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Biomedical subjects

S A Kautz

Publications and source records attributed to S A Kautz.

At least 19 recordsLinked to original sources
PubMed2007

Resistance training and locomotor recovery after incomplete spinal cord injury: a case series.

STUDY DESIGN: Longitudinal intervention case series. OBJECTIVE: To determine if a 12-week resistance and plyometric training program results in improved muscle function and locomotor speed after incomplete spinal cord injury (SCI). SETTING: University research setting. METHODS: Three ambulatory individuals with chronic (18.7+/-2.2 months post injury) motor incomplete SCI completed 12 weeks of lower extremity resistance training combined with plyometric training (RPT). Muscle maximum cross-sectional area (max-CSA) of the knee extensor (KE) and plantar flexor (PF) muscle groups was determined using magnetic resonance imaging (MRI). In addition, peak isometric torque, time to peak torque (T (20-80)), torque developed within the initial 220 ms of contraction (torque(220)) and average rate of torque development (ARTD) were calculated as indices of muscle function. Maximal as well as self-selected gait speeds were determined pre- and post-RPT during which the spatio-temporal characteristics, kinematics and kinetics of gait were measured. RESULTS: RPT resulted in improved peak torque production in the KE (28.9+/-4.4%) and PF (35.0+/-9.1%) muscle groups, as well as a decrease in T(20-80), an increased torque(220) and an increase ARTD in both muscle groups. In addition, an increase in self-selected (pre-RPT=0.77 m/s; post-RPT=1.03 m/s) and maximum (pre-RPT=1.08 m/s; post-RPT=1.47 m/s) gait speed was realized. Increased gait speeds were accompanied by bilateral increases in propulsion and hip excursion as well as increased lower extremity joint powers. CONCLUSIONS: The combination of lower extremity RPT can attenuate existing neuromuscular impairments and improve gait speed in persons after incomplete SCI.

Adolescent↗
PubMed2006

Does unilateral pedaling activate a rhythmic locomotor pattern in the nonpedaling leg in post-stroke hemiparesis?

Recent investigation in persons with clinically complete spinal cord injury has revealed that locomotor activity in one limb can activate rhythmic locomotor activity in the opposite limb. Although our previous research has demonstrated profound influences of the nonparetic limb on paretic limb motor activity poststroke, the potency of interlimb pathways for increasing recruitment of the paretic limb motor pattern is unknown. This experiment tested whether there is an increased propensity for rhythmic motor activity in one limb (pedaling limb) to induce rhythmic motor activity in the opposite limb (test limb) in persons poststroke. Forty-nine subjects with chronic poststroke hemiparesis and twenty controls pedaled against a constant mechanical load with their pedaling leg while we recorded EMG and pedal forces from the test leg. For the experimental conditions, subjects were instructed to either pedal with their test leg (bilateral pedaling) or rest their test leg while it was either stationary or moved anti-phased (unilateral pedaling). In persons poststroke, unilateral pedaling activated a complete pattern of rhythmic alternating muscle activity in the nonpedaling, test leg. This effect was most clearly demonstrated in the most severely impaired individuals. In most of the control subjects, unilateral pedaling activated some muscles in the nonpedaling leg weakly, if at all. We propose that, ipsilateral excitatory pathways associated with contralateral pedaling in control subjects are increasingly up-regulated in both legs in persons with hemiparesis as a function of increased hemiparetic severity. This enhancement of interlimb pathways may be of functional importance since contralateral pedaling induced a complete motor pattern of similar amplitude to the bilateral pattern in both the paretic and nonparetic leg of the subjects with severe hemiparesis.

Aged↗
PubMed2005

Muscle contributions to support during gait in an individual with post-stroke hemiparesis.

Walking requires coordination of muscles to support the body during single stance. Impaired ability to coordinate muscles following stroke frequently compromises walking performance and results in extremely low walking speeds. Slow gait in post-stroke hemiparesis is further complicated by asymmetries in lower limb muscle excitations. The objectives of the current study were: (1) to compare the muscle coordination patterns of an individual with flexed stance limb posture secondary to post-stroke hemiparesis with that of healthy adults walking very slowly, and (2) to identify how paretic and non-paretic muscles provide support of the body center of mass in this individual. Simulations were generated based on the kinematics and kinetics of a stroke survivor walking at his self-selected speed (0.3 m/s) and of three speed-matched, healthy older individuals. For each simulation, muscle forces were perturbed to determine the muscles contributing most to body weight support (i.e., height of the center of mass during midstance). Differences in muscle excitations and midstance body configuration caused paretic and non-paretic ankle plantarflexors to contribute less to midstance support than in healthy slow gait. Excitation of paretic ankle dorsiflexors and knee flexors during stance opposed support and necessitated compensation by knee and hip extensors. During gait for an individual with post-stroke hemiparesis, adequate body weight support is provided via reorganized muscle coordination patterns of the paretic and non-paretic lower limbs relative to healthy slow gait.

Aged↗
PubMed2005

Effect of equinus foot placement and intrinsic muscle response on knee extension during stance.

Equinus gait, a common movement abnormality among individuals with stroke and cerebral palsy, is often associated with knee hyperextension during stance. Whether there exists a causal mechanism linking equinus foot placement with knee hyperextension remains unknown. To investigate the response of the musculoskeletal system to equinus foot placement, a forward dynamic simulation of normal walking was perturbed by augmenting ankle plantarflexion by 10 degrees at initial contact. The subsequent effect on knee extension was assessed when the muscle forces were allowed, or not allowed, to change in response to altered kinematics and intrinsic force-length-velocity properties. We found that an increase in ankle plantarflexion at initial contact without concomitant changes in muscle forces caused the knee to hyperextend. The intrinsic force-length-velocity properties of muscle, particularly in gastrocnemius and vastus, diminished the effect of equinus posture alone, causing the abnormal knee extension to be less pronounced. We conclude that the effect of ankle position at initial contact on knee motion should be considered in the analysis of equinus gait.

Ankle↗
PubMed2004

Interlimb influences on paretic leg function in poststroke hemiparesis.

After stroke, paretic leg motor impairment is typically viewed as a unilateral control deficit. However, much of the neural circuitry controlling normal leg function is organized bilaterally to produce coordinated, task-specific activity in the two legs. Thus, as a result of contralesional neural control processes, paretic leg motor pattern generation may be substantially influenced by the nonparetic leg sensorimotor state during bilateral lower limb tasks. Accordingly, we investigated whether different paretic leg motor patterns are observed during mechanically equivalent bilateral and unilateral tasks and, if so, whether nonparetic leg participation improved or exacerbated paretic leg coordination deficits. A pedaling apparatus that mechanically decoupled the legs was used to present subjects with increasingly complex bi- and unilateral motor tasks: isometric force generation, discrete movement, and pedaling. Bilateral electromyographic and pedal force data were collected from 21 persons with chronic poststroke hemiparesis and 11 similarly aged controls. During isometric force generation and discrete movements, nonparetic leg influences on paretic leg coordination were similar and not markedly different from interlimb influences in controls. In bilateral pedaling, however, interlimb influences differed from controls such that paretic leg coordination deficits were exacerbated. During pedaling movements, the suppression of interlimb influences similar to those observed in isometric and discrete movement may occur in controls but may be disrupted in hemiparesis. We suggest that the coupling of pattern generation between the two legs may result in greater, albeit more impaired, paretic leg motor output during bilateral pedaling than during unilateral pedaling.

Aged↗
PubMed2004

Muscle mechanical work requirements during normal walking: the energetic cost of raising the body's center-of-mass is significant.

Inverted pendulum models of walking predict that little muscle work is required for the exchange of body potential and kinetic energy in single-limb support. External power during walking (product of the measured ground reaction force and body center-of-mass (COM) velocity) is often analyzed to deduce net work output or mechanical energetic cost by muscles. Based on external power analyses and inverted pendulum theory, it has been suggested that a primary mechanical energetic cost may be associated with the mechanical work required to redirect the COM motion at the step-to-step transition. However, these models do not capture the multi-muscle, multi-segmental properties of walking, co-excitation of muscles to coordinate segmental energetic flow, and simultaneous production of positive and negative muscle work. In this study, a muscle-actuated forward dynamic simulation of walking was used to assess whether: (1). potential and kinetic energy of the body are exchanged with little muscle work; (2). external mechanical power can estimate the mechanical energetic cost for muscles; and (3.) the net work output and the mechanical energetic cost for muscles occurs mostly in double support. We found that the net work output by muscles cannot be estimated from external power and was the highest when the COM moved upward in early single-limb support even though kinetic and potential energy were exchanged, and muscle mechanical (and most likely metabolic) energetic cost is dominated not only by the need to redirect the COM in double support but also by the need to raise the COM in single support.

Biomechanical Phenomena↗
PubMed2004

Muscle force redistributes segmental power for body progression during walking.

The ankle plantar flexors were previously shown to support the body in single-leg stance to ensure its forward progression [J. Biomech. 34 (2001) 1387]. The uni- (SOL) and biarticular (GAS) plantar flexors accelerated the trunk and leg forward, respectively, with each opposing the effect of the other. Around mid-stance their net effect on the trunk and the leg was negligible, consistent with the body acting as an inverted pendulum. In late stance, their net effect was to accelerate the leg and trunk forward, consistent with an active push-off. Because other muscles are active in the beginning and end of stance, we hypothesized that their active concentric and eccentric force generation also supports the body and redistributes segmental power to enable body forward progression. Muscle-actuated forward dynamical simulations that emulated observed walking kinematics and kinetics of young adult subjects were analyzed to quantify muscle contributions to the vertical and horizontal ground reaction force, and to the acceleration and mechanical power of the leg and trunk. The eccentric uniarticular knee extensors (vasti, VAS) and concentric uniarticular hip extensors (gluteus maximus, GMAX) were found to provide critical support to the body in the beginning of stance, before the plantar flexors became active. VAS also decelerated the forward motion of both the trunk and the leg. Afterwards when VAS shortens in mid-stance, it delivered the power produced to accelerate the trunk and also redistributed segmental power to the trunk by continuing to decelerate the leg. When present, rectus femoris (RF) activity in the beginning of stance had a minimal effect. But in late stance the lengthening RF accelerated the knee and hip into extension, which opposed swing initiation. Though RF was lengthening, it still accelerated the trunk forward by decelerating the leg and redistributing the leg segmental power to the trunk, as SOL does though it is shortening instead of lengthening. Force developed from highly stretched passive hip structures and active force produced by the uniarticular hip flexors assisted GAS in swing initiation. Hamstrings (HAM) decelerated the leg in late swing while lengthening and accelerated the leg in the beginning of stance while shortening. We conclude that the uniarticular knee and hip extensor muscles are critical to body support in the beginning of stance and redistribution of segmental power by muscles throughout the gait cycle is critical to forward progression of the trunk and legs.

Computer Simulation↗
PubMed2002

Mutability of bifunctional thigh muscle activity in pedaling due to contralateral leg force generation.

Locomotion requires uninterrupted transitions between limb extension and flexion. The role of contralateral sensorimotor signals in executing smooth transitions is little understood even though their participation is crucial to bipedal walking. However, elucidating neural interlimb coordinating mechanisms in human walking is difficult because changes to contralateral sensorimotor activity also affect the ipsilateral mechanics. Pedaling, conversely, is ideal for studying bilateral coordination because ipsilateral mechanics can be independently controlled. In pedaling, the anterior and posterior bifunctional thigh muscles develop needed anterior and posterior crank forces, respectively, to dominate the flexion-to-extension and extension-to-flexion transitions. We hypothesized that contralateral sensorimotor activity substantially contributes to the appropriate activation of these bifunctional muscles during the limb transitions. Bilateral pedal forces and surface electromyograms (EMGs) from four thigh muscles were collected from 15 subjects who pedaled with their right leg against a right-crank servomotor, which emulated the mechanical load experienced in conventional two-legged coupled-crank pedaling. In one pedaling session, the contralateral (left) leg pseudo-pedaled (i.e., EMG activity and pedal forces were pedaling-like, but pedal force was not allowed to affect crank rotation). In other sessions, the mechanically decoupled contralateral leg was first relaxed and then produced rhythmic isometric force trajectories during either leg flexion or one of the two limb transitions of the pedaling leg. With contralateral force production in the extension-to-flexion transition (predominantly by the hamstrings), rectus femoris activity and work output increased in the pedaling leg during its flexion-to-extension transition, which occurs simultaneously with contralateral extension-to-flexion in conventional pedaling. Similarly, with contralateral force production in the other transition (i.e., flexion-to-extension; predominantly by rectus femoris), hamstrings activity and work output increased in the pedaling leg during its extension-to-flexion transition. Therefore rhythmic isometric force generation in the contralateral leg supported the ongoing bifunctional muscle activity and resulting work output in the pedaling leg. The results suggest that neural interlimb coordinating mechanisms fine-tune bifunctional muscle activity in rhythmic lower-limb tasks to ensure limb flexion/extension transitions are executed successfully.

Adult↗
PubMed2001

Simulation analysis of muscle activity changes with altered body orientations during pedaling.

Testing hypotheses related to the effect of gravitational orientation on neural control mechanisms is difficult for most locomotor tasks, like walking, because body orientation with respect to gravity affects both sensorimotor control and task mechanics. To examine the mechanical effect of body orientation independently from changes in workload and posture, Brown et al. (J. Biomech. 29 p. 1349, 1996) studied pedaling at altered body orientations. They found that subjects pedaling at different orientations changed needlessly their muscle excitations, putatively to preserve body-upright pedaling kinematics. We tested the feasibility of this hypothesis using simulations based on a three biomechanical-function pair organization for control of lower limb muscles (limb extension/flexion pair, extension/flexion transition pair, and foot plantarflexion/dorsiflexion pair), where each pair consists of alternating agonistic/antagonistic muscles. Adjustment of only three parameters, one to scale the muscle excitations of each pair, was sufficient to preserve pedaling kinematics to altered body orientation. Because these adjustments produced changes in muscle excitation and net joint moments similar to those observed in pedaling subjects, the hypothesis is supported. Moreover, the effectiveness of a decoupled gain adjustment procedure where each parameter was adjusted by error in only one aspect of the pedaling trajectory during each iteration (i.e., cadence adjusted the Ext/Flex parameter; peak-to-peak variation in crank velocity over the cycle adjusted the transition parameter; average ankle angle over the cycle adjusted the foot parameter) further supports the distinct function of each muscle pair.

Bicycling↗
PubMed2001

Contributions of the individual ankle plantar flexors to support, forward progression and swing initiation during walking.

Walking is a motor task requiring coordination of many muscles. Previous biomechanical studies, based primarily on analyses of the net ankle moment during stance, have concluded different functional roles for the plantar flexors. We hypothesize that some of the disparities in interpretation arise because of the effects of the uniarticular and biarticular muscles that comprise the plantar flexor group have not been separated. Furthermore, we believe that an accurate determination of muscle function requires quantification of the contributions of individual plantar flexor muscles to the energetics of individual body segments. In this study, we examined the individual contributions of the ankle plantar flexors (gastrocnemius (GAS); soleus (SOL)) to the body segment energetics using a musculoskeletal model and optimization framework to generate a forward dynamics simulation of normal walking at 1.5 m/s. At any instant in the gait cycle, the contribution of a muscle to support and forward progression was defined by its contribution to trunk vertical and horizontal acceleration, respectively, and its contribution to swing initiation by the mechanical energy it delivers to the leg in pre-swing (i.e., double-leg stance prior to toe-off). GAS and SOL were both found to provide trunk support during single-leg stance and pre-swing. In early single-leg stance, undergoing eccentric and isometric activity, they accelerate the trunk vertically but decelerate forward trunk progression. In mid single-leg stance, while isometric, GAS delivers energy to the leg while SOL decelerates it, and SOL delivers energy to the trunk while GAS decelerates it. In late single-leg stance through pre-swing, though GAS and SOL both undergo concentric activity and accelerate the trunk forward while decelerating the downward motion of the trunk (i.e., providing forward progression and support), they execute different energetic functions. The energy produced from SOL accelerates the trunk forward, whereas GAS delivers almost all its energy to accelerate the leg to initiate swing. Although GAS and SOL maintain or accelerate forward motion in mid single-leg stance through pre-swing, other muscles acting at the beginning of stance contribute comparably to forward progression. In summary, throughout single-leg stance both SOL and GAS provide vertical support, in mid single-leg stance SOL and GAS have opposite energetic effects on the leg and trunk to ensure support and forward progression of both the leg and trunk, and in pre-swing only GAS contributes to swing initiation.

Adult↗
PubMed2001

Muscle activation and deactivation dynamics: the governing properties in fast cyclical human movement performance?

Repetitive cyclical motion and intrinsic muscle properties each impose constraints on the nervous systems to produce well-coordinated movements. We suggest that as cycle frequency increases, activation and deactivation dynamics strongly influence the neural control strategy used and may be the governing muscle property that limits performance. Pedaling and animal studies provide supporting data.

Animals↗
PubMed2000

Muscle contributions to specific biomechanical functions do not change in forward versus backward pedaling.

Previous work had identified six biomechanical functions that need to be executed by each limb in order to produce a variety of pedaling tasks. The functions can be organized into three antagonistic pairs: an Ext/Flex pair that accelerates the foot into extension or flexion with respect to the pelvis, an Ant/Post pair that accelerates the foot anteriorly or posteriorly with respect to the pelvis, and a Plant/Dorsi pair that accelerates the foot into plantarflexion or dorsiflexion. Previous analyses of experimental data have inferred that muscles perform the same function during different pedaling tasks (e.g. forward versus backward pedaling) because the EMG timing was similar, but they did not present rigorous biomechanical analyses to assess whether a muscle performed the same biomechanical function, and if so, to what degree. Therefore, the objective of this study was to determine how individual muscles contribute to these biomechanical functions during two different motor tasks, forward and backward pedaling, through a theoretical analysis of experimental data. To achieve this objective, forward and backward pedaling simulations were generated and a mechanical energy analysis was used to examine how muscles generate, absorb or transfer energy to perform the pedaling tasks. The results showed that the muscles contributed to the same primary Biomechanical functions in both pedaling directions and that synergistic performance of certain functions effectively accelerated the crank. The gluteus maximus worked synergistically with the soleus, the hip flexors worked synergistically with the tibialis anterior, and the vasti and hamstrings functioned independently to accelerate the crank. The rectus femoris used complex biomechanical mechanisms including negative muscle work to accelerate the crank. The negative muscle work was used to transfer energy generated elsewhere (primarily from other muscles) to the pedal reaction force in order to accelerate the crank. Consistent with experimental data, a phase shift was required from those muscles contributing to the Ant/Post functions as a result of the different limb kinematics between forward and backward pedaling, although they performed the same biomechanical function. The pedaling simulations proved necessary to interpret the experimental data and identify motor control mechanisms used to accomplish specific motor tasks, as the mechanisms were often complex and not always intuitively obvious.

Bicycling↗
PubMed2000

Knee joint loading in forward versus backward pedaling: implications for rehabilitation strategies.

OBJECTIVE: To use forward dynamic simulations of forward and backward pedaling in order to determine whether backward pedaling offers theoretical advantages over forward pedaling to rehabilitate common knee disorders.DESIGN. A comparison of knee joint loads was performed during forward and backward pedaling.BACKGROUND. Pedaling has been shown to be an effective rehabilitation exercise for a variety of knee disorders. Recently, backward gait has been shown to produce greater knee extensor moments and reduced patellofemoral joint loads compared to forward gait. But to date, no study has examined the efficacy of backward pedaling as a safe alternative to forward pedaling in rehabilitation programs.METHODS. A musculoskeletal model and optimization framework was used to generate simulations of forward and backward pedaling. Tibiofemoral and patellofemoral joint reaction forces were quantified.RESULTS. Lower tibiofemoral compressive loads, but higher patellofemoral compressive loads, were observed in backward pedaling. Lower protective anterior-posterior shear force was observed in backward pedaling near peak extension.CONCLUSIONS. Backward pedaling offers reduced tibiofemoral compressive loads for those patients with knee disorders such as menisci damage and osteoarthritis, but higher patellofemoral compressive loads. Therefore, backward pedaling is not recommended for patients experiencing patellofemoral pain. Further, backward pedaling should not be recommended after anterior cruciate ligament injury or reconstruction.RelevanceThe results of this study indicate that the design of rehabilitation programs including pedaling exercises should be injury specific with particular attention paid to the mechanics of the task.

Adult↗
PubMed2000

General coordination principles elucidated by forward dynamics: minimum fatique does not explain muscle excitation in dynamic tasks.

The target article presents a framework for coordination of one- and two-joint muscles in a variety of tasks. Static optimization analyses were performed that minimize muscle fatigue, and it is claimed that the predicted muscle forces account for essential features of EMG activity qualitatively well. However, static optimization analyses use the observed joint moments, which implicitly assumes that they minimize the total muscle fatigue of the task. We use a forward dynamics (i.e., relationship between muscle forces and the kinematics and kinetics of task performance) modeling approach to show that this assumption does not appear to be true in cycling (which was used as an example task in the target article). Our results challenge the hypothesized coordination framework and the underlying concept that general coordination principles for dynamic tasks can be elucidated using inverse-dynamics-based analyses.

Biomechanical Phenomena↗
PubMed2000

Contralateral movement and extensor force generation alter flexion phase muscle coordination in pedaling.

The importance of bilateral sensorimotor signals in coordination of locomotion has been demonstrated in animals but is difficult to ascertain in humans due to confounding effects of mechanical transmission of forces between the legs (i.e., mechanical interleg coupling). In a previous pedaling study, by eliminating mechanical interleg coupling, we showed that muscle coordination of a unipedal task can be shaped by interlimb sensorimotor pathways. Interlimb neural pathways were shown to alter pedaling coordination as subjects pedaling unilaterally exhibited increased flexion-phase muscle activity compared with bilateral pedaling even though the task mechanics performed by the pedaling leg(s) in the unilateral and bilateral pedaling tasks were identical. To further examine the relationship between contralateral sensorimotor state and ipsilateral flexion-phase muscle coordination during pedaling, subjects in this study pedaled with one leg while the contralateral leg either generated an extensor force or relaxed as a servomotor either held that leg stationary or moved it in antiphase with the pedaling leg. In the presence of contralateral extensor force generation, muscle activity in the pedaling leg during limb flexion was reduced. Integrated electromyographic activity of the pedaling-leg hamstring muscles (biceps femoris and semimembranosus) during flexion decreased by 25-30%, regardless of either the amplitude of force generated by the nonpedaling leg or whether the leg was stationary or moving. In contrast, rectus femoris and tibialis anterior activity during flexion decreased only when the contralateral leg generated high rhythmic force concomitant with leg movement. The results are consistent with a contralateral feedforward mechanism triggering flexion-phase hamstrings activity and a contralateral feedback mechanism modulating rectus femoris and tibialis anterior activity during flexion. Because only muscles that contribute to flexion as a secondary function were observed, it is impossible to know whether the modulatory effect also acts on primary, unifunctional, limb flexors or is specific to multifunctional muscles contributing to flexion. The influence of contralateral extensor-phase sensorimotor signals on ipsilateral flexion may reflect bilateral coupling of gain control mechanisms. More generally, these interlimb neural mechanisms may coordinate activity between muscles that perform antagonistic functions on opposite sides of the body. Because pedaling and walking share biomechanical and neuronal control features, these mechanisms may be operational in walking as well as pedaling.

Adult↗
PubMed1999

Phase reversal of biomechanical functions and muscle activity in backward pedaling.

Computer simulations of pedaling have shown that a wide range of pedaling tasks can be performed if each limb has the capability of executing six biomechanical functions, which are arranged into three pairs of alternating antagonistic functions. An Ext/Flex pair accelerates the limb into extension or flexion, a Plant/Dorsi pair accelerates the foot into plantarflexion or dorsiflexion, and an Ant/Post pair accelerates the foot anteriorly or posteriorly relative to the pelvis. Because each biomechanical function (i.e., Ext, Flex, Plant, Dorsi, Ant, or Post) contributes to crank propulsion during a specific region in the cycle, phasing of a muscle is hypothesized to be a consequence of its ability to contribute to one or more of the biomechanical functions. Analysis of electromyogram (EMG) patterns has shown that this biomechanical framework assists in the interpretation of muscle activity in healthy and hemiparetic subjects during forward pedaling. Simulations show that backward pedaling can be produced with a phase shift of 180 degrees in the Ant/Post pair. No phase shifts in the Ext/Flex and Plant/Dorsi pairs are then necessary. To further test whether this simple yet biomechanically viable strategy may be used by the nervous system, EMGs from 7 muscles in 16 subjects were measured during backward as well as forward pedaling. As predicted, phasing in vastus medialis (VM), tibialis anterior (TA), medial gastrocnemius (MG), and soleus (SL) were unaffected by pedaling direction, with VM and SL contributing to Ext, MG to Plant, and TA to Dorsi. In contrast, phasing in biceps femoris (BF) and semimembranosus (SM) were affected by pedaling direction, as predicted, compatible with their contribution to the directionally sensitive Post function. Phasing of rectus femoris (RF) was also affected by pedaling direction; however, its ability to contribute to the directionally sensitive Ant function may only be expressed in forward pedaling. RF also contributed significantly to the directionally insensitive Ext function in both forward and backward pedaling. Other muscles also appear to have contributed to more than one function, which was especially evident in backward pedaling (i.e. , BF, SM, MG, and TA to Flex). We conclude that the phasing of only the Ant and Post biomechanical functions are directionally sensitive. Further, we suggest that task-dependent modulation of the expression of the functions in the motor output provides this biomechanics-based neural control scheme with the capability to execute a variety of lower limb tasks, including walking.

Adult↗
PubMed1999

Speed-dependent reductions of force output in people with poststroke hemiparesis.

BACKGROUND AND PURPOSE: Movement is slow in people with poststroke hemiparesis. Moving at faster speeds is thought by some researchers to exacerbate abnormal or unwanted muscle activity. The purpose of this study was to quantify the effects of increased speed on motor performance during pedaling exercise in people with poststroke hemiparesis. SUBJECTS: Twelve elderly subjects with no known neurological impairment and 15 subjects with poststroke hemiparesis of greater than 6 months' duration were tested. METHODS: Subjects pedaled at 12 randomly ordered workload and cadence combinations (45-, 90-, 135-, and 180-J workloads at 25, 40, and 55 rpm). Pedal reaction forces were used to calculate work done by each lower extremity. Electromyographic activity was recorded from 7 lower-extremity muscles. RESULTS: The main finding was that net mechanical work done by the paretic lower extremity decreased as speed increased in all subjects. The occurrence of inappropriate muscle activity on the paretic side, however, was not exacerbated in that the vastus medialis muscle on the paretic side did not show a consistent further increase in its prolonged activity at higher speeds. The mechanics of faster pedaling resulted in greater net negative mechanical work because, at higher pedaling rates, the prolonged vastus medialis muscle activity is present during a greater portion of the cycle. CONCLUSION AND DISCUSSION: The lessened force output by the paretic limb is mainly the result of the inherent mechanical demands of higher-speed pedaling and not due to exacerbation of impaired neural control.

Aged↗