Joint moments' contributions to vertically accelerate the center of mass during stair ambulation in the elderly: An induced acceleration approach.

Falls are a serious problem faced by the elderly. Older adults report mostly to fall while performing locomotor activities, especially the ones requiring stair negotiation. During these tasks, older adults, when compared with young adults, seem to redistribute their lower limb joint moments. This may indicate that older adults use a different strategy to accelerate the body upward during these tasks. The purposes of this study were to quantify the contributions of each lower limb joint moment to vertically accelerate the center of mass during stair ascent and descent, in a sample of community-dwelling older adults, and to verify if those contributions were correlated with age and functional fitness level. A joint moment induced acceleration analysis was performed in 29 older adults while ascending and descending stairs at their preferred speed. Agreeing with previous studies, during both tasks, the ankle plantarflexor and the knee extensor joint moments were the main contributors to support the body. Although having a smaller contribution to vertically accelerate the body, during stair descent, the hip joint moment contribution was related with the balance score. Further, older adults, when compared with the results reported previously for young adults, seem to use more their knee extensor moment than the ankle plantarflexor moment to support the body when the COM downward velocity is increasing. By contributing for a better understanding of stair negotiation in community dwelling older adults, this study may help to support the design of interventions aiming at fall prevention and/or mobility enhancement within this population.

A mathematical analysis to address the 6 degree-of-freedom segmental power imbalance.

Segmental power is used in human movement analyses to indicate the source and net rate of energy transfer between the rigid bodies of biomechanical models. Segmental power calculations are performed using segment endpoint dynamics (kinetic method). A theoretically equivalent method is to measure the rate of change in a segment's mechanical energy state (kinematic method). However, these two methods have not produced experimentally equivalent results for segments proximal to the foot, with the difference in methods deemed the "power imbalance." In a 6 degree-of-freedom model, segments move independently, resulting in relative segment endpoint displacement and non-equivalent segment endpoint velocities at a joint. In the kinetic method, a segment's distal end translational velocity may be defined either at the anatomical end of the segment or at the location of the joint center (defined here as the proximal end of the adjacent distal segment). Our mathematical derivations revealed the power imbalance between the kinetic method using the anatomical definition and the kinematic method can be explained by power due to relative segment endpoint displacement. In this study, we tested this analytical prediction through experimental gait data from nine healthy subjects walking at a typical speed. The average absolute segmental power imbalance was reduced from 0.023 to 0.046 W/kg using the anatomical definition to ≤0.001 W/kg using the joint center definition in the kinetic method (95.56-98.39% reduction). Power due to relative segment endpoint displacement in segmental power analyses is substantial and should be considered in analyzing energetic flow into and between segments.

Does T'ai Chi Gait Reduce External Knee Adduction Moment?

INTRODUCTION: T'ai chi chuan is a beneficial exercise of improving health and function. Biomechanical insights of t'ai chi chuan are less understood. OBJECTIVES: To study t'ai chi gait (TCG), a common form of t'ai chi chuan in order to quantify external knee adduction moment (EKAM) as a key indicator of mechanical loading of the medial compartment of the knee compared with normal walking (NW). DESIGN: A quantitative biomechanics approach to determine peak EKAM for NW and TCG. RESULTS: There were a tri-modal pattern of EKAM during TCG and a bimodal pattern of EKAM during normal walking. In addition, subsequent analysis showed a 25%-47% reduction in peak EKAM during double support phases of TCG compared with NW; the peak EKAM of TCG during single-limb support phase showed significantly higher magnitude than the other two double-support phases. CONCLUSION: These results indicate that t'ai chi chuan might be a beneficial intervention for reducing the medial mechanical load at the knee joint, particularly during the first double-support phases of TCG, but the special consideration of higher peak EKAM of single-limb support phase is needed during regular t'ai chi chuan practice.

Multi-joint coordination of functional arm reaching: induced position analysis.

Functional arm reaching involves multilinked joints: shoulder, elbow, and wrist. We propose that induced position analysis is a useful analytical tool for multijoint coordination of arm reaching. This method was used to compute the contributions of the net joint moment to the hand position when reaching forward. We describe the method and give examples of validating this model with motion capture data. The shoulder and elbow were prime movers of the arm: both acted together with an "overshoot" and "undershoot" pattern respectively to move the hand forward into the final position.

Compensatory arm reaching strategies after stroke: induced position analysis.

After stroke, movement patterns of the upper limb (UL) during functional arm reaching change to accommodate altered constraints. These compensatory movement control strategies do not, however, have a one-to-one mapping with posttraining outcomes. In this study, we quantify arm movement control strategies in unilateral and bilateral reaching tasks using induced position analysis. In addition, we assess how those strategies are associated with UL residual impairments and with functional improvement after a specific bilateral arm training intervention. Twelve individuals with chronic stroke were measured while reaching to a box as part of their pre- and posttesting assessments. Other measurements included the Fugl-Meyer Upper Extremity Assessment (FM), Modified Wolf Motor Function Test (WT), and the University of Maryland Arm Questionnaire for Stroke (UMAQS). We identified arm control strategies that did not differ between unilateral and bilateral tasks but did differ by FM impairment level and by predicted gains in WT but not UMAQS. Increased shoulder relative to elbow moment contribution was associated with less impairment and greater gains of speed in functional tasks. These results suggest that one goal of training to achieve better outcomes may be to decrease the abnormal coupling of the shoulder and elbow.

A unified deformable (UD) segment model for quantifying total power of anatomical and prosthetic below-knee structures during stance in gait.

Anatomically-relevant (AR) biomechanical models are traditionally used to quantify joint powers and segmental energies of lower extremity structures during gait. While AR models contain a series of rigid body segments linked together via mechanical joints, prosthetic below-knee structures are often deformable objects without a definable ankle joint. Consequently, the application of AR models for the study of prosthetic limbs has been problematic. The purpose of this study was to develop and validate a unified deformable (UD) segment model for quantifying the total power of below-knee structures. Estimates of total below-knee power derived via the UD segment model were compared to those derived via an AR model during stance in gait of eleven healthy subjects. The UD segment model achieved similar results to the AR model. Differences in peak power, total positive work, and total negative work were 1.91±0.31%, 3.97±0.49%, and 1.39±0.33%, relative to the AR model estimates. The main advantage of the UD segment model is that it does not require the definition of an ankle joint or foot structures. Therefore, this technique may be valuable for facilitating direct comparisons between anatomical and disparate prosthetic below-knee structures in future studies.

Muscle-induced accelerations at maximum activation to assess individual muscle capacity during movement.

Analyses of muscle-induced accelerations provide insight into how individual muscles contribute to motion. In previous studies, investigators have calculated muscle-induced accelerations on a per unit force basis to assess the potential of individual muscles to contribute to motion. However, because muscle force is a function of muscle activation, length, and shortening velocity, examining induced accelerations per unit force does not take into account how the capacity of individual muscles to produce force changes during movement. Alternatively, calculating a muscle's induced accelerations at maximum activation considers the extent to which the muscle can produce force during movement, as well as the potential of the muscle to accelerate the joints at each instant due to its moment arm(s) and the dynamics of the system. We computed both quantities for the major lower extremity muscles active during the stance phase of normal gait. We found that analyzing the induced accelerations at maximum activation in some cases led to a different interpretation of the muscles' potential actions than analyzing the induced accelerations per unit force. For example, per unit force, gluteus maximus has a very large potential to accelerate the knee during single limb stance, but only a small potential to accelerate the knee at maximum activation due to this muscle operating in suboptimal regions of its force-length-velocity curve during the majority of stance. This new analysis technique will be useful in studying abnormal movement, when altered kinematics may influence the capacity of muscles to accelerate joints due to altered lengths and shortening velocities.

In situ calibration and motion capture transformation optimization improve instrumented treadmill measurements.

We increased the accuracy of an instrumented treadmill's measurement of center of pressure and force data by calibrating in situ and optimizing the transformation between the motion capture and treadmill force plate coordinate systems. We calibrated the device in situ by applying known vertical and shear loads at known locations across the tread surface and calculating a 6 x 6 calibration matrix for the 6 output forces and moments. To optimize the transformation, we first estimated the transformation based on a locating jig and then measured center-of-pressure error across the treadmill force plate using the CalTester tool. We input these data into an optimization scheme to find the transformation between the motion capture and treadmill force plate coordinate systems that minimized the error in the center-of-pressure measurements derived from force plate and motion capture sources. When the calibration and transformation optimizations were made, the average measured error in the center of pressure was reduced to approximately 1 mm when the treadmill was stationary and to less than 3 mm when moving. Using bilateral gait data, we show the importance of calibrating these devices in situ and performing transformation optimizations.

A case study of gait compensations for hip muscle weakness in idiopathic inflammatory myopathy.

BACKGROUND: The purpose of this case series was to quantify different strategies used to compensate in gait for hip muscle weakness. METHODS: An instrumented gait analysis was performed of three females diagnosed with idiopathic inflammatory myopathies and compared to a healthy unimpaired subject. Lower extremity joint moments obtained from the gait analysis were used to drive an induced acceleration model which determined each moment's contribution to upright support, forward progression, and hip joint acceleration. FINDINGS: Results showed that after midstance, the ankle plantar flexors normally provide upright support and forward progression while producing hip extension acceleration. In normal gait, the hip flexors eccentrically resist hip extension, but the hip flexor muscles of the impaired subjects (S1-3) were too weak to control extension. Instead S1-3 altered joint positions and muscle function to produce forward progression while minimizing hip extension acceleration. S1 increased knee flexion angle to decrease the hip extension effect of the ankle plantar flexors. S2 and S3 used either a knee flexor moment or gravity to produce forward progression, which had the advantage of accelerating the hip into flexion rather than extension, and decreased the demand on the hip flexors. INTERPRETATION: Results showed how gait compensations for hip muscle weakness can produce independent (i.e. successful) ambulation, although at a reduced speed as compared to normal gait. Knowledge of these successful strategies can assist the rehabilitation of patients with hip muscle weakness who are unable to ambulate and potentially be used to reduce their disability.

Using induced accelerations to understand knee stability during gait of individuals with muscle weakness.

The purpose of this case series was to quantify the effectiveness of different compensatory strategies used by individuals with muscle weakness to produce knee extension during the stance phase gait. Subjects were three males with less than anti-gravity strength in the quadriceps femoris and a variable pattern of weakness elsewhere in the lower extremity. They walked independently at a self-selected speed without assistive devices. Gait analysis of the pelvis and bilateral lower extremity segments was performed with a six-camera, two force platform motion capture system. Joint angles and net internal moments were computed from the motion data. Induced acceleration analysis was performed to quantify the relative ability of each lower extremity joint moment and gravity to produce knee angular acceleration. Results showed that a variety of adaptive strategies both within and across limbs can control knee position during gait. One subject generated knee extension almost exclusively via the hip extensor moment. Another relied less on the hip extensor moment (39%) and more on the ankle plantar flexor moment (61%) to create knee extension. The third subject used the ipsilateral hip extensor moment (24%) and the contralateral ankle plantar flexor moment (67%) to assist knee extension. The strategy selected by each subject likely was influenced by both their total pattern of impairments and the effectiveness of the available compensations. This case series demonstrates how an induced acceleration analysis can augment a traditional gait analysis to expand and enhance our understanding of compensatory movement control strategies.

Multisegment foot motion during gait: proof of concept in rheumatoid arthritis.

OBJECTIVE: To test a multisegment foot model for kinematic analysis during barefoot walking in patients with well established rheumatoid arthritis (RA) and foot impairments. METHODS: Five healthy adult subjects and 11 RA patients with advanced disease were studied. Foot impairments were assessed using standardized outcomes and clinical examination techniques. A 6-camera 60 Hz video-based motion analysis system was used to measure motion of the shank, rearfoot, forefoot, and hallux segments and the vertical displacement of the navicular. Face validity and estimates of repeatability were determined. Motion patterns were calculated and comparisons were made between healthy subjects and patients with RA. Relationships between clinical impairment and abnormal motion were determined through inspection of individual RA cases. RESULTS: Across the motion variables, the within-day and between-day coefficient of multiple correlation values ranged from 0.677 to 0.982 for the healthy subjects and 0.830 to 0.981 for RA patients. Based on previous studies, motion parameters for the healthy subjects showed excellent face validity. In RA patients, there was reduced range of motion across all segments and all planes of motion, which was consistent with joint stiffness. In the RA patients, rearfoot motion was shifted towards eversion and external rotation and peak values for these variables were increased, on average, by 7 degrees and 11 degrees, respectively. Forefoot range of motion was reduced in all 3 planes (between 31% and 53%), but the maximum and minimum angles were comparable to normal. The navicular height, during full foot contact, was on average 3 mm lower in the RA patients in comparison to normal. The hallux was less extended in the RA subjects in comparison to normal (21 degrees vs 33 degrees) during the terminal stance phase. Individual cases showed abnormal patterns of motion consistent with their clinical impairments, especially those with predominant forefoot pain or pes planovalgus. CONCLUSION: In RA, multisegment foot models may provide a more complete description of foot motion abnormalities where pathology presents at multiple joints, leading to complex and varied patterns of impairment. This technique may be useful to evaluate functional changes in the foot and to help plan and assess logical, structurally based corrective interventions.

Joint moment control of mechanical energy flow during normal gait.

The study purpose was to estimate the ability of joint moments to transfer mechanical energy through the leg and trunk during gait. A segmental power analysis of five healthy adult subjects revealed that internal joint extensor moments removed energy from the leg and added energy to the trunk, while flexor moments and gravity produced the opposite effects. The only exception to this pattern was during the push off phase of gait when the ankle plantar flexor moment added energy to both the leg and the trunk. Pairs of joint moments with opposite energetic effects (knee extensor vs gravity, hip flexor vs ankle plantar flexor) worked together to balance energy flows through the segments. This intralimb coordination suggests that moments with contradictory effects are generated simultaneously to control mechanical energy flow within the body during walking.