A Comparison of Biomechanical Variables, Neuromuscular Control and Strength during Controlled and Unexpected Falls on the Outstretched Hands in Young and Older Women

Purpose: This thesis evaluated the age differences in biomechanics and muscle activity during controlled and unexpected descents simulating a fall on the outstretched hands (FOOSH) in women. Laboratory simulation using two different protocols investigated this common mechanism of injury in older and younger women. The primary purpose of the controlled descent (FOOSH 1) was to examine the differences between young and older women to control the post-impact phase of a forward fall descent at three body angles. The primary purpose of the unexpected descent (FOOSH 2) was to examine biomechanical and muscle activity age differences in pre-impact, impact and post-impact phases of a simulated FOOSH. Methods: FOOSH 1 was a cross sectional study comparing twenty healthy young (mean 24.8±3.4 yrs.) and 18 healthy older (68.4±5.7 yrs.) women performing controlled descents on outstretched arms at three body lean angles (60, 45, and 30° from horizontal) and a muscle strength test of the non-dominant UE [isometric (ISO) concentric (CON) and eccentric (ECC)] using an isokinetic dynamometer. FOOSH 2, also a cross sectional design, evaluated twenty young (mean age 22.9 yrs., SD±3.7) and 16 older (mean age 68.1yrs., SD ±5.0) women performing five trials of unexpected FOOSHs at a body lean angle of 60° from horizontal with the same muscle strength testing protocol. A three-dimensional motion capture system (VICON Nexus, VICON, Centennial, CO) and force plate apparatus (OR6-7, AMTI, Watertown, MA) was used to determine the biomechanical measures of peak energy absorption, maximum vertical force, maximum elbow angle and maximum elbow joint extensor moment. Additional biomechanical measures of FOOSH 2 included: elbow angle and elbow angular velocity at impact, elbow joint stiffness, end elbow angle, and impulse. Surface EMG detected muscle activity of six muscle sites: anterior deltoid (AntDEL), pectoralis major (PM), triceps brachii (long head) (TRI), biceps brachii (BB), external oblique (EO) and internal oblique/transversus abdominus (IO/TrA). Results: In FOOSH 1 and FOOSH 2, older women demonstrated decreased CON elbow extensor strength compared with younger women. During FOOSH 1, at all angles, the older women had increased BB activity and decreased EO activity. In FOOSH 2 older women had significantly less IO/TrA activity prior to impact than younger women. The women differed in landing strategy in that younger women had significantly greater elbow joint angle and velocity at impact. Older women demonstrated diminished capacity to absorb energy in both the controlled (30°) and unexpected descent. Significance of findings: This is the first study to investigate biomechanical and muscle activation age differences for a simulated controlled and unexpected forward descent in women. Older women demonstrate differences that could potentially increase their risk of injury during a forward fall. The results of these studies could help clinicians develop fall injury prevention protocols by considering the neuromuscular and biomechanical factors that are important to control a forward descent. The findings suggest that UE and trunk muscle strengthening may be important components to include in a fall injury prevention training program. The modulation of energy absorption capabilities by altering elbow velocity and increasing elbow flexion angles at impact may also be an injury prevention tactic to be adopted

CO-CONTRACTION OF TIBIALIS ANTERIOR AND SOLEUS MUSCLES DURING EXERCISES WITH DIFFERENT CONDITIONS OF INSTABILITY

This study aimed to compare the level of co-contraction of shank muscles during five different types of instability. Six healthy male adults performed ten balance exercises, five double leg and five single leg, while myoelectric activity of tibialis anterior and soleus was collected. Significant differences were identified in the level of muscular co-contraction between exercises performed on the BOSU and exercises performed without instability devices. It is conclude that there is a trend toward progression of balance exercises to increase the joint ankle stability, allowing physical therapists and physical educators to develop more efficient training programs aiming at improving levels of MCC and prevent injuries

Biomechanics

Biomechanics is a vast discipline within the field of Biomedical Engineering. It explores the underlying mechanics of how biological and physiological systems move. It encompasses important clinical applications to address questions related to medicine using engineering mechanics principles. Biomechanics includes interdisciplinary concepts from engineers, physicians, therapists, biologists, physicists, and mathematicians. Through their collaborative efforts, biomechanics research is ever changing and expanding, explaining new mechanisms and principles for dynamic human systems. Biomechanics is used to describe how the human body moves, walks, and breathes, in addition to how it responds to injury and rehabilitation. Advanced biomechanical modeling methods, such as inverse dynamics, finite element analysis, and musculoskeletal modeling are used to simulate and investigate human situations in regard to movement and injury. Biomechanical technologies are progressing to answer contemporary medical questions. The future of biomechanics is dependent on interdisciplinary research efforts and the education of tomorrow’s scientists

Muscle Co-Contraction Modulates Damping and Joint Stability in a Three-Link Biomechanical Limb

Computational models of neuromotor control require forward models of limb movement that can replicate the natural relationships between muscle activation and joint dynamics without the burdens of excessive anatomical detail. We present a model of a three-link biomechanical limb that emphasizes the dynamics of limb movement within a simplified two-dimensional framework. Muscle co-contraction effects were incorporated into the model by flanking each joint with a pair of antagonist muscles that may be activated independently. Muscle co-contraction is known to alter the damping and stiffness of limb joints without altering net joint torque. Idealized muscle actuators were implemented using the Voigt muscle model which incorporates the parallel elasticity of muscle and tendon but omits series elasticity. The natural force-length-velocity relationships of contractile muscle tissue were incorporated into the actuators using ideal mathematical forms. Numerical stability analysis confirmed that co-contraction of these simplified actuators increased damping in the biomechanical limb consistent with observations of human motor control. Dynamic changes in joint stiffness were excluded by the omission of series elasticity. The analysis also revealed the unexpected finding that distinct stable (bistable) equilibrium positions can co-exist under identical levels of muscle co-contraction. We map the conditions under which bistability arises and prove analytically that monostability (equifinality) is guaranteed when the antagonist muscles are identical. Lastly we verify these analytic findings in the full biomechanical limb model

Identifying intrinsic and reflexive contributions to low-back stabilization

Motor control deficits have been suggested as potential cause and/or effect of a-specific chronic low-back pain and its recurrent behavior. Therefore, the goal of this study is to identify motor control in low-back stabilization by simultaneously quantifying the intrinsic and reflexive contributions. Upper body sway was evoked using continuous force perturbations at the trunk, while subjects performed a resist or relax task. Frequency response functions (FRFs) and coherences of the admittance (kinematics) and reflexes (sEMG) were obtained. In comparison with the relax task, the resist task resulted in a 61% decrease in admittance and a 73% increase in reflex gain below 1.1 Hz. Intrinsic and reflexive contributions were captured by a physiologically-based, neuromuscular model, including proprioceptive feedback from muscle spindles (position and velocity) and Golgi tendon organs (force). This model described on average 90% of the variance in kinematics and 39% of the variance in sEMG, while resulting parameter values were consistent over subjects

Muscular Properties and Balance Control in Older Adults

The goal of this dissertation was to understand the role of age-related changes in muscle mechanical properties in the control of upright posture in humans. First, a methodology for estimating subject-specific muscle properties in healthy young and older individuals was developed. Magnetic resonance and ultrasound imaging were used in conjunction with dynamometer experiments, musculoskeletal modeling, and numerical optimization to estimate the properties of the dorsiflexor and individual plantarflexor (gastrocnemius and soleus) muscles for 12 young and 12 older adults (balanced for gender). With aging there were declines in maximal isometric strength and increases in series-elastic stiffness in the male subjects, but no differences in the female subjects. Regardless of gender, there were age-related changes in the shape of the force-velocity relation, such that the older subjects produced less relative force during both concentric and eccentric muscle contractions. The second study tested the balance abilities of the same subjects under a variety of static (quiet stance, leaning forward/backward) and dynamic (swaying at preferred/imposed frequencies, maximal reaching, external perturbation) conditions. The older adults performed more poorly on most of the balance tasks. While maximal isometric force made a smaller than expected contribution to predicting balancing ability, the force-length, force-velocity and force-extension properties of the muscles were all predictive of the age-related declines in balance control, explaining ~40% of the variance as independent predictors and ~50% when these factors were combined. Finally, a feedback-driven inverted pendulum model of postural control was developed, which incorporated realistic representations of young and old dorsiflexor and individual plantarflexor muscles using the previously estimated mechanical properties. A sensitivity analysis was performed by manipulating the properties of the plantarflexor muscles. The balancing ability of the model was most influenced by the optimal length of the contractile component and the slack length of the series elastic component of the plantarflexor muscle models. The quiet stance model highlighted the importance of the force-length relation of muscle to the stabilization of upright posture. This dissertation demonstrated that there are age-related changes in the dorsi- and plantarflexor mechanical properties, and these changes are associated with the declines in postural control that accompany aging

Modelling of the human quiet stance with ankle joint complexity

This study derives an inverted pendulum model for quiet stance in humans around the ankle joints with 4×9-element mass-spring-damper (MSD) units as the musculoskeletal connections between the shank and foot bilaterally. The model focuses on the role played by both the stiffness and the damping parameters of muscles, tendons and ligaments about the ankle complex. This model partitions muscles, tendons and ligaments functionally. This novel model is used to study the behaviour of individual components in relation to quiet standing. The Lagrange d’ Alembert principle has been used to derive the equations of motion of the system and resulted in eighteen 2nd order differential equations with nine constraints. Four MSD units connects with the shank (tibia and fibula) and foot bilaterally. The units function passively and are representative of the mechanical functionality of muscles, tendons, and ligaments about the ankle complex. The dynamics of the MSD units are considered linear in nature and their stiffness and damping parameters are calculated by finding the slope of the force vs. deformation length curve and force vs. velocity curve reported in the literature.The simulation results revealed that the torques generated by the internal constraints through the MSD units are significantly greater than the gravitational torque. A case study has been conducted for eyes open vs. eyes closed conditions. It was found that the angular displacement of the shank varied but the overall range of motion of the ankle joint remained constant at 0.6. This was expected as there was no external perturbation applied to facilitate any amount of plantarflexion or dorsiflexion at the point of articulation of the ankle joint.In conclusion, the model derived and analysed in this study explains that the human body was able to maintain its upright posture mechanically during unperturbed quiet standing without the use of an active control system emphasising the importance of damping and its influence on postural balance. Furthermore, this sophisticated model is not limited to only considering the muscle-tendon unit and ligaments play an important role in maintaining balance during quiet stance and are therefore included in the model. This model is physiologically more realistic than previously developed postural models thus providing a deeper insight towards the passive mechanism of postural balance and providing a new approach towards future postural models

Effect of 17β Estradiol & Foot Strike Patterns on Physiological & Biomechanical Changes in Runners

It is well established that female runners are at a significantly increased risk of incurring injuries when compared to their male counterparts. Gender-specific factors such as anatomical, hormonal, and altered neuromuscular activation patterns have been implicated as causative factors. An association have been observed between hormonal fluctuation and ACL injury risk indicating potentially hormonal effect on both passive and dynamic knee stabilizer. A growing contingency believes that we were designed with all we need in our feet to be able to run with minimal shoes that mimic barefoot running striking pattern. Habitual barefoot runners tend to FFS, compared to habitually shod populations who tend to RFS. Reduced collision forces generated with FFS patterns relative to RFS account for the reduced injuries. The purpose of this study was to assess the effect of 6 weeks of a transition program of SBR on the pattern of muscle activation, spatiotemporal variables, and stance phase kinetics. These running parameters were compared and contrasted during the menstrual cycle to assess whether estrogen fluctuation has an effect on the pattern of muscle activation, and laxness of ACL. Twenty four females runner were divided into two groups. First group was tested twice across a menstrual cycle for serum levels of E, KJL and EMG activity of the quadriceps and hamstrings muscles. Second group gradually experienced SBR over 6 weeks. Kinetic analysis of running was performed during shod running, habituated SBR conditions. The results showed an observed increased in KJL in response to peak E during the ovulatory phase, which was associated with increased preactivity of the hamstring muscle. A consistent pattern was observed in the firing of the quadriceps muscle recruitment pattern throughout the follicular phase. The results of the second group indicated a significant decrease in the EMG activity of TA in the habituated SBR. A significant increase was observed in the preactivation of GAS between shod running, and habituated SBR. In conclusion, changes in KJL in response to 17β-Estradiol fluctuations changes the neuromuscular control around the knee. Changes in motor patterns in previously habitually shod runners are possible and can be accomplished within 6 weeks

Neuromechanical Tuning for Arm Motor Control

Movement is a fundamental behavior that allows us to interact with the external world. Its importance to human health is most evident when it becomes impaired due to disease or injury. Physical and occupational rehabilitation remains the most common treatment for these types of disorders. Although therapeutic interventions may improve motor function, residual deficits are common for many pathologies, such as stroke. The development of novel therapeutics is dependent upon a better understanding of the underlying mechanisms that govern movement. Movement of the human body adheres to the principles of classic Newtonian mechanics. However, due to the inherent complexity of the body and the highly variable repertoire of environmental contexts in which it operates, the musculoskeletal system presents a challenging control problem and the onus is on the central nervous system to reliably solve this problem. The neural motor system is comprised of numerous efferent and afferent pathways with a hierarchical organization which create a complex arrangement of feedforward and feedback circuits. However, the strategy that the neural motor system employs to reliably control these complex mechanics is still unknown. This dissertation will investigate the neural control of mechanics employing a “bottom-up” approach. It is organized into three research chapters with an additional introductory chapter and a chapter addressing final conclusions. Chapter 1 provides a brief description of the anatomical and physiological principles of the human motor system and the challenges and strategies that may be employed to control it. Chapter 2 describes a computational study where we developed a musculoskeletal model of the upper limb to investigate the complex mechanical interactions due to muscle geometry. Muscle lengths and moment arms contribute to force and torque generation, but the inherent redundancy of these actuators create a high-dimensional control problem. By characterizing these relationships, we found mechanical coupling of muscle lengths which the nervous system could exploit. Chapter 3 describes a study of muscle spindle contribution to muscle coactivation using a computational model of primary afferent activity. We investigated whether these afferents could contribute to motoneuron recruitment during voluntary reaching tasks in humans and found that afferent activity was orthogonal to that of muscle activity. Chapter 4 describes a study of the role of the descending corticospinal tract in the compensation of limb dynamics during arm reaching movements. We found evidence that corticospinal excitability is modulated in proportion to muscle activity and that the coefficients of proportionality vary in the course of these movements. Finally, further questions and future directions for this work are discussed in the Chapter 5