The authors thank the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2011BAK15B06) for their support.&#160;Thank Shuya Zhuang who is a master student from our laboratory for helping us complete the video shoot.

1. Observation of the Surface Structure on Water Strider Leg
<ol>
	<li>Collect water striders from local freshwater ponds using fishing landing net.</li>
	<li>Cut off at least 5 pairs of middle legs as experimental samples using scissors. Touch the bottom of the legs carefully, to prevent the surface contamination and the disrupting of microstructure in the front of legs.</li>
	<li>Dry the legs out in the air naturally.</li>
	<li>Observe the surface microstructure of legs using a scanning electron microscope with micro-level resolution as shown in Figure 1a.</li>
	<li>Observe the microsetae of the legs using a scanning electric microscope with nano-level resolution as shown inFigure 1b.</li>
</ol>
2. Prepare the Component of the Dynamic Force Measurement System
<ol>
	<li>Purchase a PVDF film sensor with a dimension of 14.9 X 10.2 mm2 X 28 &#956;m, which produces more than 10 mV per micro-strain. 
	NOTE: The PVDF film sensor is used to sense the dynamic contact force with high sensitivity.</li>
	<li>Purchase a charge amplifier with maximum 1000 mV/pC charge gain and low noise less than 5 &#956;V. 
	NOTE: The charge amplifier is used to enlarge the signal from the PVDF film sensor, in which the charge output from the transducer is converted to into a voltage.</li>
	<li>Purchase a data acquisition device, in which the analog input has the sample rates in a range from 1 S/s to 102.4 kS/s. 
	NOTE: The data acquisition device is used to read the voltage data of the charge amplifier and send them to the computer for further processing and display.</li>
	<li>Purchase several high-precise displacement stages and a servomotor. 
	NOTE: The legs depart from water at different speed driven by the servomotor.</li>
	<li>Purchase a CCD camera, whose focus length is in a range from 5 mm to 30 mm and frame rate is 30 fps. 
	NOTE: This camera is used to record and monitor the deformation of water surface and the distance between legs and water surface.</li>
	<li>Prepare a high-performance computer.</li>
</ol>
3. Assembly of all Parts of the Dynamic Force Measurement System
<ol>
	<li>Assemble the dynamic force measurement system according to the schematic illustration shown in Figure 2a and the real experiment instrument shown in Figure 2b.</li>
	<li>Fix one side of the PVDF film sensor with the electrodes to the high-precise displacement stage, which is placed on the horizontal frame, as other side is hanging. This installation method of PVDF film sensor helps improve the resolution of the measurement for the dynamic micro force.</li>
	<li>Connect the PVDF film sensor to the charge amplifier, the charge amplifier to the data acquisition device and the data acquisition device to the computer.</li>
	<li>Fix the camera to the high-precise displacement stage, which is placed on the left side of PVDF film sensor.</li>
	<li>To roughly adjust the distance between the legs and water quickly, fix a high-precise displacement stage to the frame above the PVDF film sensor, whose separation from the PVDF film sensor is about 10 cm.</li>
	<li>To lift the leg away from water surface at a precise speed, fix the servomotor below the high-precise displacement stage.</li>
</ol>
4. Calibration of the Dynamic Force Measurement System
<ol>
	<li>Use the electrostatic force system26 to generate a micro-constant force acted on the free end of PVDF film sensor, whose magnitude should be less than 0.5 &#181;N. Control the electrostatic force of the system by a voltage applied to the inner and outer electrodes of the paralleled cylindrical capacitor. 
	&#8203;Caution: The force should act in a direction normal to the PVDF film surface and the point of application is supposed to be as close as possible to the tip of the PVDF film sensor to increase sensitivity.</li>
	<li>Release the force in a short time to generate a step input.</li>
	<li>Read the voltage-time signal of step 4.2 in the computer using the software LabVIEW, which helps read the output voltage signals of the PVDF film sensor.
	<ol>
		<li>Download the software LabVIEW and the hardware driver NI-DAQmx in the official website of National Instruments.</li>
		<li>Open the demo of the continuous analog voltage measurement using the LabVIEW, as shown Figure 3.</li>
		<li>Select the physical channel of the data acquisition device connected with the charge amplifier in the module of Channel Settings.</li>
		<li>Set the sample rate to 100000 and the number of samples to 100000 in the module of Timing Settings.</li>
		<li>Select the Log and read as the logging mode and write the file path in to store the voltage data in the module of Logging Settings.</li>
		<li>Select No trigger in the module of Trigger Settings.</li>
		<li>Click the arrow-shape button in the toolbar to sample the voltage signal.</li>
	</ol>
	</li>
	<li>Analyze the voltage curve, in which the peak voltage is corresponding to force acted on the sensor.</li>
	<li>Repeat the steps 4.1-4.4 at different force input, in which a series of voltage-force points are gained.</li>
	<li>Determine the relationship about the peak output voltage and standard force in the calibration result.</li>
</ol>
5. Measurement of Adhesion Force at A Certain Speed
<ol>
	<li>Place a water drop (5 &#181;L) on the free end of the PVDF film sensor using a mechanical micropipette. 
	NOTE: The location of the droplet should be close to the tip of the PVDF film sensor.</li>
	<li>Stick a single leg to the servomotor below the high-precise displacement stage.</li>
	<li>Move the high-precise displacement stage downward until the leg contacts with the water surface, as shown in Figure 4a. Monitor the distance between the leg and water surface by the camera system mounted on the left side of the sensor.</li>
	<li>Lift the leg away from the water surface at a constant speed through the servomotor.</li>
	<li>Calculate the force corresponding to each point of voltage curve of the departing process using the model established in step 4.6, and then draw the force-time curve of departing process, as shown in Figure 4b.</li>
	<li>Record the peak adhesion of the departing process at a certain speed.</li>
</ol>
6. Measurement of Adhesion Force at Different Speeds.
<ol>
	<li>Alter the lifting speed of the legs by the servomotor and measure the adhesion force according to step 5.</li>
	<li>Plot the adhesion force versus lifting speed curve using the data gained in step 6.1.</li>
	<li>Analyze the relationship between the adhesion force and the lifting speed through the curve.</li>
</ol>

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The author has nothing to disclose.

In this protocol, a dynamic force measurement system based on the PVDF film sensor was successfully devised, assembled, calibrated to measure the adhesion force away from the water surface. Among the whole steps, it was crucial that the adhesion force was measured at different speeds by lifting the leg from the water surface as this study focused on the remarkable characteristic of the quick maneuvering on the water. The results of departing experiment showed that the adhesion force decreased when the lifting speed increased. These clarified that the water striders would feel relaxed if they moved at a high speed on the water.
The PVDF-based dynamic force measurement method is an important complement to the traditional method. In past studies, the adhesion force of the water striders in the detachment process was usually measured by the atomic force microscope (AFM) in a quasi-static mode. Compared to the AFM method, despite that the measurement accuracy is slightly inferior, the PVDF film sensor is able to measure the force of bigger macroscopic objects. In addition, due to its large frequency response characteristics, the PVDF film sensor can measure the dynamic interaction between the legs and water surface while the AFM could be only used in the quasi-static condition conversely.
The proposed method for measuring dynamic force was confined that only micro-force could be gauged. If we applied a large force to the dangling sensor, it would cause a considerable deformation of the PVDF film sensor, which would lead to imprecise result. Moreover, the sensitive area of the PVDF film sensor was small, which limited the size of measured object. However, in contrast to the conventional method, the proposed method was able to measure the dynamic force rather than merely measure the static force, which could exhibit the whole interaction process.
This method based on PVDF film sensor has broad applications in many fields due to its high sensitivity in dynamic force sensing and remarkable flexibility. For example, it has been drawn much attention to the application in structural health monitoring by monitoring the response of buildings under the vibration or big motions27,28. In addition, the PVDF film sensors are used to directly measure the interaction between two inviscid droplets in coalescence process, in which the fluid mechanics have not been fully understood. Moreover, the PVDF film sensor also plays an important role in tactile sensing in the robots29. The sensor is embedded in the robot&#39;s fingertips to measure the contact force as well as the contact temperature of an object. In the field of biological research, PVDF-based force sensors help improve the success rate of the single cell manipulation, such as DNA injection and gene therapy, through the precise mechanical feedback control with high sensitivity.

In nature, water striders possess remarkable ability to jump or glide easily and rapidly on the water surface with the help of the slender and nonwetting legs1,2,3,4,5, but seldom move slowly, which is unlike the terrestrial insects. The hierarchical structure of water strider stabilizes the superhydrophobic state, which renders dramatic reduction in contact area and adhesion force between water and the leg6,7,8,9. However, the hydrodynamic advantages of the quick disengagement of water striders from water surface remain poorly interpreted10,11,12.
The process of jumping from the water surface is mainly divided into three stages13,14,15,16. At first, water striders push the water surface downward with the middle and rear legs to convert the biological energy into the surface energy of the water until sinking to the maximum depth, which enable the insect to initialize the jumping direction and determine the detaching velocity. Followed by the ascending stage, the insect is pushed upward by the capillary force of the curved water surface until reaching the maximum velocity. In the final disengagement stage, the water strider continues to rise up by inertia until breaking away from the water surface, but the velocity is largely reduced due to the adhesion force with the water, which has principal influence on the energy consumption of the water strider. Hence, this protocol is proposed to measure the adhesion force at different take-off velocities in the disengagement stage and explain the distinct characteristic of fast moving.
There have been many studies to explore the adhesion force of water striders when propelling from water surface. Lee &#38; Kim theoretically and experimentally confirmed that the adhesion force and energy required lifting the water strider&#39;s legs decreased dramatically when the contact angle increased to 160 degrees17. Pan Jen Wei designed a hydrostatic experiment to measure the adhesion force by the TriboScope System, which was found to be 1/5 of its weight 18. Kehchih Hwang analyzed the quasi-static process of the legs detaching from the water with a 2D model and found the superhydrophobicity of the legs played a significant role in reducing the adhesion force and energy dissipation19. However, the measurement of the adhesion force in previous studies was just in condition of a quasi-static process, which was unable to monitor the adhesion force changes during the fast jumping.
In this study, we designed a dynamic force measurement system using polyvinylidene fluoride (PVDF) film sensor and other adjuvant instrument. Compared with other piezoelectric materials, PVDF is more suitable for measuring the dynamic microforce with higher sensitivity20,21,22. By integrating the PVDF film sensor into the system, the real-time adhesion force could be detected and processed when the leg was pulling up from water surface23,24,25.

<table><tbody><tr><td>PVDF film sensor</td><td>TE Connectivity</td><td>DT1-028K/L</td><td>The PVDF film sensor is used to sense the dynamic contact force .</td></tr><tr><td>Charge amplifier</td><td>Wuxi Shiao Technology co.,Ltd</td><td>YE5852B</td><td>The charge amplifier&amp;nbsp;is an electronic current integrator that produces a voltage output proportional to the integrated value of the input</td></tr><tr><td>Data acquisition device</td><td>National Instruments</td><td>USB-4431</td><td>The data acquisition device is used to read the voltage data.</td></tr><tr><td>Displacement stage</td><td>ZOLIXINSTRUMENTS CO.LTD</td><td>KSAV1010-ZF</td><td>KSAV1010/2030-ZF is a wedge vertical stage with high-resolution, high-stability and high-load.</td></tr><tr><td>CCD camera</td><td>Shenzhen Andonstar Tech Co., Ltd</td><td>digital microscope A1</td><td>Frame rate: 30 frames/sec;Focal distance: 5mm - 30mm</td></tr><tr><td>Computer</td><td>Lenovo</td><td>G480</td><td></td></tr><tr><td>Servomotor</td><td>EMAX US Inc.</td><td>ES08MD</td><td>It&#039;s not bad this servo with speed varying from 0.10 sec/60&amp;deg; / 4.8v to 0.08 sec/60&amp;deg;/6.0v.</td></tr><tr><td>Mechanical Pipettes</td><td>Dragon Laboratory Instruments Limited</td><td>YE5K693181</td><td>The pipettes cover volume range of 0.1 &amp;mu;l to 2.5 &amp;mu;l</td></tr></tbody></table>

measurement of dynamic force acted on water strider leg jumping upward by the pvdf film sensor

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, with its peak locomotion speed reaching 150 cm/s. First of all, we observed the microstructure and hierarchy of water strider legs using the scanning electron microscope. On the basis of the observed morphology of the legs, a theoretical model of the detachment from water surface was established, which explained water striders' capability to slide on water surface effortlessly in terms of energy reduction. Secondly, a dynamic force measurement system was devised using the PVDF film sensor with excellent sensitivity, which could detect the whole interaction process. Subsequently, a single leg in contact with water was pulled upward at different speeds, and the adhesion force was measured at the same time. The results of the departing experiment suggested a deep understanding of the fast jumping of water striders.

The relation between lifting speed and adhesion force is shown in Table 1. When the lifting speed increases from 0.01 m/s to 0.3 m/s, the adhesion force between the water surface and leg decreases dramatically from 0.10 to 0.03 . The results of the departing experiment showed that the peak adhesion force would decrease dramatically as the lifting speed increase, which indicated that the water striders may feel comfortable if they move quickly on water surface.
In this paper, a model of the legs departing from water surface based on the microstructure of the legs and the shape of setae is established, which could elucidate the mechanism of easy jumping from the water surface with low energy reduction. The seta of legs was a tapered post with the front part thin and the rear extremely thick, as shown in&#160;Figure 1, which resulted in the stiffness of the front part much lower than the rear. Thus, the front part of the seta tended to bend easily, while the rear did not due to the excellent rigidity. When the leg was pulled away from water, the setae on the leg would be bent as a result of the adhesion force and finally be vertical to the water surface as shown in&#160;Figure 5. Water would fall along the setae naturally with low energy dissipation, which could be neglected. The bending of the seta would reduce the contact line significantly to a circle with diameter of 0.2 m and the energy reduction could be expressed as:
<img alt="Equation 1" src="/files/ftp_upload/58221/58221eq1.jpg" /> 
where, y was surface tension coefficient, 72 mJ/m2 and D was the diameter of the seta&#39;s tip, respectively. Hence, the water striders can jump in the water easily.
The relation between the adhesion force and lifting speed was interpreted thoroughly through previous departing model. According to the conservation of energy, the total energy of lifted water due to the adhesion force was approximately equal to the energy reduction of the leg Ediss. In this model, Ediss was constant at different lifting speed. Thus, the energy of the water pulled up, including the potential energy Ep and the kinetic energy Ek, was changeless. The high lifting speed will lead to the small potential energy Ep and large kinetic energy Ek. Therefore, as the lifting speed increased, the adhesion force, proportional to potential energy Ep, would decrease drastically.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58221/58221fig1.jpg" /> 
Figure 1:&#160;The nonwetting water striderlegs. (a) The setae on the superhydrophobic leg. (b) The nanoscale grooves on the setae. <a href="https://www.jove.com/files/ftp_upload/58221/58221fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58221/58221fig2.jpg" /> 
Figure 2:&#160;The dynamic force measurement system. (a) The Schematic illustration of the dynamic force measurement system is composed of a PVDF film sensor, a CCD camera, a charge amplifier, a data acquisition device and a computer. (b) The real experiment instrument of the dynamic force measurement system. <a href="https://www.jove.com/files/ftp_upload/58221/58221fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58221/58221fig3.jpg" /> 
Figure 3: The front panel of the demo of the continuous analog voltage measurement. The demo of the LabVIEW is used to sample voltage signals of the PVDF film sensor. <a href="https://www.jove.com/files/ftp_upload/58221/58221fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/58221/58221fig4.jpg" /> 
Figure 4: The departing experiment of the legs at a certain speed. (a) The detachment of the legs away from water surface. (b) The real-time adhesion force measured by the PVDF film sensor. <a href="https://www.jove.com/files/ftp_upload/58221/58221fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/58221/58221fig5.jpg" /> 
Figure 5:&#160;Theoretical model of the water strider&#39;s leg departing from the water surface. This model demonstrates that the seta is bending peeling away from the water surface. <a href="https://www.jove.com/files/ftp_upload/58221/58221fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Table 1" src="/files/ftp_upload/58221/58221table1.jpg" /> 
Table 1: The peak adhesion force measured at different lifting speeds. The adhesion force decreases dramatically from 0.10&#160;&#956;N to 0.03&#160;&#956;N with an increase of lifting speed from 0.01 m/s to 0.3 m/s.

Watch this Scientific Journal Video about Measurement of Dynamic Force Acted on Water Strider Leg Jumping Upward by the PVDF Film Sensor at JoVE.com

Measurement of Dynamic Force Acted on Water Strider Leg Jumping Upward by the PVDF Film Sensor

The protocol here is dedicated to investigating the free and quick maneuvering of water strider on water surface. The protocol includes observing the microstructure of legs and measuring the adhesion force when departing from water surface at different speeds.

This study aimed to make an explanation for the phenomenon in nature that water strider usually jumps or glides on the water surface easily but quickly, ...

measurement-dynamic-force-acted-on-water-strider-leg-jumping-upward

Universidad Científica del Sur

Research

JoVE Journal

Engineering

6.0K Views.  Tianjin University. The protocol here is dedicated to investigating the free and quick maneuvering of water strider on water surface. The protocol includes observing the microstructure of legs and measuring the adhesion force when departing from water surface at different speeds.

Video: Measurement of Dynamic Force Acted on Water Strider Leg Jumping Upward by the PVDF Film Sensor

Quantitatively Measuring In situ Flows using a Self-Contained Underwater Velocimetry Apparatus (SCUVA)

The ability to directly measure velocity fields in a fluid environment is necessary to provide empirical data for studies in fields as diverse as oceanography, ecology, biology, and fluid mechanics. Field measurements introduce practical challenges such as environmental conditions, animal availability, and the need for field-compatible measurement techniques. To avoid these challenges, scientists typically use controlled laboratory environments to study animal-fluid interactions. However, it is reasonable to question whether one can extrapolate natural behavior (i.e., that which occurs in the field) from laboratory measurements. Therefore, in situ quantitative flow measurements are needed to accurately describe animal swimming in their natural environment.
We designed a self-contained, portable device that operates independent of any connection to the surface, and can provide quantitative measurements of the flow field surrounding an animal. This apparatus, a self-contained underwater velocimetry apparatus (SCUVA), can be operated by a single scuba diver in depths up to 40 m. Due to the added complexity inherent of field conditions, additional considerations and preparation are required when compared to laboratory measurements. These considerations include, but are not limited to, operator motion, predicting position of swimming targets, available natural suspended particulate, and orientation of SCUVA relative to the flow of interest. The following protocol is intended to address these common field challenges and to maximize measurement success.

Quantitatively Measuring In situ Flows using a Self-Contained Underwater Velocimetry Apparatus SCUVA

This protocol provides instructions on how to use a self-contained underwater velocimetry apparatus (SCUVA), which is designed for quantification of in situ animal-generated flows. In addition, this protocol addresses challenges posed by field conditions, and includes operator motion, predicting position of animals, and orientation of SCUVA.

The ability to directly measure velocity fields in a fluid environment is necessary to provide empirical data for studies in fields as diverse as ...

Bioengineering

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Kinematics and Ground Reaction Force Determination: A Demonstration Quantifying Locomotor Abilities of Young Adult, Middle-aged, and Geriatric Rats

Behavior, in its broadest definition, can be defined as the motor manifestation of physiologic processes. As such, all behaviors manifest through the motor system. In the fields of neuroscience and orthopedics, locomotion is a commonly evaluated behavior for a variety of disease models. For example, locomotor recovery after traumatic injury to the nervous system is one of the most commonly evaluated behaviors 1-3. Though locomotion can be evaluated using a variety of endpoint measurements (e.g. time taken to complete a locomotor task, etc), semiquantitative kinematic measures (e.g. ordinal rating scales (e.g. Basso Beattie and Bresnahan locomotor (BBB) rating scale, etc)) and surrogate measures of behaviour (e.g. muscle force, nerve conduction velocity, etc), only kinetics (force measurements) and kinematics (measurements of body segments in space) provide a detailed description of the strategy by which an animal is able to locomote 1. Though not new, kinematic and kinetic measurements of locomoting rodents is now more readily accessible due to the availability of commercially available equipment designed for this purpose. Importantly, however, experimenters need to be very familiar with theory of biomechanical analyses and understand the benefits and limitations of these forms of analyses prior to embarking on what will become a relatively labor-intensive study. The present paper aims to describe a method for collecting kinematic and ground reaction force data using commercially available equipment. Details of equipment and apparatus set-up, pre-training of animals, inclusion and exclusion criteria of acceptable runs, and methods for collecting the data are described. We illustrate the utility of this behavioral analysis technique by describing the kinematics and kinetics of strain-matched young adult, middle-aged, and geriatric rats.

Locomotion is often examined as a behavioural outcome in various models of disease in fields such as neuroscience and orthopedics. This video paper intends to describe a method for collecting ground reaction forces and kinematics from rats during unrestrained locomotion.

Behavior, in its broadest definition, can be defined as the motor manifestation of physiologic processes.  As such, all behaviors manifest through the ...

Neuroscience

Studying the Neural Basis of Adaptive Locomotor Behavior in Insects

Studying the neural basis of walking behavior, one often faces the problem that it is hard to separate the neuronally produced stepping output from those leg movements that result from passive forces and interactions with other legs through the common contact with the substrate. If we want to understand, which part of a given movement is produced by nervous system motor output, kinematic analysis of stepping movements, therefore, needs to be complemented with electrophysiological recordings of motor activity. The recording of neuronal or muscular activity in a behaving animal is often limited by the electrophysiological equipment which can constrain the animal in its ability to move with as many degrees of freedom as possible. This can either be avoided by using implantable electrodes and then having the animal move on a long tether (i.e. Clarac et al., 1987; Duch &amp; Pfl&uuml;ger, 1995; B&ouml;hm et al., 1997; Gruhn &amp; Rathmayer, 2002) or by transmitting the data using telemetric devices (Kutsch et al, 1993; Fischer et al., 1996; Tsuchida et al. 2004; Hama et al., 2007; Wang et al., 2008). Both of these elegant methods, which are successfully used in larger arthropods, often prove difficult to apply in smaller walking insects which either easily get entangled in the long tether or are hindered by the weight of the telemetric device and its batteries. In addition, in all these cases, it is still impossible to distinguish between the purely neuronal basis of locomotion and the effects exerted by mechanical coupling between the walking legs through the substrate. One solution for this problem is to conduct the experiments in a tethered animal that is free to walk in place and that is locally suspended, for example over a slippery surface, which effectively removes most ground contact mechanics. This has been used to study escape responses (Camhi and Nolen, 1981; Camhi and Levy, 1988), turning (Tryba and Ritzman, 2000a,b; Gruhn et al., 2009a), backward walking (Graham and Epstein, 1985) or changes in velocity (Gruhn et al., 2009b) and it allows the experimenter easily to combine intra- and extracellular physiology with kinematic analyses (Gruhn et al., 2006).
We use a slippery surface setup to investigate the timing of leg muscles in the behaving stick insect with respect to touch-down and lift-off under different behavioral paradigms such as straight forward and curved walking in intact and reduced preparations.

We describe a method to record motor activity, timed to the electrically recorded tarsal contact signal in a tethered insect, walking on a slippery surface. This is used to study the neural basis of adaptive behavior under reduced influence of mechanical interaction between legs through the substrate.

Studying the neural basis of walking behavior, one often faces the problem that it is hard to separate the neuronally produced stepping output from those ...

Image-based Lagrangian Particle Tracking in Bed-load Experiments

Image analysis has been increasingly used for the measurement of river flows due to its capabilities to furnish detailed quantitative depictions at a relatively low cost. This manuscript describes an application of particle tracking velocimetry (PTV) to a bed-load experiment with lightweight sediment. The key characteristics of the investigated sediment transport conditions were the presence of a covered flow and of a fixed rough bed above which particles were released in limited number at the flume inlet. Under the applied flow conditions, the motion of the individual bed-load particles was intermittent, with alternating movement and stillness terms. The flow pattern was preliminarily characterized by acoustic measurements of vertical profiles of the stream-wise velocity. During process visualization, a large field of view was obtained using two action-cameras placed at different locations along the flume. The experimental protocol is described in terms of channel calibration, experiment realization, image pre-processing, automatic particle tracking, and post-processing of particle track data from the two cameras. The presented proof-of-concept results include probability distributions of the particle hop length and duration. The achievements of this work are compared to those of existing literature to demonstrate the validity of the protocol.

The manuscript presents a protocol for the conduction of bed-load sediment transport experiments where the moving particles are tracked by image analysis. The experimental facility, the procedures for run realization and data processing, and finally some proof-of-concept results are presented here.

Image analysis has been increasingly used for the measurement of river flows due to its capabilities to furnish detailed quantitative depictions at a ...

Environment

Simulation of Human-induced Vibrations Based on the Characterized In-field Pedestrian Behavior

For slender and lightweight structures, vibration serviceability is a matter of growing concern, often constituting the critical design requirement. With designs governed by the dynamic performance under human-induced loads, a strong demand exists for the verification and refinement of currently available load models. The present contribution uses a 3D inertial motion tracking technique for the characterization of the in-field pedestrian behavior. The technique is first tested in laboratory experiments with simultaneous registration of the corresponding ground reaction forces. The experiments include walking persons as well as rhythmical human activities such as jumping and bobbing. It is shown that the registered motion allows for the identification of the time variant pacing rate of the activity. Together with the weight of the person and the application of generalized force models available in literature, the identified time-variant pacing rate allows to characterize the human-induced loads. In addition, time synchronization among the wireless motion trackers allows identifying the synchronization rate among the participants. Subsequently, the technique is used on a real footbridge where both the motion of the persons and the induced structural vibrations are registered. It is shown how the characterized in-field pedestrian behavior can be applied to simulate the induced structural response. It is demonstrated that the in situ identified pacing rate and synchronization rate constitute an essential input for the simulation and verification of the human-induced loads. The main potential applications of the proposed methodology are the estimation of human-structure interaction phenomena and the development of suitable models for the correlation among pedestrians in real traffic conditions.

A protocol is presented for the characterization of the in-field pedestrian behavior and the simulation of the resulting structural response. Field-tests demonstrate that the in situ identified pacing rate and synchronization rate among the participants constitute an essential input for the simulation and verification of the human-induced loads.

For slender and lightweight structures, vibration serviceability is a matter of growing concern, often constituting the critical design requirement. With ...

Sit-to-stand-and-walk from 120% Knee Height: A Novel Approach to Assess Dynamic Postural Control Independent of Lead-limb

Individuals with sensorimotor pathology e.g., stroke have difficulty executing the common task of rising from sitting and initiating gait (sit-to-walk: STW). Thus, in clinical rehabilitation separation of sit-to-stand and gait initiation - termed sit-to-stand-and-walk (STSW) - is usual. However, a standardized STSW protocol with a clearly defined analytical approach suitable for pathological assessment has yet to be defined.
Hence, a goal-orientated protocol is defined that is suitable for healthy and compromised individuals by requiring the rising phase to be initiated from 120% knee height with a wide base of support independent of lead limb. Optical capture of three-dimensional (3D) segmental movement trajectories, and force platforms to yield two-dimensional (2D) center-of-pressure (COP) trajectories permit tracking of the horizontal distance between COP and whole-body-center-of-mass (BCOM), the decrease of which increases positional stability but is proposed to represent poor dynamic postural control.
BCOM-COP distance is expressed with and without normalization to subjects&#39; leg length. Whilst COP-BCOM distances vary through STSW, normalized data at the key movement events of seat-off and initial toe-off (TO1) during steps 1 and 2 have low intra and inter subject variability in 5 repeated trials performed by 10 young healthy individuals. Thus, comparing COP-BCOM distance at key events during performance of an STSW paradigm between patients with upper motor neuron injury, or other compromised patient groups, and normative data in young healthy individuals is a novel methodology for evaluation of dynamic postural stability.

Here, we present a novel protocol to measure positional stability at key events during the sit-to-stand-to-walk using the center-of-pressure to the whole-body-center-of-mass distance. This was derived from the force platform and three-dimensional motion-capture technology. The paradigm is reliable and can be utilized for the assessment of neurologically compromised individuals.

Individuals with sensorimotor pathology e.g., stroke have difficulty executing the common task of rising from sitting and initiating gait (sit-to-walk: ...

Behavior

A Novel Application of Musculoskeletal Ultrasound Imaging

Ultrasound is an attractive modality for imaging muscle and tendon motion during dynamic tasks and can provide a complementary methodological approach for biomechanical studies in a clinical or laboratory setting. Towards this goal, methods for quantification of muscle kinematics from ultrasound imagery are being developed based on image processing. The temporal resolution of these methods is typically not sufficient for highly dynamic tasks, such as drop-landing. We propose a new approach that utilizes a Doppler method for quantifying muscle kinematics. We have developed a novel vector tissue Doppler imaging (vTDI) technique that can be used to measure musculoskeletal contraction velocity, strain and strain rate with sub-millisecond temporal resolution during dynamic activities using ultrasound. The goal of this preliminary study was to investigate the repeatability and potential applicability of the vTDI technique in measuring musculoskeletal velocities during a drop-landing task, in healthy subjects. The vTDI measurements can be performed concurrently with other biomechanical techniques, such as 3D motion capture for joint kinematics and kinetics, electromyography for timing of muscle activation and force plates for ground reaction force. Integration of these complementary techniques could lead to a better understanding of dynamic muscle function and dysfunction underlying the pathogenesis and pathophysiology of musculoskeletal disorders.

We describe a new ultrasound-based vector tissue Doppler imaging technique to measure muscle contraction velocity, strain and strain rate with sub-millisecond temporal resolution during dynamic activities. This approach provides complementary measurements of dynamic muscle function and could lead to a better understanding of mechanisms underlying musculoskeletal disorders.

Ultrasound is an attractive modality for imaging muscle and tendon  motion during dynamic tasks and can provide a complementary methodological  approach ...

Medicine

A Rapidly Incremented Tethered-Swimming Maximal Protocol for Cardiorespiratory Assessment of Swimmers

Incremental exercise testing is the standard means of assessing cardiorespiratory capacity of endurance athletes. While the maximal rate of oxygen consumption is typically used as the criterion measurement in this regard, two metabolic breakpoints that reflect changes in the dynamics of lactate production/consumption as the work rate is increased are perhaps more relevant for endurance athletes from a functional standpoint. Exercise economy, which represents the rate of oxygen consumption relative to performance of submaximal work, is also an important parameter to measure for endurance-athlete assessment. Ramp incremental tests comprising a gradual but rapid increase in work rate until the limit of exercise tolerance is reached are useful for determining these parameters. This type of test is typically performed on a cycle ergometer or treadmill because there is a need for precision with respect to work-rate incrementation. However, athletes should be tested while performing the mode of exercise required for their sport. Consequently, swimmers are typically assessed during free-swimming incremental tests where such precision is difficult to achieve. We have recently suggested that stationary swimming against a load that is progressively increased (incremental tethered swimming) can serve as a &#34;swim ergometer&#34; by allowing sufficient precision to accommodate a gradual but rapid loading pattern that reveals the aforementioned metabolic breakpoints and exercise economy. However, the degree to which the peak rate of oxygen consumption achieved during such a protocol approximates the maximal rate that is measured during free swimming remains to be determined. In the present article, we explain how this rapidly incremented tethered-swimming protocol can be employed to assess the cardiorespiratory capacity of a swimmer. Specifically, we explain how assessment of a short-distance competitive swimmer using this protocol revealed that his rate of oxygen uptake was 30.3 and 34.8 mL&#8729;min-1&#8729;kg-1BM at his gas-exchange threshold and respiratory compensation point, respectively.

As opposed to measurement during free swimming, which presents inherent challenges and limitations, determination of important parameters of cardiorespiratory function for swimmers can be made using a more feasible and easier to administer tethered-swimming rapidly incremented protocol with gas exchange and ventilatory data collection.

Incremental exercise testing is the standard means of assessing cardiorespiratory capacity of endurance athletes. While the maximal rate of oxygen ...

Biochemistry

Comparison of Kinetic Characteristics of Footwork during Stroke in Table Tennis: Cross-Step and Chasse Step

The cross-step and chasse step are the basic steps of table tennis. This study presents a protocol to investigate the ground reaction force characteristics between cross-step and chasse step during stroke in table tennis. Sixteen healthy male national level 1 table tennis players (Age: 20.75 &plusmn; 2.06 years) volunteered to participate in the experiment after understanding the purpose and details of the experiment. All participants were asked to hit the ball into the target zone by cross-step and chasse step, respectively. The ground reaction force in the anterior-posterior, medial-lateral, and vertical directions of the participant was measured by a force platform. The key finding of this study was that: the posterior ground reaction force of cross-step footwork (0.89 &plusmn; 0.21) was significantly large (P = 0.014) than the chasse step footwork (0.82 &plusmn; 0.18). However, the lateral ground reaction force of cross-step footwork (-0.38 &plusmn; 0.21) was significantly lower (P &lt; 0.001) than chasse step footwork (-0.46 &plusmn; 0.29) as well as the vertical ground reaction force of cross-step footwork (1.73 &plusmn; 0.19) was significantly lower (P &lt; 0.001) than chasse step footwork (1.9 &plusmn; 0.33). Based on the mechanism of the kinetic chain, the better lower limb dynamic performance of sliding stroke may be conducive to energy transmission and thus bring gain to the swing speed. Beginners should start from the chasse step to hit the ball technically, and then practice the skill of cross-step.

This study presents a protocol to investigate the ground reaction force characteristics between cross-step and chasse step during stroke in table tennis.