Effects of Stiffness Changes on Ankle Kinematics and Muscle ActivityNicholas Bira, Julieth Ochoa, Cameron Arnett, Prof. Neville Hogan

Massachusetts Institute of Technology, Amgen Scholars Program, Hogan Lab

ABSTRACTThe human ankle is an essential aspect of human locomotion, and understanding the subtle nuances of

how the ankle behaves under different stressors is of interest to researchers studying locomotion. Previous studies involving both single and multiple degrees of freedom have analyzed the mobility and motility of the ankle in healthy individuals. This study aims to examine how different ankle stiffnesses affect gait. Individuals with low mobility in the ankle (high stiffness) stride differently than people with high mobility, or poor muscular control (low stiffness). We applied varying stiffnesses to healthy individuals during the walking cycle to examine their responses and muscle activity. The results of these measurements will further define ankle behavior and how various factors affect locomotion.

METHODS

Processing Raw Data Workflow

Anklebot

Butterworth Filter

Segmentation based on

knee angle

Averaging and Stan.

Deviations

Zero data using initial

values

EMG

Rectification

Running Average window (200ms)

Segmentation based on heel

strikes

Averaging and Stan.

Devs.

Swing Phase

Swing Phase

Swing Phase

Stance Phase

Stance Phase

Stance Phase

Typical Knee Angle Stride Behavior Typical D/P Stride BehaviorTypical I/E Stride Behavior

• ~22 minutes of walking occurred at various stiffnesses: 1 min of starting 0, 5 min at +40, 5 min at 0, 1 min at -5, 5 min at -10, and another 5 min at 0.

• Data collected included knee angle, ankle inversion/eversion, ankle dorsiflexion/plantarflexion, and EMG signals for the Tibialis Anterior, Soleus, Gastrocnemius, and Peroneus Longus (200Hz for Anklebot, 1kHz for EMGs).

• 2 Degrees of Freedom in the ankle examined: Inversion/Eversion (I/E), and Dorsiflexion/Plantarflexion (D/P). Knee angles and lower leg muscle EMGs were also examined.

• Mobile Anklebot setup for Overground Trials (on a 200m track)

Eversion Inversion Plantarflexion

Dorsiflexion

Anklebot actuators, with knee brace. Kinematic data was collected from the two actuators and potentiometer.

Results - Kinematics

• Dependent variable: maximum knee flexion during swing phase

• +40 produced significantly different results than neutral or -10; reduced for OG and increased for TM

• Overall greater flexion in OG than TM

• Dependent variable: Ankle inversion/eversion maximum knee flexion during swing phase

• No significant difference between neutral stiffnesses in OG or TM

• During -10 stiffness, I/E was increased in both TM and OG

• During +40, I/E was decreased in both TM and OG

• Dependent variable: Stride Duration• In both OG and TM walking, +40 elicited

significantly greater stride duration• In OG, stride duration decreased after

+40 and remained constant regardless• In TM, stride duration decreased in each

epoch before jumping back up in 0_end

• Dependent variable: Ankle dorsi-/plantar-flexion at maximum knee flexion during swing phase

• No significant difference between +40 and following 0 in OG or TM

• During -10, DP was significantly greater in OG and lesser in TM

• This trend continued for OG but returned to normal for TM

• Two-factor ANOVAs over 800 strides

• Knee, DP, and IE angle were extracted from the moment of maximum knee flexion during swing phase

• Stride duration is the elapsed time between heel strikes

• Independent variables: • Walking Environment

(TM, OG)• Stiffness Value (+40,

0_middle, -10, 0_end)• 0.05 significance level was set

for all statistical tests• A significant interaction was

observed between walking environment and stiffness (p<0.05)

• Post hoc analyses using Tukey’s HSD test were conducted to examine marginal means for stiffness condition

• Overall statistically greater values for OG when compared to TM for knee and D/P, and lesser for I/E and stride duration.

• -10 stiffness had greatest effect on all studied aspects.

Inversion

Eversion

Dorsiflexion

Plantarflexion

Treadmill and Overground Mean Stride Behaviors per Epoch

TMOG

Results – Overground EMGs

• Two-factor ANOVAs were conducted on 420 strides

• Examining the effect of imposed ankle stiffness on maximum EMG activation levels during 4 different segments of the gait cycle in OG walking: 0-10% (initial contact & loading response), 40-60% (terminal stance and pre-swing), 60-80% (initial and mid swing), and 80-100% (mid and terminal swing)

• A 0.05 significance level was set for all statistical tests.

• Independent variables: Stiffness Value (0_initial, +40, 0_middle, -10, 0_end) and Muscle Type (TA, PL, SOL, GAS)

• Dependent variable: Maximum EMG Activation Levels

• Soleus alone displayed significant differences during the 0-10% epoch

• Increased between stiffness changes, and only for the -10 and following 0 stiffness

• SOL and GAS statistically increased across all epochs, except GAS decreased following the -10 epoch

• TA activation levels decreased significantly for all stiff• No effects in PL

• PL and SOL increased for each of the progressive epochs (especially SOL)

• TA displayed decreased magnitude as trial proceeded• No significant effects in GAS

• SOL displayed significant increases for each of the progressive epochs

• No significant effects in PL, TA, or GAS

TA

SOLSOLSOL

GASSOL

PL

All four muscles examined play important roles in normal walking and stabilization of the ankle.

Conclusions/Future Work

• Anklebot restricts rotation about the I/E plane with applied stiffnesses, but not in the D/P plane.• While variations in the I/E plane are apparent for the different stiffnesses, there are also statistically significant differences in D/P angle

as well.• This suggests coupling between I/E and D/P

• The EMG data from overground suggests that the Soleus is profoundly affected by different stiffnesses, in particular, that proceeding from a positive stiffness to a negative one generates increased muscle activity that lasts beyond the cause

• The TA seems to be noticeably affected by the negative 10 stiffness during the swing phase of the stride• The TA is involved in lifting the toes, and this behavior may be the result of the ankle attempting to stabilize and compensate for the

negative stiffness.• Treadmill and Overground demonstrate markedly different magnitudes in angle magnitude

• This indicates that treadmill and overground may be different in how they induce walking, unlike some previous studies that indicate the two produce equivalent results

• On treadmill, steps seem to be less pronounced as a possible result of the instability felt while walking • Overground may encourage longer strides and more pronounced movements as a result of better posture and forward facing

movement

• Distinct effort will be taken to collect further trials from multiple subjects using the processing methods in this presentation• Muscle EMGs will be calibrated to maximum voluntary contraction from standardized procedures in future trials• A more thorough examination of the differences between TM and OG, eliminating differences such as walking speed, stride length, etc.