Hypothesis / aims of study
Physiotherapists typically use digital palpation to determine the extent to which a relaxed muscle resists movement in response to an applied force. The muscle resistance is commonly referred to as muscle tone, tension, activity, tightness, or stiffness. The viscoelastic properties of the connective tissue, and the contractile components of the muscle will contribute to the muscle stiffness.
These digital assessments are subjective, relying on the experience and skill of the clinician, with little known about their accuracy or repeatability. There is no standardised palpation scale for stiffness, nor any scale that has been validated against an objective standard. Despite this, it is standard practice globally to use digital palpation to assess muscle stiffness, and to base clinical treatment on these findings.
The primary aim of this study was to assess physiotherapists’ ability to assign a seven-point palpation scale to stiffness values generated by a novel device that can objectively measure force (N) and displacement (mm), over a range of specified stiffness (N/m) values.
Study design, materials and methods
This was a prospective observational study. Qualified physiotherapists, who use digital palpation to assess muscle stiffness for a minimum of 30 minutes per week, were invited to participate via advertisements. Participants were excluded if they did not have conversational level of English. Their gender, years of clinical experience, and area of clinical focus (whether they identified themselves as a ‘musculoskeletal or a ‘pelvic floor physiotherapist’) were collected. Local ethics committee approval was obtained.
A seven-point arbitrary scale graded from -3 to +3 was modified from a scale published by Reissing (1). This scale was chosen as it offered a similar number of scale categories to the well utilised Modified Oxford Scale for muscle strength (2). The author modified the scale by adding specific force and displacement wording to ensure participants understood the measurement boundaries at each level of the scale. The scale ranged from 3 (highest stiffness) to -3 (lowest stiffness), with 0 being ‘normal’.
A custom designed ‘palpation instrument’ was developed by the University research institute to replicate the haptic feedback that clinicians use to grade muscle stiffness. The electromechanical instrument can accurately measure the force applied to displace a moveable probe a certain distance. The end of the probe was covered with a compressible silicone material to resemble the soft tissue covering underlying muscle. Stiffness calculations were obtained from the force and displacement measurements.
The study was divided into two stages, with a different method for data collection used in each stage. In stage one, participants were given a number on the scale between 3 and -3 and requested the author increase or decrease the stiffness provided by the instrument until they determined the plunger stiffness matched the scale category they were identifying. Participants completed three randomized trials of the seven-point scale, resulting in 21 measures per person. In stage two, the author set the instrument stiffness to random values between 0 N/m and 1050 N/m (the range of stiffness values generated from the results of stage one), and asked the participant to assign the stiffness to a scale category between 3 and -3. Participants completed three randomized trials of seven measures, resulting in 21 measures per person. Measurements of displacement, force and stiffness were also recorded for each stage, with participants blinded to the device controls and all measurement values.
The probability of assigning a particular stiffness value to each of the scale categories was analysed using a cumulative frequency distribution of the stiffness values. The participants’ ability to consistently apply a particular stiffness value to a scale category was analysed using a coefficient of variation for each scale category.
Results
A total of 125 participants, of whom 83 % were female, took part in the study over a period of nine months, with 42 musculoskeletal and 42 pelvic floor physiotherapists taking part in both stages. Years of clinical experience ranged from three months to 56 years. Participants reported receiving their physiotherapy undergraduate training from 11 different countries.
1764 stiffness data points were recorded for both stages. Figure 1 illustrates the range of stiffness values that were assigned to each scale category in stage two. It demonstrates the large overlap between each category, this overlap also occurred with stage one results. Stage two is more representative of how clinicians assess muscle stiffness, so further analysis was completed using these data.
The probability of a stiffness value being assigned to the same scale category by different participants varied widely. Probabilities of the mean stiffness values for each scale category being assigned to that category are shown in Table 1. At scale levels -3, 2, and 3, there was a slightly higher probability of the scale been chosen (46 %, 42 %, and 41 % respectively).
None of the participants were consistently able to categorise a stiffness value to a scale category when using the whole scale (i.e. within a 10% margin of the stiffness values presented). 72 % of participants varied between 11 % to 30 %, with 28 % of participants showing variability over 31 %. When individual scale categories were analysed, the participants ability to consistently assign a stiffness to that individual category ranged from 1 % to 79 %, indicating large variations in ability.
Participants’ application of force in estimating stiffness progressively increased in a linear fashion from -3 to 3 (0.6 N ± 0.7 N at -3; to 3.4 N ± 1.8 N at 3). During stiffness measurement, the plunger was moved between 4 mm and 6 mm on average, with increased movement of the plunger at lower ends of the scale compared to the higher end in a linear fashion (6.3 mm ± 1.7 mm at -3 to 3.8 mm ± 1.7 mm at 3).
Interpretation of results
To the authors’ knowledge, this is the first study that has attempted to quantify physiotherapists to assign stiffness values to a digital palpation scale. While the scale appears to have lower stiffness values assigned to -3 on the scale, and higher stiffness values assigned to 3 on the scale, there are large overlaps between each scale category. The probability of assigning a stiffness value to the same scale category is low and the consistency in applying the scale is poor.