine deficits that need addressing.

Measurement reliability is affected by many factors including:

• Instrument factors: Calibration issues and pre-set test parameters.
• Patient factors: Motivation, posture, kinetic chain involvement (“cheating”), fatigue and recovery status from previous exercise.
• Tester factors: Tester strength, positioning, and instructions.
• Test protocol factors: Make vs. break testing, limb movement, number of repetitions, and rest periods.

 
The distinction between ‘make’ and ‘break’ test protocols is important. ‘Make’ testing involves the patient pushing against a fixed resistance, while ‘break’ testing involves the tester overpowering the patient’s force. In ‘break’ testing the amount of force applied depends on the strength of the tester. Break tests also involve eccentric forces which produce higher peak forces than concentric contractions. Understanding these differences and standardizing your approach is key to improving the consistency of your measures.

How reliable is your data?
Do you know your measurement variability? If you don’t, there is no way for you to know your "minimum detectable change" (MDC). Without knowing your absolute reliability (magnitude of the measurement error), there is no way of knowing whether the change you are seeing is simply due to measurement variation or a true change in force/strength. Research reports of reliable test measures for various tests and protocols don't guarantee your measurement reliability either. You need to test this yourself. 

Assessing your own absolute reliability (vs relative reliability using ICC values) can reveal the level of variability inherent in your measurements and help you identify your MDC and interpret your results more accurately. Many devices will do this for you. If not, there are several ways of doing this. One simple way is to calculate the mean difference between your measures, and the 95% standard deviation of the mean difference. Any force measures taken must lie outside this value to be (95%) sure the difference is real and not just related to measurement variability.

3. Clinical Decision-Making
The value of force testing depends not only on the quality of the data, but also many other clinical decisions:

• Safety:  is it safe to perform this test? (e.g post-op rotator cuff repair surgery).
• Test selection: what is my variable of interest? (peak force, force fatigue, RFD).
• Test position: where in the ROM am I interested in testing and can I get reliable measures in this position? (e.g end range abduction-external rotation (90/90) for anterior shoulder instability).
• Test quality: is the patient ‘cheating’ by using other agonist muscles (e.g deltoid, pectorals) or the kinetic chain to compensate during the test.
• Interpreting deficits:

          - What is clinically relevant? 15% might be relevant for an athlete but not for a sedentary individual.
          - What is the cause of this deficit? Can you unpack and ‘reverse engineer’ the test to identify where the system is ‘weak’? (e.g in the ASh test or a countermovement/plyometric push up – what is the cause of reduced force or RFD?)

• Informing rehabilitation: how will I target specific tissues in the rehabilitation programme to improve tissue adaptation or performance based on these results?

I’m going to expand on the last two points.

Reverse engineering the observed deficits:
When deficits are present, we need to be able to reverse-engineer or unpack the deficits to assess specific impairments that may be contributing to the observed deficit. Has the patient ticked the necessary ‘clinical test’ boxes around the glenohumeral joint and scapula to be able to safely and effectively perform a countermovement or plyometric push-up?

• No symptoms
• Full ROM
• Adequate motor control
• Adequate strength in key muscle groups
• Adequate power (rate of force development) in key muscle groups.

 
Failing to detect underlying clinical impairments risks returning someone to higher level participation with functional strength, but with underlying untreated deficits that may put them at risk for re-injury.

Informing rehabilitation:
Understanding how your data informs your rehabilitation requires a fundamental knowledge of the physics of force production and tissue mechanics (contractile and non-contractile components) during various contraction types (concentric, eccentric and isometric).

For example: Rate of force development (RFD) is often used as a proxy for power. Power = Force x Velocity. If you want to increase power, you either increase the force (using heavy slow training) or the velocity (light, high velocity training). Which one do you choose and how do you programme that? Does the patient have sufficient tissue capacity to do that safely? How do you target specific bone, muscle, tendon and other connective tissues specifically in your rehabilitation to optimize adaptation or performance? Some questions to ponder.

Summary
Force technology has been a game-changer for physiotherapists supporting our clinical reasoning and providing objective markers of progress for patients and funders.  However, their value depends on the knowledge and skill of the tester in selecting the appropriate test, the quality of the data and what we do with the results.

​Here is a checklist of questions to ask yourself when using force technology to help you get the best value for your testing buck:

• What am I measuring this and how will I use this information?
• What is the quality of my data?
• What is a clinically relevant deficit?
• Can I reverse engineer this movement to assess its components and identify and rehabilitate contributing impairments?
• Do I understand the tissue mechanics and mechanics of force production in this test?

 
A lot of questions that I hope will point you in the direction of some answers, or some targeted CPD. If nothing else, hopefully some food for thought.

May the force be with you. 

RESOURCES
Cadogan A, Laslett M, Hing W, McNair P, Williams M. Reliability of a new hand-held dynamometer in measuring shoulder range of motion and strength. Man Ther. 2011;16(1):97-101.​ (Article link)

Force Plate Fundamentals for Physiotherapists (Free Webinar).

​Enhancing Muscular Performance (Online Learning). 

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Author

Dr Angela Cadogan is a registered Specialist Physiotherapist (MSK) based in Christchurch, New Zealand. She has a PhD and clinical sub-specialty in diagnosis and management of shoulder conditions and works in clinical consultancy, education and clinical governance roles. 

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