Accuracy and Reliability of Single-Camera Measurements of Ankle Clonus and Quadriceps Hyperreflexia

Abstract

Objective: To evaluate the accuracy and reliability of a simple, single-camera smartphone-based method, named the Reflex Tracker (RT) system, for measuring reflex threshold angles related to ankle clonus and quadriceps hyperreflexia.

Design: A prospective comparison study using a high-fidelity reference standard was constructed employing a 2 × 2 × 2 factorial design, with factors of rater (tester) type (student and experienced physical therapist), joint (ankle and knee), and repetition (2 per condition).

Setting: This multicenter study was conducted at 4 outpatient rehabilitation clinics.

Participants: A convenience sample of 14 individuals with a neurologic condition presented with 20 lower limbs that exhibited ankle clonus and/or quadriceps hyperreflexia and were included in the study. Also participating in the study were 8 student and 8 experienced physical therapist raters (testers) (N=16).

Interventions: Not applicable.

Main outcome measures: The plantar flexor reflex threshold angle (PFRTA) related to ankle clonus and the quadriceps reflex threshold angle (QRTA) related to quadriceps hyperreflexia were quantified.

Results: PFRTA and QRTA results were compared between the smartphone RT method and synchronous 3-dimensional inertial measurement unit (IMU) sensor motion capture. Mean difference (bias) was minimal between RT and IMU measurements for PFRTA (bias≤0.2°) and QRTA (bias≤1.2°). Intrarater reliability for PFRTA ranged from 0.85-0.90 using RT and from 0.85-0.87 using IMU; QRTA ranged from 0.97-0.98 using RT and from 0.96-0.99 using IMU. Intersensor reliability for PFRTA and QRTA was 0.97 and 0.99, respectively. Minimum detectable change for PFRTA ranged from 7.1°- 8.7° and for QRTA ranged from 6.1°-8.3°.

Conclusions: RT performed comparable to IMU for accurate and reliable measurement of PFRTA and QRTA to quantify ankle clonus and quadriceps hyperreflexia in clinical settings.

Keywords: CI, confidence interval; ICC, intraclass correlation coefficient; IMU, inertial measurement unit; LSD, least significant difference; LoA, limit of agreement; MDC, minimum detectable change; PFRTA, plantar flexor reflex threshold angle; Plantar flexor; QRTA, quadriceps reflex threshold angle; RMS, root mean square; RT, Reflex Tracker; RTA, reflex threshold angle; Reflex threshold angle; Rehabilitation; Smartphone; Spasticity.

Fig 1

Experimental setups and single-camera perspective. (A) Ankle clonus drop test setup, (B) quadriceps pendulum test setup, (C) ankle drop test setup after applying color masking, (D) stationary marker, and (E) marker experiencing motion blur.

Fig 2

Participant testing and analysis flow diagram. *Problems identified during analysis included the tester contacting the shank IMU during the test, a knee marker coming loose, and magnetic interference. ^†Problems later identified in knee pendulum test data included a misrecording and magnetic interference.

Fig 3

Representative results in 1 participant of the (A) ankle clonus drop test and (B) quadriceps pendulum test measured by the RT system and IMUs. Differences in PFRTA and QRTA measurements by each system are computed relative to baseline values. Hip internal rotation measured by IMUs illustrates 1 source of differences between RT and IMU plantar flexion angle.

Fig 4

Bland-Altman plots of RTA measurements calculated with the RT system and IMUs. The shaded regions represent separate 95% CIs of the bias and 95% CIs of the LoA.