The relation between neuromechanical parameters and Ashworth score in stroke patients

Abstract

Background: Quantifying increased joint resistance into its contributing factors i.e. stiffness and viscosity ("hypertonia") and stretch reflexes ("hyperreflexia") is important in stroke rehabilitation. Existing clinical tests, such as the Ashworth Score, do not permit discrimination between underlying tissue and reflexive (neural) properties. We propose an instrumented identification paradigm for early and tailor made interventions.

Methods: Ramp-and-Hold ankle dorsiflexion rotations of various durations were imposed using a manipulator. A one second rotation over the Range of Motion similar to the Ashworth condition was included. Tissue stiffness and viscosity and reflexive torque were estimated using a nonlinear model and compared to the Ashworth Score of nineteen stroke patients and seven controls.

Results: Ankle viscosity moderately increased, stiffness was indifferent and reflexive torque decreased with movement duration. Compared to controls, patients with an Ashworth Score of 1 and 2+ were significantly stiffer and had higher viscosity and patients with an Ashworth Score of 2+ showed higher reflexive torque. For the one second movement, stiffness correlated to Ashworth Score (r2 = 0.51, F = 32.7, p < 0.001) with minor uncorrelated reflexive torque. Reflexive torque correlated to Ashworth Score at shorter movement durations (r2 = 0.25, F = 11, p = 0.002).

Conclusion: Stroke patients were distinguished from controls by tissue stiffness and viscosity and to a lesser extent by reflexive torque from the soleus muscle. These parameters were also sensitive to discriminate patients, clinically graded by the Ashworth Score. Movement duration affected viscosity and reflexive torque which are clinically relevant parameters. Full evaluation of pathological joint resistance therefore requires instrumented tests at various movement conditions.

Figure 1

Measurement set-up. The subject's ankle was fixated on the footplate that was rotated by an electrically powered single axis actuator. Ankle reaction torque, ankle angle and EMG were measured during imposed ramp-and-hold movements.

Figure 2

Range of motion. Range of motion (RoM) of all subject groups (mean and standard deviation). The asterisk denotes significant difference (see Results).

Figure 3

Imposed ramp-and-hold movement profiles, joint torque and IEMG. Rows from top to bottom: Ankle joint angle showing the imposed (dorsiflexion) ramp-and-hold (RaH) joint rotation profiles at four different movement durations (columns: 0.25, 0.5, 1.0, 2.0 s), corresponding joint torque responses and IEMG signals from all four muscles. Traces are shown over a five second time frame for an AS3 patient. Positive values indicate to dorsiflexion.

Figure 4

Model fit. Typical model fits at 0.5 s dorsiflexion duration. Left column: patient (AS3). Right column: control subject. A-B: imposed ankle movement; C-D: measured joint torque (grey) and torque as predicted from the model (black); E-F: reflex torque from triceps surae and tibialis anterior muscles; G-H; torque due to stiffness (solid) and viscosity (dashed); I-J: inertial (solid) and gravitational torque (dashed).

Figure 5

Estimated IEMG activity. Same patient (left column) and control subject (right column) and conditions as in Figure 4. Traces in grey are the IEMG signals from all muscles (C-D and G-L). The black traces (E-F and M-N) are the estimated (synthesized) muscle activity of the TA and triceps surae (sum of GL, SL and GM) respectively. The estimated signals were obtained from multiplication of the IEMG signals with the optimized weighting factors (e₁-e₄) and served as inputs to the muscle activation filters to produce the reflexive torque such as shown in Figure 4 (E-F).

Figure 6

Parameter covariance. Covariance matrix P (top) and SEM values (bottom) of all estimated model parameters. Only the upper part of P is shown because of its symmetry. For normalization, see Method Section. Averages over all conditions and subjects (solid bars) ± 1 s.d. (grey error bars). The auto-covariance is on the diagonal of P. The off-diagonal terms of P are the relative cross-covariances between two different corresponding parameters. Percentages at the right are measures of interdependence, i.e. the number of times the auto-covariance was smaller than any of the corresponding cross-covariance values. The SEM is equal to the square root of the auto-covariance, divided by the corresponding mean parameter value.

Figure 7

Intertrial difference. Intertrial parameter difference (solid bars: mean; error bars ± 1 s.d.) relative to the mean value of both measurements (one repetition), and then averaged over all conditions and subjects and for all parameters (horizontal axis). Asterisk denotes statistical difference from zero value.

Figure 8

Ankle Joint Viscosity and Stiffness. Viscosity (top) and stiffness (bottom) for all subject groups against dorsiflexion duration. Subject groups (C, AS0, AS1, AS2+) from left to right for each cluster, denoted by c, 0, 1 and 2+ respectively. Joint viscosity and stiffness were taken at the same ankle angle for all subjects (controls and patients) being 3.03 degrees dorsiflexion (see Methods).

Figure 9

Reflexive torque. Stretch reflex torque (r.m.s.) for all subject groups against movement duration for triceps surae (top) and tibialis anterior (botttom) muscles. Subject groups (C, AS0, AS1, AS2+) from left to right for each cluster, denoted by c, 0, 1 and 2+ respectively.