Mark W. Rogers

Effects of arm acceleration and behavioral conditions on the organization of postural adjustments during arm flexion

SummaryNine standing subjects performed unilateral arm flexion movements over an eight-fold range of speeds, under two behavioral conditions. In the visually-guided condition, a visual target informed subjects about the correct movement speed. Seven subjects also made movements of different speeds during a self-paced condition, without a visual target. Angular displacement and acceleration of the arm, and EMG activity from the hamstrings (HM), erector spinae (ES) and the anterior deltoid (AD) muscles were measured. The following results were observed. (1) Mean rectified amplitudes of EMG activity in HM and ES were typically correlated with the average arm acceleration and presumably the disturbance to posture and/or balance. HM and ES amplitudes were correlated for only six subjects. Functions relating the ratios of HM/ES EMG amplitudes to acceleration varied between subjects. (2) HM onset latencies were highly variable for slow movements and usually lagged movement. For movements above a threshold-like point in acceleration, HM latencies were correlated with arm acceleration and recruited before movement. ES latencies were constant for fast movements, and negatively correlated with acceleration for slower movements. (3) The recruitment order of HM and AD was influenced by the behavioral condition but not by arm acceleration for fast movements. HM and AD were recruited coincidentally for visually-guided movements, while for self-paced movements, HM was recruited before AD. We conclude that for the arm flexion task: (1) HM and ES are not tightly coupled; (2) both behavioral and mechanical conditions affect the recruitment of postural muscles; and (3) postural and focal components of the movement are probably organized by parallel processes.

Lateral Stability, Sensorimotor Function and Falls in Older People

AIMS: To design simple tests of lateral stability for assessing balance in older people and to determine whether poor performances in these tests are associated with impaired vision, lower limb sensation, quadriceps strength, simple reaction time, and falling in this group.

Lateral Stability and Falls in Older People

ROGERS, M. W., and M.-L. MILLE. Lateral stability and falls in older people. Exerc. Sport Sci. Rev., Vol. 31, No. 4, pp. 182–187, 2003. Aging changes in specific neuromusculoskeletal factors affecting protective stepping and other balance functions may precipitate lateral instability and falls. Identification of these factors provides directives for novel therapeutic interventions to reduce fall risk in older people.

Dynamic transitions in stance support accompanying leg flexion movements in man

SummaryThe control processes underlying dynamic transitions in stance support during single leg flexion movements were investigated in human subjects as a function of the intended speed of movement, by examining the vertical and lateral horizontal components of the ground reaction forces, the frontal plane trajectory of the body center of mass (CM) recorded via motion analysis, and the electromyographic (EMG) recordings of selected lower limb muscles. For the slowest movements, the measured vertical force beneath the flexing and single stance limbs closely matched the vertical force-time history predicted by a quasi-static mechanical model, whereas, the more rapid natural and fast speeds showed progressively larger discrepancies between measured and predicted forces. The initial resultant horizontal force component was exerted in the flexing to stance limb direction but was proportionately greater (4∶1) beneath the flexing versus the stance limb during fast and natural speeds, and became equivalent for slow movements. Speed related EMG differences included an early phasic recruitment of the lateral hip muscle of the flexing limb which always preceded the ground reaction force changes for fast and natural but not slow movements, and a considerably earlier onset of the stance leg knee extensor relative to the flexing limb knee flexor for slow versus fast and natural speeds. Overall, the findings suggested two different speed related strategies for linking the postural and intentional movement components, where the choice of the strategy selected appeared to reflect the mechanical requirements needed to overcome the inertial force of the body mass during transitions from bipedal to single limb stance support.

Control of body mass transfer as a function of speed of ascent in sit-to-stand.

The purpose of this study was to test the hypothesis that a progressive variation in the speed of ascent would result in differences in the horizontal and vertical motions of the body center of mass (CM) and in the governing impulse-momentum relationship. A motion analysis system and two force platforms were used to examine the STS task among 10 healthy adults at each of three self-selected speeds. As the speed of ascent became faster, a progressively earlier time to the maximum vertical linear momentum and an increase in its magnitude occurred. In contrast, a relatively constant time to the maximum linear momentum, which was also the time when the propulsive impulse became the braking impulse, was found in the horizontal direction, and the propulsive impulse showed a disproportionately (1:3) smaller increase from slow to fast speeds than its vertical counterpart. The relative invariance in the horizontal motion suggested that different neuromuscular control strategies may have been employed in the horizontal and vertical directions to accomplish the different tasks of balance control in one direction and changing the gravitational potential energy in the other direction.

Lateral Balance Factors Predict Future Falls in Community-Living Older Adults

OBJECTIVE To prospectively determine the capacity of measures of mediolateral (ML) protective stepping performance, maximum hip abduction torque, and trunk mobility, in order to predict the risk of falls among community-living older people. DESIGN Cross-sectional study. SETTING A balance and falls research laboratory. PARTICIPANTS Medically screened and functionally independent community-living older adult volunteers (N=51). INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Measures included: (1) protective stepping responses: percentage of trials with multiple balance recovery steps and sidestep/crossover step recovery patterns, and first step length following motor-driven waist-pull perturbations of ML standing balance; (2) hip abduction strength and axial mobility: (3) peak isokinetic hip abduction joint torque and trunk functional axial rotation (FAR) range of motion; and (4) fall incidence: monthly mail-in reporting of fall occurrences with follow-up contact for 1 year post-testing. One- and 2-variable logistic regression analysis models determined which single and combined measures optimally predicted fall status. RESULTS The single variable model with the strongest predictive value for falls was the use of multiple steps in all trials (100% multiple steps) (odds ratio, 6.2; P=.005). Two-variable models, including 100% multiple steps and either hip abduction torque or FAR variables, significantly improved fall prediction over 100% multiple steps alone. The hip abduction and FAR logistic regression optimally predicted fall status. CONCLUSIONS The findings identify new predictor variables for risk of falling that underscore the importance of dynamic balance recovery performance through ML stepping in relation to neuromusculoskeletal factors contributing to lateral balance stability. The results also highlight focused risk factors for falling that are amenable to clinical interventions for enhancing lateral balance function and preventing falls.

Assessment of abdominal and back extensor function. A quantitative approach and results for chronic low-back patients.

A method was developed to obtain static and dynamic measures of trunk flexor and extensor strength and endurance. The method was evaluated using 32 normal subjects. Variables of trunk strength and endurance were used to compare 24 normals (12 men and 12 women) and 24 patients (16 men and eight women) with chronic low-back dysfunction. The Iowa Trunk Dynamometer is acceptably reliable and provides for assessment of isolated function of the abdominal and back muscles. For peak abdominal and back extensor strength, the range of superiority of men over women was 39–57%, and the range of superiority of normals over patients with chronic low-back dysfunction was 48–82%. Using time to percent decrement of peak strength as a criterion, the abdominals were more susceptible to fatigue than the back extensors, women demonstrated more endurance than men, and the endurance for normals was less than those patients who were able to perform dynamic reciprocal trunk movements.

Speed variation and resultant joint torques during sit-to-stand

The purpose of this study was to test the hypothesis that a progressively faster speed of ascent requires significantly greater peak resultant joint torque (RJT) at major load-bearing joints of the lower limb during the sit-to-stand (STS) transfer. Eight healthy adults performed the STS at slow, natural, and fast speeds. A motion analysis system and two force platforms were employed to record kinetic data, and equations of motion were applied to compute the RJT for the ankle, knee, and hip. The results of the study supported the hypothesis that when the speed of ascent increased progressively, the peak hip flexion, knee extension, and ankle dorsiflexion RJTs increased disproportionately. However, the peak hip extension and ankle plantar flexion RJTs remained relatively constant across the range of the speeds. Implications for clinical practice pertaining to the timing and magnitude of RJT, as well as for interventions that emphasize the adaptive characteristics of movements, are suggested.

Segmental contributions to total body momentum in sit-to-stand

In a previous investigation, we reported that the maximum linear momentum of the body center of mass (CM) during a sit-to-stand task showed a relative invariance in the horizontal vs the vertical direction of motion as the speed of ascent increased from natural to fast. The present study investigated the segmental origin of this directionally specific difference by examining the linear momentum of the shank, thigh, and head-arm-trunk segments for ten healthy young adults at slow, natural, and fast self-selected speeds. Findings indicated that the head-arm-trunk was the major contributor to the horizontal maximum linear momentum of the CM and accounted for the relative invariance in its magnitude. In contrast, the thigh was the major contributor to the vertical maximum linear momentum of the CM and was responsible for the progressive increase in its magnitude across the range of speeds. Moreover, the compatibility between the motions of the head-arm-trunk and of the shank in their general profile and peak magnitudes further suggested that a simplifying strategy may have been employed to reduce the overall number of degrees of freedom associated with the sit-to-stand movement.

Muscle weakness impairs the proprioceptive control of human standing

The leg muscles have two distinct roles in human standing. They are a principle source of the sensory input used to detect body sway and they also produce the contractile force that corrects body sway. In this population study, we provide evidence for a link between these contractile and sensory functions of muscle. In subjects classified as having weak or strong leg muscles, we compared body sway with and without vision. Subjects (17) with weakness through prior-polio were compared with age-matched controls (34) and from 174 subjects aged 60-69, those classified weak (<15 Nm ankle dorsiflexion) were compared with the strong (>or=15 Nm). The weaker and stronger groups from these populations had equivalent visual acuity and lower-limb sensory function. However, the weaker swayed disproportionately more than the stronger on closing the eyes. Strength alone could not cause this increased sway of the weaker subjects because they were as stable as the strong subjects when the eyes were open. This effect of strength was not apparent in an older group (>or=70 years, n = 276), where eye closure increased sway by similar amounts in the weak and strong. This appears to be related to visual and somatosensory impairments and increased morbidity in the weak of this group, an association not present in the younger groups. We conclude that there is a relative failure of proprioceptive postural control associated with muscle weakness. This indicates a functional link between contractile and sensory muscular processes and shows that multiple sensory inputs are more important for people with muscle weakness.