The System

Inertial Measurement Units (IMU)

Our sensors are designed and assembled in house.  The wireless IMUs allow for the capturing of movement data through the use of an accelerometer, a gyroscope, and a magnetometer.  Using Bluetooth Low Energy, a sensor communicates with an iPad on a set interval, allowing the iOS application to process the incoming samples and provide feedback to the user.

A key factor in how our sensors work is the virtual alignment.  Data from an alignment trial is processed using a Gauss Newton minimizer to virtually align the sensors with the knee joint axis. This method is preferable to manual alignment because it standardizes sensor placement across subjects regardless of anatomical differences while allowing sensors to be attached at a location with minimal soft tissue artifact.

Data Acquisition

• Body slicing synchronization

Feature Calculators

• Our system uses validated novel algorithms to calculate high-level clinical gait features from raw IMU data
• Work in real-time to process incoming sample data from our BLE sensors and output useful features that can be used in our classifier

Classification System

• Using the calculated features, the classifier identifies which gait deviations are present
• It does this using an ensemble learning method

Feedback/Audio Engine

• The feedback engine, used specifically in the ReLOAD project, classifies each step and works in tandem with the audio engine to map a deviation to an audio effect
• The audio engine, built on top of Apple’s AVAudioEngine API, contains custom built audio effects used for real-time feedback

Region Of Limb Stability (ROLS)

The Region of Limb Stability (ROLS) is a novel metric created to quantify the stability of the supporting lower limb by measuring bi-directional excursion via inertial sensors during the single limb stance test. Maximum thigh and shank segment excursions in the anterior-posterior and medial-lateral planes were used to illustrate the ROLS excursion diagram and calculate the ROLS value. The ROLS SI (in %) is a single metric generated to quantify differences within individuals by comparing two ROLS values calculated during left and right SLS where 100% suggests absolute symmetry. Clinicians and coaches can utilize the ROLS results to develop an individualized balance-training program to decrease the risk of injury in athletes who present increased lower limb segmental excursion or asymmetry. Also the use of ROLS provides a practical form of objective testing for concussion injuries.

Gyroscope Only (GO) algorithm for calculating knee angles

Tele-rehabilitation of patients with gait abnormalities could benefit from continuous monitoring of knee joint angle in the home and community. Continuous monitoring with mobile devices can be restricted by the number of body-worn sensors, signal bandwidth, and the complexity of operating algorithms. Therefore, this paper proposes a novel algorithm for estimating knee joint angle using lower limb angular velocity, obtained with only two leg-mounted gyroscopes. This gyroscope only (GO) algorithm calculates knee angle by integrating gyroscope-derived knee angular velocity signal, and thus avoids reliance on noisy accelerometer data. To eliminate drift in gyroscope data, a zero-angle update derived from a characteristic point in the knee angular velocity is applied to every stride.

Noise-Zero Crossing Algorithm

NZC is based on the inverted pendulum model of gait, which states that the leg alternates between advancing as a pendulum during swing phase (pivot at the hip) and as an inverted pendulum during the stance phase (pivot at the foot). Under this mode, heel-strike is identified by the noise spike in the trough following mid-stance swing, and toe-off is identified by the moment of zero velocity following the trough before mid-stance swing.

Heel-Strike

The trough after MSw has been associated with a region of increased noise that has been observed across studies involving healthy and pathologic populations (Aminian et al., 2002; Bötzel et al., 2016; Greene et al., 2010 ; Salarian et al., 2004) in which wavelet transforms and filtering techniques were used to reduce noise to reliably identify the trough. NZC proposes that the universality of this noise can be exploited to identify HS without filtering.

Toe-Off

Bötzel et al. note that TO occurs after the second trough in the shank angular velocity signal, but prior to forward velocity. They explain their findings with the physiologic argument that the minimum of the second trough physically represents the point at which the shank begins to decelerate. They argue that TO should occur sometime after that deceleration begins, but before the limb begins to advance about the opposite pivot, represented by the inception of positive angular velocity. We posit that TO can thus be estimated as the zero-crossing after MSt, which is the precise moment of zero shank angular velocity.

NZC algorithm implementation

The NZC algorithm is outlined in the figure above. NZC waits for the first zero-crossing after the onset of walking. If the slope is negative, the algorithm waits for the onset of noise, defined as the first sample where the slope of the signal becomes positive, checks whether that sample is the minimum swing period from the last transition point (τ_ST), and stores that point as a swing-to-stance heel-strike event (SWtoST). If the slope is positive, the algorithm checks whether the current sample is at least the minimum stance period from the last transition point (τ_SW) and stores this point as a stance to swing toe-off event (STtoSW). Transition points consist of STtoSW and SWtoST events. Minimum swing and stance periods were defined a priori using knowledge of gait phase knowledge and manually verified for accuracy. For each sample, gait phase is assigned as stance if the most recent transition point was SWtoST or swing if it was STtoSW. GCT, SLS, and DLS time intervals are computed using the gait phase from both limbs.

Missing Sample Algorithm

This paper presents a novel, practical, and effective routine to reconstruct missing samples from a time-domain sequence of wirelessly transmitted IMU data during high-level mobility activities. Our work extends previous approaches involving empirical mode decomposition (EMD)-based and auto-regressive (AR) model-based interpolation algorithms in two aspects: 1) we utilized a modified sifting process for signal decomposition into a set of intrinsic mode functions with missing samples, and 2) we expand previous AR modeling for recovery of audio signals to exploit the quasi-periodic characteristics of lower-limb movement during the modified Edgren side step test. To verify the improvements provided by the proposed extensions, a comparison study of traditional interpolation methods, such as cubic spline interpolation, AR model-based interpolations, and EMD-based interpolation is also made via simulation with real inertial signals recorded during high-speed movement. The evaluation was based on two performance criteria: Euclidian distance and Pearson correlation coefficient between the original signal and the reconstructed signal. The experimental results show that the proposed method improves upon traditional interpolation methods used in recovering missing samples.

TUG Test

• The Timed Up and Go (TUG) test is used to assess a person’s mobility. The subject starts seated in a chair, and is asked to stand up, walk around a cone placed 3 meters away, and sit back down into the chair. The test general takes 10 to 20 seconds to complete.

AMP Test

• The Amputee Mobility Predictor test is used to assess determinants of an amputee’s lower-limb ability to walk

mCTSIB Test

• The Modified Clinical Test of Sensor Interaction and Balance (mCTSIB) test is used to assess both vestibular and non-vestibular balance

6 Minute Walk Test

• The 6MWT is used to assess the distance walked over 6 minutes as an approximate test of aerobic capacity and endurance

10 Meter Walk Test

• The 10MWT is used to assess the speed at which a subject walks over a short duration, 10 meters

Self Report

• A collection of standardized instruments for the subjective reporting of prosthetic satisfaction, perceived mobility, and other measures of pain and independence