Coronal holes are solar regions with low soft X-ray or low extreme ultraviolet intensities. The magnetic fields from coronal holes extend far away from the Sun, and thus they are identified as regions with open magnetic field lines. Coronal holes are concentrated in the polar regions during the sunspot minimum phase, and spread to lower latitude during the rising phase of solar activity. In this work, we identify coronal holes with outward and inward open magnetic fluxes being in the opposite poles during solar quiet period. We find that during the sunspot rising phase, the outward and inward open fluxes perform pole-to-pole trans-equatorial migrations in opposite directions. The migration of the open fluxes consists of three parts: open flux areas migrating across the equator, new open flux areas generated in the low latitude and migrating poleward, and new open flux areas locally generated in the polar region. All three components contribute to the reversal of magnetic polarity. The percentage of contribution from each component is different for different solar cycle. Our results also show that the sunspot number is positively correlated with the lower-latitude open magnetic flux area, but negatively correlated with the total open flux area. Theoretically, the coronal holes are defined as the regions in which the magnetic field lines extend far away from the Sun , in other words, the regions with open magnetic field lines (OMF).Coronal holes are often observed as the regions with low optical intensity (LOI) in soft X-ray or extreme ultraviolet (EUV) images. They are found to locate inside regions that are of weak and predominantly unipolar magnetic field (Altschuler 3). Polar coronal holes (PCHs) are mostly seen above latitude 65° during the sunspot minimum phase. Despite their name, PCHs are not restricted to polar regions, but can extend to lower latitudes and even the other hemisphere during the decreasing phase of solar activity. The PCHs are observed to have quasi-rigid rotation7. In contrast, non-polar coronal holes  mostly appear within latitude ±30° during solar maximum phase, and show significant differential rotation8.Based on their distribution on the solar surface, coronal holes are often divided into polar and non-polar coronal holes (Sanchez-Ibarra & Barraza-Paredes9). Hess Webber et al.10 measured the area of the polar coronal holes over 1996–2010, and found that the areas in the two hemispheres did not reach maximum at the same time. Karna et al.11 investigated the relationship between the area of polar coronal holes and solar magnetic fields over solar cycle 24, and found inverse correlation between the area of PCH and sunspot number. Their results also showed connection from the magnetic fields of active regions to those of the polar regions. Karachik et al.12 reported the formation of two non-polar coronal holes from the dissipation of the magnetic fields of four active regions. Ikhsanov & Ivanov8 analyzed different properties of equatorial and polar coronal holes from 1970 to 1995, including numbers, locations, areas and polarities, and reported that both types of coronal holes can be further divided into multiple sub-classes based on the temporal variation of their properties. A recent study by Bilenko & Tavastsherna13 examined the evolution of coronal holes and solar global magnetic fields over three solar cycles from 1976 to 2012. By analyzing the temporal variation of coronal hole location, they reported that non-polar coronal holes exhibit a poleward motion and a sinusoidal motion. These studies have all identified coronal holes as the dark regions in EUV or X-ray images . Obridko & Shelting2 pointed out that LOI coronal holes and open field regions are not completely identical physical objects although the two are often associated with each other in statistical sense. They studied the evolution of open field regions from 1970 to 1996. Their comparison between the sunspot locations and open flux ratio  showed a close relationship between mid-latitude coronal holes and active regions.Since coronal holes are predominantly unipolar regions on the Sun, they are good tracers of the change of the solar magnetic-field polarities, and many works have been conducted to examine the variations of different properties of coronal holes. The evolution of the number and locations of coronal holes was found to be correlated with solar magnetic fields  193 Å and radial field magnetograms from Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) were used to create synoptic maps. The time period of the AIA and HMI data is from June 2010 to December 2014, corresponding to Carrington rotation number 2099 to 2158. The LOI CHs were identified from the synoptic maps based on the procedures described by Krista & Gallagher17. To identify OMF coronal holes, we used the radial-field synoptic maps from the Wilcox Solar Observatory (WSO) from May 1970 to December 2014, corresponding to Carrington rotation number 1642 to 2158. Each synoptic map consists of 60 by 30 pixels in longitude (ϕ) and sine-latitude (sin λ) coordinate, resulting in a spatial resolution of Δϕ = 2π/60 in longitudinal direction, and Δ(sin λ) = 2/30 in sine-latitude. The area of a pixel in the map thus equals to rΔϕΔ(sin λ), and is same at all latitudes. To determine whether the field line of a pixel would extend to infinity, we need to first know the three-dimensional magnetic field above the solar surface. Earlier studies on the magnetic field above the photosphere indicate that the inner corona can be considered current free  and that the current is only significant during rapid development of active region (Harvey19). Since the magnetic field structure of interest in our study is the large-scale global fields that do not change over one solar rotation, the structure can be assumed to be current free, and a Potential Field Source Surface (PFSS) model18 was applied to the synoptic map to construct the 3-D magnetic field. The constructed magnetic field is current free between the solar surface and an upper boundary (source surface), where all field lines become radial. In this work, the source surface is placed at 20, Obridko & Shelting2). After the 3D magnetic fields were constructed, the field lines were traced from the source surface to the solar surface, and the footpoints of the open magnetic field lines on the solar surface were identified as OMF CHs.Coronal holes have generally been identified as regions of either low optical intensity  or open magnetic field lines . Since coronal holes (CHs) are major source regions of high-speed solar wind streams (HSSs)et al.21 to identify HSS events, and excluded the events that may be caused by solar flare-related coronal mass ejections by examining the X-ray data from the GOES-12 satellite. A total number of 118 high-speed solar wind stream events were selected.The solar wind speed data were obtained from the Solar Wind Electron Proton Alpha Monitor (SWEPAM) onboard the Advanced Composition Explorer (ACE) from June 2010 to December 2014. We followed the criteria described by Xystouris 22). This assumption is based on the fact that HSS is moving at a speed much higher than the Alfvén speed and sound speed in the interplanetary space and that in-situ measurements of solar wind at 1 AU often show that the radial velocity is much higher than the transverse components. We caution that this simplistic assumption inevitably introduced some errors to the determined source longitudes.To determine the source longitudes of the HSS events, we need to trace the trajectories of the HSSs back to the solar surface. The exact trajectory and velocity profile of HSS through the interplanetary space are currently not observable. As a first order approximation, we assumed that the solar wind is propagating radially at constant speed , Ψ+, and Ψ−, where ΨOMF = Ψ+ + Ψ−) by summing the number of pixels of unsigned, outward, and inward open magnetic field over longitude in each Carrington rotation  t. The result is plotted in the first three panels of Fig. λj, the maximum values of To study the temporal variation of the polarities and area of the CHs at different latitudes, we constructed the unsigned, outward, and inward open magnetic flux map .Figure rΔϕΔ(sin λ)).To better understand the cause of the PPTE migration pattern, we use the outward open flux area as an example, and plotted the total area in different latitudinal ranges in Fig. Comparing panels (b) and (c), we can see that the maximum areas in the polar region are larger than those in  latitude range for all three solar cycles, indicating that some open flux areas in the polar region are locally generated. The difference between the maximum areas in the polar region and in the  region indicate that the amount of open flux area locally generated at the polar region in each solar cycle is at least 36%, 70% and 57%, respectively, of the total open flux area in the polar region. Therefore, except for the first cycle, majority of the polar region flux is locally generated. For the first cycle, the migration of the open flux from lower latitude to polar region can be clearly seen in Fig. Applying similar analysis to the comparison between 30°–60° (panel c) and 0°–30° (panel d), we can see that in first cycle the maximum area in 30°–60° is larger that that in 0°–30°, indicating some new open flux have been generated in the latitude between 30° and 60°. The amount of the locally generated open flux is at least 30% of the total open flux in 30°–60°. In contrast, the maximum area in 30°–60° is lower than that in 0°–30° for the next two cycles, indicating that some open flux in 0°–30° have dissipated locally. The difference indicates that the amount of dissipation is at least 15% and 11% for the second and third cycle, respectively. The maximum amount of open flux that can migrate from 0°–30° to 30°–60° in these two cycles can be estimated as the maximum area in 30°–60°. In short, the maximum amount of migrated open flux area is 73, 56, and 72 pixels.In panel (d), comparison between the dashed (before crossing the equator) and solid lines (after crossing the equator) show that the peak areas in all three cycles are larger after crossing, indicating that new open fluxes have been generated after crossing the equator. The open fluxes locally generated between the equator and 30° after crossing the equator are at least 18%, 36% and 15% for the three cycles.In summary, our analysis indicates that the PPTE pattern consists of four components: (1) open fluxes migrating across the equator; (2) locally generated open fluxes dissipating locally without migrating to higher latitudes; (3) locally generated open fluxes migrating to higher latitudes; and (4) open flux locally generated in the polar region. The contribution from each component differ from cycle to cycle. The situation for the inward open flux is expected to be similar except that the direction of migration is opposite.In contrast to the PPTE migration of the open magnetic flux, sunspots are rarely present at the equator or at latitude higher than ≈35°. This is because the sunspots are formed by the stretching and enhancing of magnetic field lines by solar differential rotation, the latter peaks at the mid latitude but is minimum at the equator and at high latitudes.In a short period approaching the maximum phase of sunspot number, open flux at the two poles almost completely disappear , and can also be seen in the results of later observational studies .This phenomenon was first reported by Waldmeieret al.10, Karna et al.11). As described in the introduction, some polar coronal holes can extend to low latitude and even the other hemisphere during the decreasing phase of solar activity. Therefore, the low-latitude open flux area in Fig. Figure 26. The basic idea of the mechanism is that the decay of the tilt of sunspot groups generates the poloidal field of the Sun. The Babcock-Leighton model has since been developed into a range of more elaborated dynamo models  is based on the assumption that the polar open fluxes come from the edge of active regions in the lower latitudes. By including the effects of differential rotation, magnetic diffusion, and meridional flow for the formation of polar open fluxes, the model demonstrated that part of the leading-polarity fluxes of the active regions diffuse to the other hemisphere, while the remaining fluxes are carried pole-ward by the meridional flow. A different model based on observational results was proposed by Ikhsanov & Tavastsherna31 and Bilenko & Tavastsherna13. They suggested that the polar open fluxes are generated by the poleward transportation of the open fluxes at around 35° latitude.Many current solar dynamo models are based on Babcock-Leighton mechanismThese models may partly explain the poleward migration of the open flux, but cannot explain the pole-to-equator portion of the migration as shown in Fig.


KennedyThe Journal of the Belgian Society of Radiology (JBSR) has been subject to profound modifications within a short time frame during the past year. First, the journal is now operated fully online for submission, review and publication. This means the last printed regular issue of the journal was the JBR-BTR 98/1 issue in 2015  to become JBSR. One may consider the aforementioned changes are way too much and that the upcoming year should be less hectic, but thinking so is probably wrong in the post-internet era, characterized by rapid transnational communications. Amongst many other changes to come, the metrics and article automatic indexation of the JBR-BTR will be allocated to the JBSR after all due verifications. Meanwhile, we will define and implement the article types we want to encourage and how we want these to be disseminated and debated in and outside our community. Of course we will commit ourselves to education and scientific promotion that are among the goals of the BSR. By reciprocity, we expect from the BSR a greater contribution to the journal’s quality and scientific content through its scientific subsections, starting by nominating new editorial board members, ready to take all ongoing changes. Actually, if we do not take the opportunity to change things, we just surrender to changes imposed by others. It is well established that for sustainable progress, change is just a minimal requirement that prepares others to change. In this sense, the change of our journals’ name means more than a simple cosmetic facelift and epitomizes the difficult choices we are facing when implementing changes.Under its previous denomination, the JBSR already published articles in English, the indisputable standard of communication for biomedical research. By removing references to our main national languages in its official journal’s title, the board of the BSR made a strong statement towards opening its interaction and broadening its base. However, in this political realm of anxiety to lose traditions caused by too much openness, the board of the BSR has also decided that the name of our national society will remain in the journal’s title. This does not only affirm our need to be recognized as such by all, but also subliminally tells who we are: Whatever the changes we are facing, we remain strongly attached to Belgian traditions, among which, a political sense of consensus where people often wisely decide to arrange the goat and the cabbage. Whatever its name, the need to define our journal as fully open to the world is just an example of the many decisions and changes we have to implement this year and in the years to come. Isn’t the change continuous?The author declares that they have no competing interests.


To investigate the long-term anatomical and functional outcomes of Descemet stripping automated endothelial keratoplasty (DSAEK).  Prospective follow-up of 114 eyes (95 subjects) after DSAEK for endothelial dysfunction. Measurements included best spectacle-corrected visual acuity (BSCVA), straylight, endothelial cell density (ECD), and graft thickness. 2 (33%) immediately after surgery and, thereafter, steadily decreased at a rate of 11 cells/mm2 per month. No significant correlation was observed between graft thickness at 3 years and BSCVA.  The mean follow-up time was 5.1 ± 1.5 years. Four grafts ultimately failed (after 5 to 7 years). From baseline up to 1 year after DSAEK, mean BSCVA improved by 0.30 logMAR. This beneficial effect remained until the last follow-up (LFU). After DSAEK, straylight was reduced. ECD sharply dropped by 900 cells/mm We observed a low graft failure rate and a normalization of graft thickness. Postoperative straylight remained elevated relative to the normal population. The sharp initial and the subsequent more gradual ECD decline are consistent with other studies. A significant and prolonged functional gain can be achieved by posterior lamellar grafting for endothelial dysfunction. Conditions such as Fuchs' endothelial dystrophy (FED) or pseudophakic bullous keratopathy (PBK) may lead to irreversible corneal edema and, consequently, loss of vision. Currently, FED is the most frequently registered indication for corneal transplants . Full-thIn 1956, endothelial keratoplasty was introduced for the first time . Due to A possible side effect of the favourable results of this type of corneal transplantation as compared to penetrating grafts is that ophthalmologists and their patients may be prone to consider such an intervention as desirable at a substantially earlier stage of the condition's progression. This demands a continuous, close, and long-term surveillance of postoperative proceedings in order to allow an adequate evaluation of the benefit/risk ratio of the surgical intervention. In this study, we assessed the anatomical and ophthalmic characteristics of the donor grafts and the recipient eyes that underwent DSAEK up to 7 years after surgery.Eligible for inclusion were patients with FED, PBK, or secondary endothelial decompensation. The subjects in this study underwent DSAEK in the Rotterdam Eye Hospital (REH). All clinical data were collected prospectively. For 56 patients, these data were retrieved from the ongoing national survey of corneal transplants of the Netherlands Organ Transplant Registry (NOTR). The ethical committee concluded that the Dutch Medical Research Involving Humans Act (WMO) did not apply to this part of the study, and therefore, official approval was not required. For 39 patients, data were collected as part of a prospective study for which approval was obtained from the Medical Ethical Committee of the Erasmus Medical Center, Rotterdam. All these 39 subjects gave written informed consent in advance. The study adhered to the tenets of the Declaration of Helsinki.μm head and a Barron Punch trephination system  were used for the preparation of the donor lamellae. The graft was inserted either with the aid of a 10-0 prolene suture [Surgery was performed by 3 experienced surgeons  between June 2007 and October 2011. Donor corneas (diameter 8.5 mm) were supplied by the Euro Cornea Bank  and prepared by the surgeon. A Moria microkeratome  equipped with a 350 e suture  or with e suture . For a de suture , 15.After surgery, the following regimens of dexamethasone (0.1%) topical eye drops were prescribed: week 1–4, 6 gtt daily; week 5–12, 4 gtt daily; week 13–52, 3 gtt daily; and thereafter, 1 gtt per day.The best spectacle-corrected visual acuity (BSCVA) was measured using an ETDRS chart. If such data were not available, supplemental visual acuity outcomes (assessed on an angular chart projector and converted to logMAR scores) were used.Intraocular straylight, expressed as log(s), was measured with the C-Quant instrument  by means of the compensation comparison method . If the Baseline endothelial cell density (ECD) from the donor cornea was provided by the Euro Cornea Bank: the trypan blue stained tissue was inspected by light microscopy, and cells were manually counted . PostopeCentral corneal thickness was estimated from rotating Scheimpflug images . When Scheimpflug images were missing, data were supplemented with ultrasound pachymetry .Postoperative lamellar thickness was obtained from confocal microscopic scans. The interface between a donor lamella and a recipient cornea is designated by a layer of highly reflective particles; for establishing graft thickness, the confocal image was chosen that showed this layer most distinctly. As it is assumed that the donor tissue will have reached a stable state after a sufficient elapse of time, lamellar thickness was examined at 36 months or, if not available from that visit, at the previous or next moment of follow-up .Data were analysed with SPSS  and are presented—on a per eye basis—at baseline and consecutive postoperative visits. Postoperative visits were scheduled at 1, 3, and 6 months and at 1, 2, 3, 5, and 7 years. Nominal data are given as the numbers and percentages; continuous data are reported as the means together with their standard deviation (SD) or 95% confidence interval (CI).ρ was calculated to inspect correlation between parameters. For the comparison of patient subgroups, an independent t-test or one-way ANOVA was used.For each separate eye and for each outcome parameter, the final follow-up value was selected to obtain a maximally complete set of “last follow-up” (LFU) data. Spearman's n = 108) of the eyes had Fuchs' endothelial dystrophy. Before surgery, the mean visual acuity was 0.48 ± 0.27 logMAR, the mean corneal thickness was 683 ± 95 μm, and the mean preoperative ECD of the donor cornea was 2736 ± 153 mm−2  that underwent DSAEK were included in this study. Demographic (ocular-based) characteristics at baseline are listed in 153 mm−2 . Strayli153 mm−2 .The mean follow-up time for visual acuity was 5.1 ± 1.5 years. On average, follow-up times for ECD, corneal thickness, and straylight were of similar duration. F = 3.06; p = 0.01), no clear separation of homogenous subsets was observed.Visual acuity improved substantially after DSAEK and appeared to stabilize after about 6 months postoperatively. In particular, before transplantation and during the first months of follow-up, many straylight measurements are missing or unreliable (supposedly due to the combination of poor vision and the concentration which this subjective method requires). Although one-way ANOVA analysis of the straylight outcomes  suggests a significant effect (2 (33%) immediately after surgery, a more gradual decline is observed of 11 cells/mm2 per month which continues rather linearly (R2 = 0.18) during the entire follow-up. Three years after transplantation (range 1.9 to 5.3 years), the thickness of the donor lamella was 109 ± 35 μm . After slow endothelial decompensation, four grafts ultimately failed at 59, 60, 73, and 84 months. Primary graft failures did not occur in this study group. Concomitant ocular disorders potentially affecting visual acuity were observed in 24 eyes: age-related macular degeneration , glaucoma (n = 3), and macular hole (n = 1). In two of these eyes (one with AMD and the other with glaucoma), graft failure occurred.After a sharp drop of the mean ECD of about 900 cells/mmn = 110); ECD, 1296 ± 569 mm−2 (n = 108); total corneal thickness (including the graft), 601 ± 53 μm (n = 110); and straylight, log(s) = 1.43 ± 0.19 . A comparison of the visual outcome of patients with and without concomitant ocular pathology showed that vision is affected indeed: 0.32 ± 0.24 logMAR (n = 22) versus 0.15 ± 0.16 logMAR . The correlation (Spearman's ρ) between the following parameters was determined: preoperative ECD, lamellar thickness at 3 years, LFU BSCVA, LFU ECD, LFU corneal thickness, and LFU straylight. With the exceptions of ρ = –0.22 (p = 0.02) between BSCVA and corneal thickness and ρ = 0.44 (p = 0.000003) between donor pachymetry and LFU pachymetry, no significant correlation was observed.After exclusion of graft failures, the LFU analysis results were as follows: visual acuity, 0.18 ± 0.19 logMAR  and anatomical outcomes . Our study was designed to acquire both long-term functional and anatomical outcomes of DSAEK in a large cohort.As can be expected from the primary indication for DSAEK being the restoration of visual acuity, a significant beneficial effect on BSCVA was observed. The BSCVA values we measured at 1 year were at least comparable to those reported by other investigators , 19, 20.Relative to preoperative values, postoperative straylight is reduced. Similar results, both pre- and postoperatively, have been reported before for endothelial keratoplasty with FED . CompareFor most eyes of this study, reliable postoperative images of the endothelium could be obtained by means of confocal microscopy. Such images permit manual ECD assessment  which isμm) to ultrathin DSAEK (mean graft thickness 101 μm) reports a clear effect of thickness on BSCVA [Thickness measurements from the confocal scan images show that after DSAEK the recipient cornea returns to normal dimensions, indicating adequate corneal endothelial pump function. We did not find any significant correlation between donor lamella thickness at 3 years and LFU BSCVA. This is in line with the conclusion of a meta-analysis  that graon BSCVA .Close inspection of The early postoperative decline in graft endothelial cell density we observed is reported by other studies as well. The (subjective) straylight measurements appear to indicate no more than just a weak improvement after DSAEK. With respect to visual function, however, we conclude that our results present additional evidence for the substantial gain that can be achieved by posterior lamellar grafting for endothelial dysfunction, in particular, in the eyes with Fuchs' endothelial dystrophy. The average visual gain was 0.3 logMAR after 1 year, a clinically beneficial effect which appears to last for at least up to 7 years. Graft failure was low (3.5%).


We investigate the relation between grip force and grip stiffness for the human hand with and without voluntary cocontraction. Apart from gaining biomechanical insight, this issue is particularly relevant for variable-stiffness robotic systems, which can independently control the two parameters, but for which no clear methods exist to design or efficiently exploit them. Subjects were asked in one task to produce different levels of force, and stiffness was measured. As expected, this task reveals a linear coupling between force and stiffness. In a second task, subjects were then asked to additionally decouple stiffness from force at these force levels by using cocontraction. We measured the electromyogram from relevant groups of muscles and analyzed the possibility to predict stiffness and force. Optical tracking was used for avoiding wrist movements. We found that subjects were able to decouple grip stiffness from force when using cocontraction on average by about 20% of the maximum measured stiffness over all force levels, while this ability increased with the applied force. This result contradicts the force–stiffness behavior of most variable-stiffness actuators. Moreover, we found the thumb to be on average twice as stiff as the index finger and discovered that intrinsic hand muscles predominate our prediction of stiffness, but not of force. EMG activity and grip force allowed to explain 72 ± 12% of the measured variance in stiffness by simple linear regression, while only 33 ± 18% variance in force. Conclusively the high signal-to-noise ratio and the high correlation to stiffness of these muscles allow for a robust and reliable regression of stiffness, which can be used to continuously teleoperate compliance of modern robotic hands. A soft system (low stiffness) will yield to external perturbation forces, while a stiff system will withstand them. For example, when brushing one’s teeth, the grip on the toothbrush needs to be soft enough for following the shape of the jaw without hurting the gum, but firm enough (high stiffness) for keeping the handle within a stable pose without losing it and for guiding the head of the toothbrush in the desired direction.Stiffness is defined as a ratio of a force change to a corresponding displacement. However, additional criteria need to be fulfilled for a force–displacement relation to be considered stiffness  and velocity (damping forces). Conclusively, the stiffness we measure is not a quasi-stiffness, reflexive stiffness, nor apparent stiffness [see also Latash and Zatsiorsky ].static, intrinsic, or a-reflexive stiffness and is close to the stiffness of mechanical springs. It is a combination of passive stiffness stemming from the muscles, tendons, surrounding tissue, and ligaments and short-range stiffness originating from the crossbridges.In biomechanics and neuroscience, our definition of stiffness is commonly referred to by using the terms It has been shown that (a) the stiffness of a muscle increases linearly with increasing muscle force Zajac,  and the Each of the stiffness modulation methods has different advantages: while changing kinematics is energy efficient, external force modulation and cocontraction allow for posture maintenance. Among these methods, we choose to investigate cocontraction as stiffness modulation mechanism, because it raises open biomechanical questions and its results can be directly applied to variable-stiffness actuators in robots. By using a perturbation device that can measure human grip stiffness related to grip force  allowing to compensate high-frequency impacts and increasing system robustness, VSAs offer valuable properties such as (b) enriching dynamic capabilities by allowing to frequently store energy in reversal points or (c) embodying the desired behavior of a task into the mechanical structure of the robot —similar to the notion pretension used for VSAs in robotics. Since we focus on robotics, we will ask subjects for an isometric cocontraction only and will give them a visual feedback about the applied force and stiffness. Furthermore, by referring to the notation cocontraction, we mean the simultaneous contraction of flexor and extensor muscles of thumb and index finger, which results in stable pinch grip force but increased pinch grip stiffness. A simultaneous contraction of all flexor muscles of thumb and index finger opposing each other in a pinch is not considered as cocontraction in this article.In literature, it remains unclear what the notion cocontraction exactly means. Sometimes it refers to a decoupling, we naturally imply an increase of stiffness from its usual coupling to force. We will refrain from analyzing the possibility to decrease stiffness from its normal coupling to force—since it is expected to be impossible.Moreover, by referring to the notation Different studies simulated, measured, and analyzed the role of cocontraction for the human locomotor system. Hogan  analyzedCocontraction increases the stiffness of arm joints, at least in the absence of external forces  is an unnatural task, but allows to command different combinations of contraction and cocontraction including those leading to non-zero net force. However, so far it has been used only to investigate different levels of cocontraction at zero net force.Two ways of forcing subjects to cocontract are acknowledged, either by (a) the application of unstable force fields  and present visual feedback of the applied force and stiffness of each 2We measured stiffness in subject experiments with and without voluntary cocontraction using a device that applies a fast position perturbation to a thumb–index finger grip. We used optical tracking to observe and prevent changes in kinematics and electromyography to analyze and investigate the regression of force and stiffness from muscular activity.2.1The grip perturbator we used in this experiment is presented in Figure 2.1.1Delsys Trigno Wireless System have an internal amplification of 1 kV/V and provide an analog signal at 4 kHz with a constant delay of 48 ms. These electrodes complies with the requirements put forth by the Medical Device Directive 93/42/EEC, and we comply with its intended use. The EMG electrodes were attached in accordance with the recommendations of the SENIAM project .Furthermore, we tested measuring the adductor pollicis muscle as well. Due to strong sweating and large movement of the underlying skin for the pinch grip, the electrodes took off very rapidly, which makes it impossible for us to measure this muscle. The electrodes were placed close to the six corresponding muscles . The cameras have an optimal resolution of 659  × 494  pixels at 242 frames per second, and we used them at a frequency of 400 Hz. After positioning the EMG sensors, marker sets for tracking the position and orientation of wrist and forearm and single markers to track the positions of the distal phalanx of index finger and thumb were positioned , seven right and three left-handed , age 22–27 years, and all initially fully naive to the experiment, performed the two experimental protocols, with and without isometric cocontraction, as described below. For all subjects and experiments, the right hand was used, be they right or left handed, which is restricted by the design of the perturbator with its fans and optical markers. To further assist the subjects in holding their wrist and arm orientation stable during the measurements, a vacuum cushion was used, which was adjusted to each subject. Subjects were seated in all experimental conditions.The whole procedure lasted between 90 and 120 min per participant. No subject had a history of neurological disorder or neuromuscular injury affecting the CNS or the muscles. All subjects participated voluntarily and gave written consent to the procedures, which were conducted in partial accordance with the principles of the Helsinki agreement . Approval was received from the works council of the German Aerospace Center, as well as its institutional board for data privacy ASDA; the collection and processing of experimental data were approved by both committees.EMG base noise level for 5 s (see maximum voluntary contraction (MVC), i.e., to grip as strongly as they were able to, three times for 5 s each, while the maximum grip force and corresponding EMG levels were measured. The MVC was used to set the prescribed force levels in the following two main tasks.At first, subjects were asked to lay their arm relaxed on the table to measure the  5 s see . Further2.2.1task 1, subjects were asked to stably hold six different visually presented force levels using the vertical red lines  within a range of ±5% of MVC without using any kind of voluntary cocontraction. The force levels were given to them in a randomized order four times each, leading to a total of 24 perturbations. The perturbation is a small and fast displacement of 7.5 mm of the pinch grip, and force is measured to calculate stiffness using its difference before and after perturbation. Since active response is not our scope, the measurement is finished within 40 ms. This procedure is similar to the one in our previous studies  given to them in a randomized order within a range of ±5% of MVC 15 times each and use cocontraction to produce higher stiffness at a similar force than in task 1, leading to 75 perturbations. In other words, they had to keep the red solid line between the two red dashed lines and always produce stiffness higher than the black dashed line in Figure In 2.3From the measured force data and the known position perturbation, we calculated the grip stiffness. We found out from the optical tracking data how the perturbation length is distributed to thumb and index finger. We evaluated whether and how well stiffness and force values could be predicted from EMG data and how EMG–force and EMG–stiffness relationships vary within and across subjects. We analyzed whether and how much voluntary cocontraction and the grip force before the perturbation influenced stiffness, EMG values, and kinematics.2.3.1TbP before and TaP after the perturbation are similar to the one introduced in our previous study , which has a standard deviation (SD) below 5 ⋅ 10−4 N. This number was empirically determined and led to stable results. The force before the perturbation was calculated using TbP. Assuming that neuromuscular feedback does not have any measurable influence within 40 ms  for sample size n assessing low sample sizes with a higher SE; the coefficient of variation is a normalized measure of the SD and compensates for the sample mean μ(⋅). Since the objective function equation (σ(⋅) for both. The minimum of this cost function minimizes the variation of resulting stiffness values k measured under exactly the same conditions  and the oscillations in force within time interval TaP of both tasks. Since subjects cannot produce the exact same cocontraction level twice angdper se, we refrained from analyzing the influence of kinematics on stiffness and from drawing wrong conclusions. Thus, its remaining influence is still part of the measurement noise.Since the kinematic position was controlled to be kept stable and not commanded 2.3.3The oversampled EMG signal  was filtered offline using a delay-free second-order Butterworth bandpass filter between 25 and 450 Hz. The produced muscular activity was evaluated using the average rectified value (ARV) over a time frame of 200 ms before the perturbation. From the relaxation task, a steady time window of about 500 ms was chosen manually , representing the EMG base noise level. The base noise of each electrode was subtracted from the EMG data subjectwise. EMG data were normalized by their maximum values and divided by their SDs for each electrode and each subject.2.3.4R2 was used. For calculating the intrasubject R2 cross-validated values the number of required models equals the number of perturbations per subject  and for the intersubject R2 cross-validated values the number of required models equals the number of subjects (leave-one-subject-out) were used. Since we expected a non-linear dependency between measured EMG and force, we tested if taking the square root  test to reveal significant patterns of the two factors. Moreover, for testing significance of a correlation, we used a standard function in MATLAB, which provides a p value based on results of a t-test testing differences in variances. Equality of variances was tested using a two-sample F-test. Finally, Steiger’s z-test was used to investigate differences between correlations  and their interaction (p ≤ 0.05) have a significant influence on the obtained results.We performed a multivariate two-way repeated-measure MANOVA—including the dependent variables task 1. There is a significant positive correlation for subject S6 between trial number and stiffness and a significant negative correlation for subject S5 between trial number and force for the experimental condition of task 2.Concerning effects of learning and fatigue, we found no significant correlation between trial number to both force and stiffness for the experimental condition of 3.1task 1 to task 2. Results of Figure stiffness showed that both factors task and force level (p ≤ 0.01) are significant, but their interaction is not significant. Post hoc THSD tests revealed a significantly larger stiffness within task 2 and—as might be expected—an always increasing stiffness with force level (p ≤ 0.0001). Moreover, two measures for the ability to decouple stiffness from force are given in Table task 1, task 2, The linear regressions in Figure 3.2Beside minimizing the variation in kinematic orientation and position during the experiments, the kinematic data reveal insights on how the total perturbation length of 7.5 mm is distributed between thumb and index finger and give an indication of the relative stiffnesses of the two digits. Table thumb and index finger displacements showed that the factor force level is significant for both variables (p ≤ 0.05), but the factor task is significant for the index finger displacement (p ≤ 0.01), only. Post hoc THSD tests revealed no significant pattern for both factors and variables.Performing univariate two-way repeated-measure ANOVAs for the dependent variables For details about how subjects performed in keeping the predefined position, please have a look into the Appendix.3.3t-test on the Fisher-transformed correlation coefficients on whether the correlation of EMG to force and stiffness significantly differs across subjects. The results show that only for the two intrinsic muscles in the hand, the correlation of EMG to stiffness significantly differs in comparison to its correlation to force (p ≤ 0.001). A detailed analysis of the correlations between force, stiffness, and muscular activities for each of the two tasks can be found in the Appendix. Moreover, an overview on the contributions from the three groups of muscles—extrinsic extensors and flexors and interossei—can be found here.We performed an intersubject regression of stiffness and force from EMG (see Tables p < 0.001) and stiffness (p < 0.05) than taking the square of muscular activation. Moreover, no clear improvement can be found by taking the square root in comparison with plain muscular activity. Finally, taking the square root of muscular activity in comparison to the square clearly improves its correlation to force (p < 0.01), but not to stiffness. Conclusively, all reported results and analyses focusing on regressing stiffness and force from EMG use the plain muscular activity.Since literature inconsistently reports, we tested whether taking the square root or square of EMG data improves the quality of the linear fits of force and stiffness to EMG using Steiger’s z-test Steiger, . The tesFor details about intrasubject regression, please have a look into the Appendix.4In this article, we analyzed the role of voluntary cocontraction for decoupling grip stiffness from its natural increase with grip force. To measure influences from cocontraction only, we minimized effects of variabilities in kinematics by providing the subject a visual feedback of the current hand and arm posture. In a first task, we asked subjects to apply a set of force levels several times without the use of cocontraction to measure the basic force–stiffness coupling. In a second task, we asked subjects to decouple stiffness from force using voluntary cocontraction while holding a specific force level. We measured EMG to investigate the possibility of regressing stiffness and force from the measurement of muscular activity.4.1r = 0.30, p < 0.05). Milner and Franklin reported in the study by Burdet et al. . Similarly, Kearney and Hunter  reportedper se rather than a combined EMG level.To have similar cocontraction ranges at all force levels, we commanded in a former version of the experiments a combination of applied force and EMG similar to the work done by Osu and Gomi , Osu et Anyway, similar to our results reported above, Akazawa et al.  found thFinally, it needs to be mentioned that our finding of an increasing ability for decoupling force and stiffness by cocontraction is opposing the torque–stiffness plots of existing VSA mechanisms  meaning that these muscles provide a stable correlation across subjects. This is possibly a result of higher signal-to-noise ratio (SNR) for the intrinsic muscles. Since the measured surface EMG signal involves the EMG pattern from other, deep, muscles—which we interpret as a lower SNR for the extrinsic muscles—the correlation of forearm muscles dropped, while the one of the intrinsic muscles in the hand did not. Similarly, Maier and Hepp-Reymond . Even if the subject is unknown, these models provide surprisingly good results for the regression of stiffness. However, this holds for the regression of stiffness, only, and not for force. What is the reason? The significances of the coefficients for these two regressions show that the two intrinsic muscles in the hand had an unexpectedly high influence on the modeling of stiffness, while all muscles contributed almost equally to the regression of force. Looking into correlations between stiffness, force, and muscular activities shows a comparatively high correlation of the intrinsic muscles to stiffness  or just correlation  from the conducted experiments. But the result can be interpreted from a biomechanical point of view: coactivating extrinsic flexor and extensor muscles introduces high forces on the finger joints. This may lead to instability at—in particular—the metacarpophalangeal joint: it could reduce the strain by an uncontrolled sideways, abduction-like, movement. The interossei muscles, connecting the proximal and metacarpal bones at each side of the metacarpophalangeal joint, can be used to stabilize this movement—and apparently do. A somewhat similar mechanism can be found, e.g., during pinch grip: extrinsic extensor muscles—namely extensor carpi ulnaris and extensor carpi radialis longus/brevis—are activated simultaneously with flexor muscles to prevent the wrist from moving; i.e., the intent is to contract the flexor muscles, and the extensor muscles are activated involuntarily to provide support.First, we need to acknowledge that prestudies led us to the wrong conclusion of a predominant role of the index finger on the measured stiffness, based on which we decided to exclude muscles activating the thumb from the EMG measurements. But since we find the thumb to be Moreover, a publication from Milner et al.  argues iIt needs to be acknowledged that the SDI does neither control index finger nor thumb and controls the movement of the middle finger, only (see Table Note that we investigated the use of non-linear regression models, as well, to improve the results: Gaussian processes (Rasmussen and Williams, On the basis of the results in the studies by Joyce and Rack  and Vrentask-dependent stiffness during activities of daily living: Leidner et al. (Compliant Manipulation Tasks into a task taxonomy, e.g., by classifying tasks of contact/no contact, in-hand manipulation/external manipulation tasks, or tasks with and without deformation of the environment. By continuously measuring stiffness of the hand, it will be possible to measure a task dependent stiffness during activities of daily living, such as cutting an onion, cleaning with a sponge or connecting a plug (Leidner et al., All in all, the intrinsic muscles in the hand are found to dominate our regression of stiffness and not of force, while the experiment design does not allow us to reveal whether the stiffness itself is dominated by these muscles. A good possibility to answer this question might be the use of functional electrical stimulation placed on respective extrinsic and intrinsic muscles as performed for the human hand (Lauer et al., r et al.  started HH developed the idea and device, researched the literature, analyzed the data, and contributed to the acquisition of data. MG-D contributed to acquisition of data and the analysis of data. GS contributed to the acquisition and analysis of data and revised the work. JB contributed to the regression analysis and testing of machine-learning methods. PS contributed to the interpretation of data and revised the work.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.