Tanya Onushko

Stroke-related changes in neuromuscular fatigue of the hip flexors and functional implications.

ObjectiveThe aim of this study was to compare stroke-related changes in hip flexor neuromuscular fatigue of the paretic leg during a sustained isometric submaximal contraction with those of the nonparetic leg and controls and to correlate fatigue with clinical measures of function. DesignHip torques were measured during a fatiguing hip flexion contraction at 20% of the hip flexion maximal voluntary contraction in the paretic and nonparetic legs of 13 people with chronic stroke and 10 age-matched controls. In addition, the participants with stroke performed a fatiguing contraction of the paretic leg at the absolute torque equivalent to 20% maximal voluntary contraction of the nonparetic leg and were tested for self-selected walking speed (10-m Walk Test) and balance (Berg). ResultsWhen matching the nonparetic target torque, the paretic hip flexors had a shorter time to task failure compared with the nonparetic leg and controls (P < 0.05). The time to failure of the paretic leg was inversely correlated with the reduction of hip flexion maximal voluntary contraction torque. Self-selected walking speed was correlated with declines in torque and steadiness. Berg-Balance scores were inversely correlated with the force fluctuation amplitude. ConclusionsFatigue and precision of contraction are correlated with walking function and balance after stroke.

Abnormal Volitional Hip Torque Phasing and Hip Impairments in Gait Post Stroke

The purpose of this study was to quantify how volitional control of hip torque relates to walking function poststroke. Volitional phasing of hip flexion and extension torques was assessed using a load-cell-instrumented servomotor drive system in 11 chronic stroke subjects and 5 age-matched controls. Hips were oscillated from approximately 40 degrees of hip flexion to 10 degrees of hip extension at a frequency of 0.50 Hz during three movement conditions [hips in phase (IP), 180 degrees out of phase (OP), and unilateral hip movement (UN)] while the knees and ankles were held stationary. The magnitude and phasing of hip, knee, and ankle torques were measured during each movement condition. Surface electromyography was measured throughout the legs. Over ground gait analysis was done for all stroke subjects. During robotic-assisted movement conditions, the paretic limb produced peak hip torques when agonist hip musculature was stretched instead of midway through the movement as seen in the nonparetic and control limbs (P < 0.012). However, mean torque magnitudes of the paretic and nonparetic limbs were not significantly different. Abnormalities of paretic hip torque phasing were more pronounced during bilateral movement conditions and were associated with quadriceps overactivity. The magnitude of flexion torque produced during maximal hip extension was correlated with the Fugl Meyer Score, self-selected walking speed, and maximal hip extension during over ground walking. These results suggest that hyperexcitable stretch reflexes in the paretic limb impair coordinated hip torque phasing and likely interfere with walking function post stroke.

Force Control Is Related to Low-Frequency Oscillations in Force and Surface EMG

Force variability during constant force tasks is directly related to oscillations below 0.5 Hz in force. However, it is unknown whether such oscillations exist in muscle activity. The purpose of this paper, therefore, was to determine whether oscillations below 0.5 Hz in force are evident in the activation of muscle. Fourteen young adults (21.07±2.76 years, 7 women) performed constant isometric force tasks at 5% and 30% MVC by abducting the left index finger. We recorded the force output from the index finger and surface EMG from the first dorsal interosseous (FDI) muscle and quantified the following outcomes: 1) variability of force using the SD of force; 2) power spectrum of force below 2 Hz; 3) EMG bursts; 4) power spectrum of EMG bursts below 2 Hz; and 5) power spectrum of the interference EMG from 10–300 Hz. The SD of force increased significantly from 5 to 30% MVC and this increase was significantly related to the increase in force oscillations below 0.5 Hz (R 2 = 0.82). For both force levels, the power spectrum for force and EMG burst was similar and contained most of the power from 0–0.5 Hz. Force and EMG burst oscillations below 0.5 Hz were highly coherent (coherence = 0.68). The increase in force oscillations below 0.5 Hz from 5 to 30% MVC was related to an increase in EMG burst oscillations below 0.5 Hz (R 2 = 0.51). Finally, there was a strong association between the increase in EMG burst oscillations below 0.5 Hz and the interference EMG from 35–60 Hz (R 2 = 0.95). In conclusion, this finding demonstrates that bursting of the EMG signal contains low-frequency oscillations below 0.5 Hz, which are associated with oscillations in force below 0.5 Hz.

Effects of Multijoint Spastic Reflexes of the Legs During Assisted Bilateral Hip Oscillations in Human Spinal Cord Injury

OBJECTIVE To investigate the timing and magnitude of muscle activation during an active-assist bilateral hip motor task in human spinal cord injury (SCI). DESIGN A single test session using a novel robotic system to alternately flex and extend the hips from 40 degrees of hip flexion to 10 degrees of hip extension at 1 of 3 frequencies (.25, .50, .75Hz). Subjects were asked either to actively assist the movements or to remain relaxed during the imposed oscillations. SETTING All data were collected in a research laboratory. PARTICIPANTS Ten subjects with motor incomplete (American Spinal Injury Association grade C or D) SCI and 10 individuals without neurologic injury participated in this study. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Electromyograms and joint torques were recorded from the lower extremities of SCI subjects and compared with electromyograms and joint torque patterns recorded from 10 neurologically healthy individuals completing the same tasks. RESULTS In trials involving active assistance of the imposed hip oscillations, SCI subjects produced muscle activation patterns that were phased differently from muscle activity of neurologically intact subjects. SCI subjects generated peak torque at the end ranges of movement (ie, 40 degrees hip flexion, 10 degrees extension), whereas control subjects generated the greatest torque midway through the movements. Moreover, the phasing of active-assist hip torque in SCI subjects was similar to the phasing of reflexive hip torques produced during the unassisted condition (ie, SCI subjects instructed to relax), while control subjects produced no reflexive torques during unassisted trials. CONCLUSIONS The differences in the timing of muscle activity during the active-assist task in controls and SCI subjects highlights problems in generating appropriately timed muscle activity during ongoing movements. The similarity in muscle activity patterns for the active-assist and unassisted trials in SCI subjects further suggests that reflex feedback from hip afferents contributes substantially to muscle activation during active-assist movements. These findings demonstrate the disruptions in reflex regulation of movement in people with incomplete SCI and suggest that spastic reflexes might disrupt motor control.

Bilateral Oscillatory Hip Movements Induce Windup of Multijoint Lower Extremity Spastic Reflexes in Chronic Spinal Cord Injury

After spinal cord injury (SCI), alterations in intrinsic motoneuron properties have been shown to be partly responsible for spastic reflex behaviors in human SCI. In particular, a dysregulation of voltage-dependent depolarizing persistent inward currents (PICs) may permit sustained muscle contraction after the removal of a brief excitatory stimulus. Windup, in which the motor response increases with repeated activation, is an indicator of PICs. Although windup of homonymous stretch reflexes has been shown, multijoint muscle activity is often observed following imposed limb movements and may exhibit a similar windup phenomenon. The purpose of this study was to identify and quantify windup of multijoint reflex responses to repeated imposed hip oscillations. Ten chronic SCI subjects participated in this study. A custom-built servomotor apparatus was used to oscillate the legs about the hip joint bilaterally and unilaterally from 10° of extension to 40° flexion for 10 consecutive cycles. Surface electromyograms (EMGs) and joint torques were recorded from both legs. Consistent with a windup response, hip and knee flexion/extension and ankle plantarflexion torque and EMG responses varied according to movement cycle number. The temporal patterns of windup depended on the muscle groups that were activated, which may suggest a difference in the response of neurons in different spinal pathways. Furthermore, because windup was seen in muscles that were not being stretched, these results imply that changes in interneuronal properties are also likely to be associated with windup of spastic reflexes in human SCI.

Hip proprioceptors preferentially modulate reflexes of the leg in human spinal cord injury

Stretch-sensitive afferent feedback from hip muscles has been shown to trigger long-lasting, multijoint reflex responses in people with chronic spinal cord injury (SCI). These reflexes could have important implications for control of leg movements during functional activities, such as walking. Because the control of leg movement relies on reflex regulation at all joints of the limb, we sought to determine whether stretch of hip muscles modulates reflex activity at the knee and ankle and, conversely, whether knee and ankle stretch afferents affect hip-triggered reflexes. A custom-built servomotor apparatus was used to stretch the hip muscles in nine chronic SCI subjects by oscillating the legs about the hip joint bilaterally from 10° of extension to 40° flexion. To test whether stretch-related feedback from the knee or ankle would be affected by hip movement, patellar tendon percussions and Achilles tendon vibration were delivered when the hip was either extending or flexing. Surface electromyograms (EMGs) and joint torques were recorded from both legs. Patellar tendon percussions and Achilles tendon vibration both elicited reflex responses local to the knee or ankle, respectively, and did not influence reflex responses observed at the hip. Rather, the movement direction of the hip modulated the reflex responses local to the joint. The patellar tendon reflex amplitude was larger when the perturbation was delivered during hip extension compared with hip flexion. The response to Achilles vibration was modulated by hip movement, with an increased tonic component during hip flexion compared with extension. These results demonstrate that hip-mediated sensory signals modulate activity in distal muscles of the leg and appear to play a unique role in modulation of spastic muscle activity throughout the leg in SCI.

Stroke‐related effects on maximal dynamic hip flexor fatigability and functional implications

Introduction: Stroke‐related changes in maximal dynamic hip flexor muscle fatigability may be more relevant functionally than isometric hip flexor fatigability. Methods: Ten chronic stroke survivors performed 5 sets of 30 hip flexion maximal dynamic voluntary contractions (MDVC). A maximal isometric voluntary contraction (MIVC) was performed before and after completion of the dynamic contractions. Both the paretic and nonparetic legs were tested. Results: Reduction in hip flexion MDVC torque in the paretic leg (44.7%) was larger than the nonparetic leg (31.7%). The paretic leg had a larger reduction in rectus femoris EMG (28.9%) between the first and last set of MDVCs than the nonparetic leg (7.4%). Reduction in paretic leg MDVC torque was correlated with self‐selected walking speed (r2 = 0.43), while reduction in MIVC torque was not (r2 = 0.11). Conclusions: Reductions in maximal dynamic torque of paretic hip flexors may be a better predictor of walking function than reductions in maximal isometric contractions. Muscle Nerve 51: 446–448, 2015

Locomotor adaptations to prolonged step-by-step frontal plane trunk perturbations in young adults

The purpose of this study was to quantify the magnitude and time course of dynamic balance control adaptations to prolonged step-by-step frontal plane forces applied to the trunk during walking. Healthy young participants (n = 10, 5 female) walked on an instrumented split-belt treadmill while an external cable-driven device applied frontal plane forces to the trunk. Two types of forces were applied: 1) forces which accentuated COM movement in the frontal plane (destabilizing) and 2) forces which resisted COM movement in the frontal plane (stabilizing). We quantified dynamic balance control using frontal plane measures of (1) the extent of center of mass (COM) movement over a gait cycle (COM sway), (2) the magnitude of base of support (step width), and (3) cadence. During destabilizing force conditions, COM sway, step width, and cadence increased. In response to stabilizing force conditions, COM sway decreased. In addition, during destabilizing balance conditions participants made quicker adaptations to their step width compared to the time to adapt to stabilizing forces. Taken together, these results provide important insight into differences in dynamic balance control strategies in response to stabilizing and destabilizing force fields.

Step length and width variability while walking on a motion simulator mounted treadmill

While devices which allow scientists to perturb normal walking are becoming increasingly common, postural adaptations to these perturbations have not been fully quantified. One way to quantify postural responses to perturbations are through the assessment of variability of step length and width. In the present study we determined variability of both step length and width while subjects walked under perturbations of varying amplitude in roll, pitch, yaw, anteroposterior, lateral, and combined roll, pitch, yaw directions. Step kinematics were quantified using motion analysis. The majority of changes in step length variability occurred in Pitch (p<;0.01), mediolateral (p<;0.05) and anteroposterior (p<;0.01) directions. Changes in step width variability were most apparent in combined Roll-Pitch-Yaw (p<;0.01) as well as Roll (p<;0.05), and Yaw (p<;0.05) directions. These data demonstrate that sinusoidal perturbations while walking on a treadmill are sufficient to disrupt normal postural control. These conditions therefore may be useful in constructing rehabilitation programs to improve dynamic balance.While devices which allow scientists to perturb normal walking are becoming increasingly common, postural adaptations to these perturbations have not been fully quantified. One way to quantify postural responses to perturbations are through the assessment of variability of step length and width. In the present study we determined variability of both step length and width while subjects walked under perturbations of varying amplitude in roll, pitch, yaw, anteroposterior, lateral, and combined roll, pitch, yaw directions. Step kinematics were quantified using motion analysis. The majority of changes in step length variability occurred in Pitch (p<;0.01), mediolateral (p<;0.05) and anteroposterior (p<;0.01) directions. Changes in step width variability were most apparent in combined Roll-Pitch-Yaw (p<;0.01) as well as Roll (p<;0.05), and Yaw (p<;0.05) directions. These data demonstrate that sinusoidal perturbations while walking on a treadmill are sufficient to disrupt normal postural control. These conditions therefore may be useful in constructing rehabilitation programs to improve dynamic balance.

The Effect of Antagonist Muscle Sensory Input on Force Regulation

The purpose of this study was to understand how stretch-related sensory feedback from an antagonist muscle affects agonist muscle output at different contraction levels in healthy adults. Ten young (25.3 ± 2.4 years), healthy subjects performed constant isometric knee flexion contractions (agonist) at 6 torque levels: 5%, 10%, 15%, 20%, 30%, and 40% of their maximal voluntary contraction. For half of the trials, subjects received patellar tendon taps (antagonist sensory feedback) during the contraction. We compared error in targeted knee flexion torque and hamstring muscle activity, with and without patellar tendon tapping, across the 6 torque levels. At lower torque levels (5%, 10%, and 15%), subjects produced greater knee torque error following tendon tapping compared with the same torque levels without tendon tapping. In contrast, we did not find any difference in torque output at higher target levels (20%, 30%, and 40%) between trials with and without tendon tapping. We also observed a load-dependent increase in the magnitude of agonist muscle activity after tendon taps, with no associated load-dependent increase in agonist and antagonist co-activation, or reflex inhibition from the antagonist tapping. The findings suggest that at relatively low muscle activity there is a deficiency in the ability to correct motor output after sensory disturbances, and cortical centers (versus sub-cortical) are likely involved.