Accepted Manuscript Title: Individuals with Diminished Hip Abductor Muscle Strength Exhibit Altered Ankle Biomechanics & Neuromuscular Activation during Unipedal Balance Tasks Author: Szu-Ping Lee Christopher M. Powers PII: DOI: Reference:
S0966-6362(13)00699-1 http://dx.doi.org/doi:10.1016/j.gaitpost.2013.12.004 GAIPOS 4102
To appear in:
Gait & Posture
Received date: Revised date: Accepted date:
5-8-2013 3-12-2013 4-12-2013
Please cite this article as: Lee S-P, Powers CM, Individuals with Diminished Hip Abductor Muscle Strength Exhibit Altered Ankle Biomechanics & Neuromuscular Activation during Unipedal Balance Tasks, Gait and Posture (2013), http://dx.doi.org/10.1016/j.gaitpost.2013.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Individuals with Diminished Hip Abductor Muscle Strength Exhibit Altered Ankle Biomechanics & Neuromuscular Activation
us
Szu-Ping Lee, PT, PhD1
cr
ip t
during Unipedal Balance Tasks
M
an
Christopher M. Powers, PT, PhD2
d
1. Department of Physical Therapy, University of Nevada, Las Vegas, Las Vegas, USA
Ac ce p
te
2. Division of Biokinesiology & Physical Therapy, University of Southern California, Los Angeles, USA
Address for Correspondence: Szu-Ping Lee, PT, PhD Assistant Professor
Department of Physical Therapy
University of Nevada, Las Vegas 4505 S. Maryland Parkway, Box 453029, Las Vegas, NV 89154-3029, USA Phone: (702)895-3086 Email: [email protected]
Page 1 of 22
Abstract Coordinated control of the hip and ankle is important for maintaining postural stability. The
ip t
purpose of the study was to compare postural stability between individuals with contrasting hip abductor strength during unipedal balance tasks and to determine whether diminished hip
cr
abductor strength results in greater utilization of the ankle strategy to maintain balance. Fortyfive females (27.6±3.5 yrs.) participated in the study. Participants were ranked based on their
us
isometric hip abductor muscle strength. The top 33% of the participants were categorized as the
an
strong group (n=15) and the lower 33% as the weak group (n=15). Each subject performed a static and a dynamic unipedal balance task, during which mean COP displacement, peak ankle
M
invertor and evertor moments, and neuromuscular activation of the lower leg muscles were assessed. Two-way mixed analyses of variance tests with task as a repeated factor were
d
performed to detect the effects of task and group on the variables of interest. When averaged
te
across tasks, mean medial-lateral COP displacement was significantly greater in the weak group
Ac ce p
(13.6±11.7 vs. 9.8±6.0 mm, p=0.05). The weak group also exhibited greater peak ankle invertor and evertor moments (0.31±0.10 vs. 0.25±0.11 Nm/kg, p=0.03; 0.04±0.06 vs. -0.02±0.07 Nm/kg, p=0.01), and increased peroneus longus activation (46±12 vs. 36±15 %, p 100 dB at
te
65Hz, gain at 1 kHz x 20 ±1%, input impedance > 100,000,000 ohms, and a signal bandwidth
Ac ce p
from 20Hz to 3,000Hz (MA-420, Motion Lab System). EMG signals were transmitted from the first stage pre-amplifier to a second stage receiver unit attached to the back of the subject. From the receiver unit, the signal was hardwired to a 16-bit analog to digital converter. Ankle kinematics, ground reaction force, COP and EMG data were collected simultaneously using a motion analysis program (Qualisys Track Manager ver. 2.5, Qualisys Motion Capture Systems, Gothenburg, Sweden).
Procedures
Page 5 of 22
Data were collected at the Jacquelin Perry Musculoskeletal Biomechanics Research Laboratory at the University of Southern California. To control for the influence of footwear,
ip t
participants were fitted with standardized athletic shoes (New Balance Inc., MA, USA). Each participant participated in two testing sessions. Hip abductor strength was assessed
cr
during the first visit. To minimize the potential effect of muscle fatigue induced from the muscle strength assessment, the biomechanical and balance testing took place on a separate day (average
an
us
interval = 5.7 ± 4.4 days).
M
Hip Abductor Muscle Strength Testing
Hip abductor strength was evaluated using a previously described weight bearing method.
d
Detailed description as well as the reliability data of this method have been published in a
te
previous study.[10] Briefly, the hip muscle strength testing was performed utilizing a force
Ac ce p
transducer connected to a non-stretchable fabric belt wrapping around the distal ends of both femurs. Hip abductor muscle strength was assessed with subjects in a weight bearing squat position (50° knee flexion and 30° hip flexion). Participants were instructed to maintain their natural lordotic curvature, and place their feet parallel to each other, shoulder-width apart. Participants were then instructed to push outward against the resistance belt “as hard as possible” for 5 seconds. A 1 minute rest period was given between trials. A total of 3 trials were collected.
Balance Testing
Page 6 of 22
The skin surface above TA and PL was shaved and clean with isopropyl alcohol. Electrodes for the PL were placed on the muscle belly inferior from the fibular head at a distance approximately 1/3 of the total muscle length. Electrodes were oriented so they were parallel to
ip t
the PL muscle fibers. Electrodes for the TA were placed on the muscle belly approximately 3 cm inferior and 2 cm lateral to the tibial tuberosity, and were parallel to the TA muscle fibers. The
cr
electrodes and pre-amplifiers were secured to the skin with pre-wrap to minimize movement
us
artifacts during testing. To standardize the level of EMG activity, subjects performed two 5second maximal voluntary isometric contractions (MVIC) for each muscle. Resisted contraction
an
of the TA (ankle dorsiflexion) was performed against an immobile object in a standing position with the tested ankle in neutral plantarflexion/dorsiflexion, and the hip and knee in 0 degrees of
M
flexion. Resisted contraction of the PL (ankle eversion) was performed with subjects in a seated
d
position with the hip and knee flexed to 90 degrees and the ankle in neutral
te
plantarflexion/dorsiflexion). Resistance to ankle eversion was provided by a non-stretchable
Ac ce p
strap placed around the forefoot.
Following EMG preparation, opto-reflective markers (spheres, 14 mm diameter) were placed on the following anatomical landmarks: end of second toes, first and fifth metatarsal heads, medial and lateral malleoli, medial and lateral epicondyles of the femurs, greater trochanters, anterior superior iliac spines, iliac crests, and the L5-S1 junction. Tracking cluster markers were attached to neoprene bands secured around the thigh and shank. Additional tracking cluster markers were secured to the heel counter of the shoe. Once all markers were attached, a standing calibration trial was captured. Following collection of the calibration trial, all markers were removed except for the tracking clusters and the markers on the iliac crest and L5-S1 junction.
Page 7 of 22
For the static balance task, participants were instructed to stand on one leg with the hip and knee of the stance leg extended, and the arms folded across the front of the chest (Figure 1). Only the dominant leg (defined as participant’s preferred leg for jumping and landing) was tested.
ip t
Care was taken to ensure that the stance foot was oriented in line with the fore-aft direction of the lab coordinate system. During this task, participants were instructed to prevent the legs from
cr
bracing against each other and to look ahead at an object on the wall about five meters away.
us
Subjects were informed that the goal of the task was to stand as steady as possible for 20 seconds. A total of 3 static balance trials were collected.
an
Dynamic postural stability was assessed during a unipedal step-down task. Participants were instructed to lower themselves from an elevated force platform, touch their heel on the
M
lower step, then return to the starting position over a 2-second period (Figure 2). The height of
d
the step was normalized to the each subjects’ height (10% of body height).[11] A metronome (set
te
at 30 beats per minute) was used to guide the rate of the stepping task. One trial consisted of 5 consecutive up-and-down repetitions (10 seconds). A total of 3 dynamic balance trials were
Ac ce p
obtained.
Data Analysis
Postural stability during the balance tasks was assessed using stabilogram analyses. COP data obtained during the balance tasks were analyzed using a customized Matlab® program. The COP signals were passed through a 4th-order Butterworth low-pass digital filter with a 5-Hz cutoff frequency. Postural stability variable of interest was the mean COP displacement in the medial-lateral direction, which was computed according to the procedures described by Prieto et al.[12] The mean values from 3 trials for each condition were used for statistical analysis.
Page 8 of 22
EMG data were filtered (20-500 Hz bandpass) and rectified. For each muscle, the activation level was normalized to the highest average 2-second EMG amplitude obtained during the MVIC trials. The EMG variable computed was the average EMG amplitude of PL and TA
ip t
during the sampled duration of the balance tasks. The mean values from 3 trials for each
cr
condition were used for statistical analysis.
Three-dimensional marker and ground reaction force data were imported into a
us
biomechanical analysis software program (Visual 3D® v4.75, C-Motion, MD, USA). Marker
an
trajectories were filtered using a 4th-order Butterworth low-pass filter with cut-off frequency of 12 Hz. The local coordinate system and joint center locations were computed based on the
M
standing calibration trial. Three-dimensional ankle joint kinematics were calculated using Euler angles with the following order of rotations: plantarflexion/dorsiflexion, eversion/inversion and
d
internal/external rotation. Internal ankle joint moments were computed with inverse-dynamics.
te
Joint moment values were normalized to each subject’s body mass. The biomechanical variables
Ac ce p
of interests were peak ankle invertor and evertor moments during the balance tasks. To match the 2 seconds of data obtained during each trial of the dynamic balance task, COP and moment data sampled during the 10th and 11th seconds of the static balance task were used for analysis. For the dynamic balance task, COP and moment data obtained during the 3rd repetition were used for analysis. The mean values from the 3 trials for each task were analyzed statistically.
Statistical Analysis
Page 9 of 22
Participants were ranked based on their hip abductor muscle strength. The top 33% of the participants were categorized as the strong group (n = 15) and the lower 33% as the weak group (n = 15). On average, strength values in the weak group were 19% lower than the previously
ip t
reported reference values for females using the same weight bearing method described above, while strength values in the strong group were on average 15% higher than the reference
cr
values.[10] Normality of the hip abductor muscle strength data was confirmed prior to the group
us
dichotomization.
an
Differences in age, height, weight and physical activity level between the strong and the weak group were evaluated using independent t-tests. To determine if postural stability, ankle
M
biomechanics, and neuromuscular activation of the ankle muscles differ between groups across the two balance tasks, 2 x 2 (group x task) mixed analyses of variance (ANOVAs) tests with task
d
as a repeated factor were performed for each dependent variable of interest. F-statistics were
te
computed for the main effects of group and task, as well as the interaction between the two
Ac ce p
independent variables. If a significant interaction was found, the individual effects were analyzed separately. All statistical tests were performed using SPSS® software (ver. 19.0, International Business Machines Corp. New York, USA). The significance level for all statistical procedures was set at 0.05. The effect size of the between-group differences for each dependent variable was presented using a Cohen’s D value.
Results
Page 10 of 22
Average age, height and weight of the participants dichotomized to the strong and weak groups were not significantly different (Table 1). On average, the hip abductor strength of the
ip t
weak group was 30% lower than the strong group (2.1 ± 0.2 vs. 3.0 ± 0.2 N/kg, p
S0966-6362(13)00699-1 http://dx.doi.org/doi:10.1016/j.gaitpost.2013.12.004 GAIPOS 4102
To appear in:
Gait & Posture
Received date: Revised date: Accepted date:
5-8-2013 3-12-2013 4-12-2013
Please cite this article as: Lee S-P, Powers CM, Individuals with Diminished Hip Abductor Muscle Strength Exhibit Altered Ankle Biomechanics & Neuromuscular Activation during Unipedal Balance Tasks, Gait and Posture (2013), http://dx.doi.org/10.1016/j.gaitpost.2013.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Individuals with Diminished Hip Abductor Muscle Strength Exhibit Altered Ankle Biomechanics & Neuromuscular Activation
us
Szu-Ping Lee, PT, PhD1
cr
ip t
during Unipedal Balance Tasks
M
an
Christopher M. Powers, PT, PhD2
d
1. Department of Physical Therapy, University of Nevada, Las Vegas, Las Vegas, USA
Ac ce p
te
2. Division of Biokinesiology & Physical Therapy, University of Southern California, Los Angeles, USA
Address for Correspondence: Szu-Ping Lee, PT, PhD Assistant Professor
Department of Physical Therapy
University of Nevada, Las Vegas 4505 S. Maryland Parkway, Box 453029, Las Vegas, NV 89154-3029, USA Phone: (702)895-3086 Email: [email protected]
Page 1 of 22
Abstract Coordinated control of the hip and ankle is important for maintaining postural stability. The
ip t
purpose of the study was to compare postural stability between individuals with contrasting hip abductor strength during unipedal balance tasks and to determine whether diminished hip
cr
abductor strength results in greater utilization of the ankle strategy to maintain balance. Fortyfive females (27.6±3.5 yrs.) participated in the study. Participants were ranked based on their
us
isometric hip abductor muscle strength. The top 33% of the participants were categorized as the
an
strong group (n=15) and the lower 33% as the weak group (n=15). Each subject performed a static and a dynamic unipedal balance task, during which mean COP displacement, peak ankle
M
invertor and evertor moments, and neuromuscular activation of the lower leg muscles were assessed. Two-way mixed analyses of variance tests with task as a repeated factor were
d
performed to detect the effects of task and group on the variables of interest. When averaged
te
across tasks, mean medial-lateral COP displacement was significantly greater in the weak group
Ac ce p
(13.6±11.7 vs. 9.8±6.0 mm, p=0.05). The weak group also exhibited greater peak ankle invertor and evertor moments (0.31±0.10 vs. 0.25±0.11 Nm/kg, p=0.03; 0.04±0.06 vs. -0.02±0.07 Nm/kg, p=0.01), and increased peroneus longus activation (46±12 vs. 36±15 %, p 100 dB at
te
65Hz, gain at 1 kHz x 20 ±1%, input impedance > 100,000,000 ohms, and a signal bandwidth
Ac ce p
from 20Hz to 3,000Hz (MA-420, Motion Lab System). EMG signals were transmitted from the first stage pre-amplifier to a second stage receiver unit attached to the back of the subject. From the receiver unit, the signal was hardwired to a 16-bit analog to digital converter. Ankle kinematics, ground reaction force, COP and EMG data were collected simultaneously using a motion analysis program (Qualisys Track Manager ver. 2.5, Qualisys Motion Capture Systems, Gothenburg, Sweden).
Procedures
Page 5 of 22
Data were collected at the Jacquelin Perry Musculoskeletal Biomechanics Research Laboratory at the University of Southern California. To control for the influence of footwear,
ip t
participants were fitted with standardized athletic shoes (New Balance Inc., MA, USA). Each participant participated in two testing sessions. Hip abductor strength was assessed
cr
during the first visit. To minimize the potential effect of muscle fatigue induced from the muscle strength assessment, the biomechanical and balance testing took place on a separate day (average
an
us
interval = 5.7 ± 4.4 days).
M
Hip Abductor Muscle Strength Testing
Hip abductor strength was evaluated using a previously described weight bearing method.
d
Detailed description as well as the reliability data of this method have been published in a
te
previous study.[10] Briefly, the hip muscle strength testing was performed utilizing a force
Ac ce p
transducer connected to a non-stretchable fabric belt wrapping around the distal ends of both femurs. Hip abductor muscle strength was assessed with subjects in a weight bearing squat position (50° knee flexion and 30° hip flexion). Participants were instructed to maintain their natural lordotic curvature, and place their feet parallel to each other, shoulder-width apart. Participants were then instructed to push outward against the resistance belt “as hard as possible” for 5 seconds. A 1 minute rest period was given between trials. A total of 3 trials were collected.
Balance Testing
Page 6 of 22
The skin surface above TA and PL was shaved and clean with isopropyl alcohol. Electrodes for the PL were placed on the muscle belly inferior from the fibular head at a distance approximately 1/3 of the total muscle length. Electrodes were oriented so they were parallel to
ip t
the PL muscle fibers. Electrodes for the TA were placed on the muscle belly approximately 3 cm inferior and 2 cm lateral to the tibial tuberosity, and were parallel to the TA muscle fibers. The
cr
electrodes and pre-amplifiers were secured to the skin with pre-wrap to minimize movement
us
artifacts during testing. To standardize the level of EMG activity, subjects performed two 5second maximal voluntary isometric contractions (MVIC) for each muscle. Resisted contraction
an
of the TA (ankle dorsiflexion) was performed against an immobile object in a standing position with the tested ankle in neutral plantarflexion/dorsiflexion, and the hip and knee in 0 degrees of
M
flexion. Resisted contraction of the PL (ankle eversion) was performed with subjects in a seated
d
position with the hip and knee flexed to 90 degrees and the ankle in neutral
te
plantarflexion/dorsiflexion). Resistance to ankle eversion was provided by a non-stretchable
Ac ce p
strap placed around the forefoot.
Following EMG preparation, opto-reflective markers (spheres, 14 mm diameter) were placed on the following anatomical landmarks: end of second toes, first and fifth metatarsal heads, medial and lateral malleoli, medial and lateral epicondyles of the femurs, greater trochanters, anterior superior iliac spines, iliac crests, and the L5-S1 junction. Tracking cluster markers were attached to neoprene bands secured around the thigh and shank. Additional tracking cluster markers were secured to the heel counter of the shoe. Once all markers were attached, a standing calibration trial was captured. Following collection of the calibration trial, all markers were removed except for the tracking clusters and the markers on the iliac crest and L5-S1 junction.
Page 7 of 22
For the static balance task, participants were instructed to stand on one leg with the hip and knee of the stance leg extended, and the arms folded across the front of the chest (Figure 1). Only the dominant leg (defined as participant’s preferred leg for jumping and landing) was tested.
ip t
Care was taken to ensure that the stance foot was oriented in line with the fore-aft direction of the lab coordinate system. During this task, participants were instructed to prevent the legs from
cr
bracing against each other and to look ahead at an object on the wall about five meters away.
us
Subjects were informed that the goal of the task was to stand as steady as possible for 20 seconds. A total of 3 static balance trials were collected.
an
Dynamic postural stability was assessed during a unipedal step-down task. Participants were instructed to lower themselves from an elevated force platform, touch their heel on the
M
lower step, then return to the starting position over a 2-second period (Figure 2). The height of
d
the step was normalized to the each subjects’ height (10% of body height).[11] A metronome (set
te
at 30 beats per minute) was used to guide the rate of the stepping task. One trial consisted of 5 consecutive up-and-down repetitions (10 seconds). A total of 3 dynamic balance trials were
Ac ce p
obtained.
Data Analysis
Postural stability during the balance tasks was assessed using stabilogram analyses. COP data obtained during the balance tasks were analyzed using a customized Matlab® program. The COP signals were passed through a 4th-order Butterworth low-pass digital filter with a 5-Hz cutoff frequency. Postural stability variable of interest was the mean COP displacement in the medial-lateral direction, which was computed according to the procedures described by Prieto et al.[12] The mean values from 3 trials for each condition were used for statistical analysis.
Page 8 of 22
EMG data were filtered (20-500 Hz bandpass) and rectified. For each muscle, the activation level was normalized to the highest average 2-second EMG amplitude obtained during the MVIC trials. The EMG variable computed was the average EMG amplitude of PL and TA
ip t
during the sampled duration of the balance tasks. The mean values from 3 trials for each
cr
condition were used for statistical analysis.
Three-dimensional marker and ground reaction force data were imported into a
us
biomechanical analysis software program (Visual 3D® v4.75, C-Motion, MD, USA). Marker
an
trajectories were filtered using a 4th-order Butterworth low-pass filter with cut-off frequency of 12 Hz. The local coordinate system and joint center locations were computed based on the
M
standing calibration trial. Three-dimensional ankle joint kinematics were calculated using Euler angles with the following order of rotations: plantarflexion/dorsiflexion, eversion/inversion and
d
internal/external rotation. Internal ankle joint moments were computed with inverse-dynamics.
te
Joint moment values were normalized to each subject’s body mass. The biomechanical variables
Ac ce p
of interests were peak ankle invertor and evertor moments during the balance tasks. To match the 2 seconds of data obtained during each trial of the dynamic balance task, COP and moment data sampled during the 10th and 11th seconds of the static balance task were used for analysis. For the dynamic balance task, COP and moment data obtained during the 3rd repetition were used for analysis. The mean values from the 3 trials for each task were analyzed statistically.
Statistical Analysis
Page 9 of 22
Participants were ranked based on their hip abductor muscle strength. The top 33% of the participants were categorized as the strong group (n = 15) and the lower 33% as the weak group (n = 15). On average, strength values in the weak group were 19% lower than the previously
ip t
reported reference values for females using the same weight bearing method described above, while strength values in the strong group were on average 15% higher than the reference
cr
values.[10] Normality of the hip abductor muscle strength data was confirmed prior to the group
us
dichotomization.
an
Differences in age, height, weight and physical activity level between the strong and the weak group were evaluated using independent t-tests. To determine if postural stability, ankle
M
biomechanics, and neuromuscular activation of the ankle muscles differ between groups across the two balance tasks, 2 x 2 (group x task) mixed analyses of variance (ANOVAs) tests with task
d
as a repeated factor were performed for each dependent variable of interest. F-statistics were
te
computed for the main effects of group and task, as well as the interaction between the two
Ac ce p
independent variables. If a significant interaction was found, the individual effects were analyzed separately. All statistical tests were performed using SPSS® software (ver. 19.0, International Business Machines Corp. New York, USA). The significance level for all statistical procedures was set at 0.05. The effect size of the between-group differences for each dependent variable was presented using a Cohen’s D value.
Results
Page 10 of 22
Average age, height and weight of the participants dichotomized to the strong and weak groups were not significantly different (Table 1). On average, the hip abductor strength of the
ip t
weak group was 30% lower than the strong group (2.1 ± 0.2 vs. 3.0 ± 0.2 N/kg, p
