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Ge Wu and Weifeng Zhao

This study examined the role of feedback from cutaneous mechanoreceptors in the stability of human upright posture. A two-link, one degree of freedom, inverted pendulum model was constructed for the human body with ankle joint torque proportional to the delayed outputs from muscle receptors, joint receptors, and cutaneous mechanoreceptors in the foot. Theoretical analysis and numerical simulations indicated that the use of mechanoreceptive information reduced the frequency range and the maximum peak-peak value of the dynamic response of the system. However, without the use of muscle receptors, the mechanoreceptive feedback could not stabilize the system. In addition, body movement of human subjects was measured when their balanced upright posture was disturbed by a transient, forward/backward movement of a supporting platform. The loss of or change in cutaneous mechanoreceptive sense in their feet was induced by (a) having healthy subjects stand on a soft surface and (b) testing neuropathic patients with loss of vibratory sensation in their feet. The results showed significant increases in frequency range and maximum peak-peak value of ankle rotation and velocity for subjects standing on a soft (vs. hard) surface and for neuropathic patients (vs. age- and gender-matched healthy subjects).

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Joseph F. Seay, Jeffery M. Haddad, Richard E.A. van Emmerik and Joseph Hamill

Increases in movement variability have previously been observed to be a hallmark property of cooraination changes between coupled oscillators that occur as movement frequency is scaled. Prior research on the walk-run transition in human locomotion has also demonstrated increases in variability around the transition region, supporting predictions of nonequilibrium phase transitions (Diedrich & Warren, 1995). The current study examined the coordinative patterns of both intra- and inter-limb couplings around the walk-run transition using two different temporal manipulations of locomotor velocity as a control parameter in healthy young participants (N = 11). Coordination variability did not increase before the transition. The nature of the change in continuous relative phase variability between gait modes was coupling-specific, and varying the time spent at each velocity did not have an overall effect on gait transition dynamics. Lower extremity inter-limb coordination dynamics were more sensitive to changes in treadmill velocity than intra-limb coordination. The results demonstrate the complexity of segmental coordination change in human locomotion, and question the applicability of dynamical bimanual coordination models to human gait transitions.

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Jürgen Konczak, Kai Brommann and Karl Theodor Kalveram

Knowledge of how stiffness, damping, and the equilibrium position of specific limbs change during voluntary motion is important for understanding basic strategies of neuromotor control. Presented here is an algorithm for identifying time-dependent changes in joint stiffness, damping, and equilibrium position of the human forearm. The procedure requires data from only a single trial. The method relies neither on an analysis of the resonant frequency of the arm nor on the presence of an external bias force. Its validity was tested with a simulated forward model of the human forearm. Using the parameter estimations as forward model input, the angular kinematics (model output) were reconstructed and compared to the empirically measured data. Identification of mechanical impedance is based on a least-squares solution of the model equation. As a regularization technique and to improve the temporal resolution of the identification process, a moving temporal window with a variable width was imposed. The method's performance was tested by (a) identifying a priori known hypothetical time-series of stiffness, damping, and equilibrium position, and (b) determining impedance parameters from recorded single-joint forearm movements during a hold and a goal-directed movement task. The method reliably reconstructed the original angular kinematics of the artificial and human data with an average positional error of less than 0.05 rad for movement amplitudes of up to 0.9 rad, and did not yield hypermetric trajectories like previous procedures not accounting for damping.

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Toshimasa Yanai and James G. Hay

The purpose of this study was to test the hypothesis that, in human running at a given speed, runners select the combination of cycle rate (CR) and cycle length (CL) that minimizes the power generated by the muscles. A 2-D model of a runner consisting of a trunk and two legs was defined. A force actuator controlled the length of each leg, and a torque actuator controlled the amplitude and frequency of the backward and forward swing of each leg. The sum of the powers generated by the actuators was determined for a range of CRs at each of a series of speeds. The CR and CL vs. speed relationships selected for the model were derived from a series of CR and CL combinations that required the least power at each speed. Two constraints were imposed: the maximum amplitude of the forward and backward swing of the legs (±50°) and the minimum ground contact time needed to maintain steady-state running (0.12 sec). The CR vs. speed and CL vs. speed relationships derived on the basis of a minimum power strategy showed a pattern similar to those reported for longitudinal (within-subjects) analyses of human running. The anatomical constraint set a limit on the maximum CL attainable at a given speed, and the temporal constraint made CL decrease at high speeds. It was concluded that the process for selecting CL-CR combinations for human running has characteristics similar to the process for solving a constrained optimization problem.

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Jacob Buus Andersen and Thomas Sinkjaer

Due to the complexity of applying a well-defined stretch during human walking, most of our knowledge about the short latency stretch reflex modulation in humans is based on H-reflex studies. To illuminate the difference between the two methodologies, both types of reflexes were evoked in the same subjects, same experiment. Stretch reflexes were evoked via a stretch device capable of evoking stretch reflexes of the human soleus muscle during walking. H-reflexes were elicited by an electrical stimulation of the tibial nerve at the popliteal fossa at the knee. A significantly different modulation of the two reflexes was found in the late stance where the stretch reflex decreased in relation to the H-reflex. This was consistent with an unloading of the muscle spindles during the push-off in late stance, suggesting a complex alpha-gamma coactivation, if any, at this time of the step. The soleus stretch reflex and H-reflex were compared during the stance phase of walking and sitting at matched soleus EMG activity. No difference was found in the amplitude of the stretch reflex. However, there was a significant decrease of the H-reflex during the stance phase of walking, consistent with a task-specific presynaptic mediated reflex control. It is proposed that the short latency stretch reflex during walking is not sensitive to such a presynaptic inhibition.

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William R. Leonard

This paper examines the evolutionary origins of human dietary and activity patterns, and their implications for understanding modern health problems. Humans have evolved distinctive nutritional characteristics associated the high metabolic costs of our large brains. The evolution of larger hominid brain size necessitated the adoption of foraging strategies that both provided high quality foods, and required larger ranges and activity budgets. Over time, human subsistence strategies have become ever more efficient in obtaining energy with minimal time and effort. Today, populations of the industrialized world live in environments characterized by low levels of energy expenditure and abundant food supplies contributing to growing rates of obesity. Analyses of trends in dietary intake and body weight in the US over the last 50 years indicate that the dramatic rise in obesity cannot be explained solely by increased energy consumption. Rather, declines in activity are also important. Further, we find that recent recommendations on physical activity have the potential to bring daily energy expenditure levels of industrialized societies surprisingly close to those observed among subsistence-level populations. These findings highlight the importance of physical activity in promoting nutritional health and show the utility of evolutionary approaches for developing public health recommendations.

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Jaclyn B. Caccese and Thomas W. Kaminski

Context:

The Balance Error Scoring System (BESS) is the current standard for assessing postural stability in concussed athletes on the sideline. However, research has questioned the objectivity and validity of the BESS, suggesting that while certain subcategories of the BESS have sufficient reliability to be used in evaluation of postural stability, the total score is not reliable, demonstrating limited interrater and intrarater reliability. Recently, a computerized BESS test was developed to automate scoring.

Objective:

To compare computerderived BESS scores with those taken from 3 trained human scorers.

Design:

Interrater reliability study.

Setting:

Athletic training room.

Patients:

NCAA Division I student athletes (53 male, 58 female; 19 ± 2 y, 168 ± 41 cm, 69 ± 4 kg).

Interventions:

Subjects were asked to perform the BESS while standing on the Tekscan (Boston, MA) MobileMat® BESS. The MobileMat BESS software displayed an error score at the end of each trial. Simultaneously, errors were recorded by 3 separate examiners. Errors were counted using the standard BESS scoring criteria.

Main Outcome Measures:

The number of BESS errors was computed for the 6 stances from the software and each of the 3 human scorers. Interclass correlation coefficients (ICCs) were used to compare errors for each stance scored by the MobileMat BESS software with each of 3 raters individually. The ICC values were converted to Fisher Z scores, averaged, and converted back into ICC values.

Results:

The double-leg, single-leg, and tandem-firm stances resulted in good agreement with human scorers (ICC = .999, .731, and .648). All foam stances resulted in fair agreement.

Conclusions:

Our results suggest that the MobileMat BESS is suitable for identifying BESS errors involving each of the 6 stances of the BESS protocol. Because the MobileMat BESS scores consistently and reliably, this system can be used with confidence by clinicians as an effective alternative to scoring the BESS.

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Scott O. Cloyd, Mont Hubbard and LeRoy W. Alaways

Feedback control of a human-powered single-track bicycle is investigated through the use of a linearized dynamical model in order to develop feedback gains that can be implemented by a human pilot in an actual vehicle. The object of the control scheme is to satisfy two goals: balance and tracking. The pilot should be able not only to keep the vehicle upright but also to direct the forward motion as desired. The two control inputs, steering angle and rider lean angle, are assumed to be determined by the rider as a product of feedback gains and “measured” values of the state variables: vehicle lean, lateral deviation from the desired trajectory, and their derivatives. Feedback gains are determined through linear quadratic regulator theory. This results in two control schemes, a “full” optimal feedback control and a less complicated technique that is more likely to be usable by an inexperienced pilot. Theoretical optimally controlled trajectories are compared with experimental trajectories in a lane change maneuver.

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Alan Hreljac, Rodney T. Imamura, Rafael F. Escamilla, W. Brent Edwards and Toran MacLeod

The primary purpose of this project was to examine whether lower extremity joint kinetic factors are related to the walk–run gait transition during human locomotion. Following determination of the preferred transition speed (PTS), each of the 16 subjects walked down a 25-m runway, and over a floor-mounted force platform at five speeds (70, 80, 90, 100, and 110% of the PTS), and ran over the force platform at three speeds (80, 100, and 120% of the PTS) while being videotaped (240 Hz) from the right sagittal plane. Two-dimensional kinematic data were synchronized with ground reaction force data (960 Hz). After smoothing, ankle and knee joint moments and powers were calculated using standard inverse dynamics calculations. The maximum dorsiflexor moment was the only variable tested that increased as walking speed increased and then decreased when gait changed to a run at the PTS, meeting the criteria set to indicate that this variable influences the walk–run gait transition during human locomotion. This supports previous research suggesting that an important factor in changing gaits at the PTS is the prevention of undue stress in the dorsiflexor muscles.

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Jeffrey J. Brault, Theodore F. Towse, Jill M. Slade and Ronald A. Meyer

Short-term creatine supplementation is reported to result in a decreased ratio of phosphocreatine (PCr) to total creatine (TCr) in human skeletal muscle at rest. Assuming equilibrium of the creatine kinase reaction, this decrease in PCr:TCr implies increased cytoplasmic ADP and decreased Gibbs free energy of ATP hydrolysis in muscle, which seems contrary to the reported ergogenic benefits of creatine supplementation. This study measured changes in PCr and TCr in vastus lateralis muscle of adult men (N = 6, 21–35 y old) during and 1 day after 5 d of creatine monohydrate supplementation (0.43 g·kg body weight−1·d−1) using noninvasive 31P and 1H magnetic-resonance spectroscopy (MRS). Plasma and red-blood-cell creatine increased by 10-fold and 2-fold, respectively, by the third day of supplementation. MRS-measured skeletal muscle PCr and TCr increased linearly and in parallel throughout the 5 d, and there was no significant difference in the percentage increase in muscle PCr (11.7% ± 2.3% after 5 d) vs. TCr (14.9% ± 4.1%) at any time point. The results indicate that creatine supplementation does not alter the PCr:TCr ratio, and hence the cytoplasmic Gibbs free energy of ATP hydrolysis, in human skeletal muscle at rest.