Finger stability in precision grips

Stable precision grips using the fingertips are a cornerstone of human hand dexterity. Occasionally, however, our fingers become unstable and snap into a hyper-extended posture. This is because multi-link mechanisms, like our fingers, can buckle under tip forces. Suppressing this instability is crucial for hand dexterity, but how the neuromuscular system does so is unknown. Here we show that finger stability is due to the stiffness from muscle contraction and likely not feedback control. We recorded maximal force application with the index finger and found that most buckling events lasted less than 50ms, too fast for sensorimotor feedback to act. However, a biomechanical model of the finger predicted that muscle-induced stiffness is also insufficient for stability at maximal force unless we add springs to stiffen the joints. We tested this prediction in 39 volunteers. Upon adding stiffness, maximal force increased by 34±3%, and muscle electromyography readings were 21±3% higher for the finger flexors (mean±standard error). Hence, people refrain from applying truly maximal force unless an external stabilizing stiffness allows their muscles to apply higher force without losing stability. Muscle recordings and mathematical modeling show that the splint offloads the demand for muscle co-contraction and this reduced co-contraction with the splint underlies the increase in force. But more stiffness is not always better. Stiff fingers would interfere the ability to passively adapt to complex object geometries and precisely regulate force. Thus, our results show how hand function arises from neurally tuned muscle stiffness that balances finger stability with compliance.

Estimation of Finite Finger Joint Centers of Rotation Using 3D Hand Skeleton Motions Reconstructed from CT Scans

The present study proposed a method to estimate the finite finger joint centers of rotation (CoRs) with high accuracy using 3D hand skeleton motions reconstructed from CT scans. Ten hand postures starting from a fully extended posture and ending at a fist posture with about 10° difference in flexion between the adjacent postures were captured by a CT scanner for 15 male participants, and their 3D hand skeletons were reconstructed using the CT scans. Each bone segment from the full extension posture was registered to the corresponding bone segments of the remaining hand postures. The proximal bone segments of a joint from two postures were aligned to estimate the finite CoR of the joint between the two postures. Centerlines of the distal bone segments of the joint were then identified using the principal component analysis method, and the finite CoR of the joint was determined as the intersection point of the identified centerlines. The proposed method reduced the variation of estimated finite joint CoRs by 16.0% to 67.0% among the finger joints compared to the existing methods. The variation of estimated finite joint CoRs decreased as the rotation angle of the joint increased. The proposed method can be used for the simulation of finger movement with high accuracy.

«SLEEPING» POSITIONS IN THE BRONZE AGE BURIALS

Авторы предлагают проверить трактовку скорченной на боку скелетной позы как символа спящего человека в погребениях эпохи ранней бронзы степной зоны Восточной Европы. В выборке из 77 изображений современных людей в состоянии утреннего сна летом в полупустынном климате юга России отмечено безусловное доминирование встречаемости скорченных поз, несмотря на возможность сна в вытянутой позе. Сравнение поз современных спящих людей с положением скелетов в погребениях эпохи ранней бронзы Прикубанья выявило ряд совпадений в соотношении основных типов поз, в частоте скорченных поз в целом и в более мелких деталях положения рук и поворота туловища. Трактовка скорченной позы как типичной позы спящего человека признана допустимой. The authors propose to check the interpretation of the contracted position of the skeleton lying on the side as a symbol of a sleeping individual in the Early Bronze Age burials in the steppe belt of Eastern Europe. The sample of 77 images featuring modern people in the state of morning dream in summer from the areas in the Russian South with a semi-desert climate demonstrates indisputable prevalence of contracted positions despite the fact that people can sleep in an extended posture. Comparison of positions in which modern persons sleep with the positions of the skeletons found in the Early Bronze Age burials of the Kuban region revealed a number of coincidences regarding the main types of positions, incidence of contracted positions as a whole and minor details of hand and arm positions as well as the turn of the body. The interpretation of the contracted position as a typical posture of a sleeping person has been recognized as acceptable.

Bifocal Femoral Deformity Correction and Lengthening Using a Circular Fixator Construct in a Dog

A 7 mo old male rottweiler was evaluated for a right hind limb lameness caused by malunion of a Salter-Harris type II fracture. Radiographs and computed tomography (CT) revealed that the right distal femur had valgus, procurvatum, external rotation, and was 35% (70 mm) shorter than the contralateral femur. Distal femoral wedge ostectomies were performed to acutely correct the angular and rotational deformities. Lengthening of the femur was accomplished by distraction osteogenesis performed over 53 days at a second, proximal diaphyseal osteotomy using a circular fixator construct. This bifocal approach yielded approximately 30 mm of femoral lengthening and a confluent column of regenerate bone that bridged the distraction gap. When evaluated 14 mo after surgery, the dog held the right hip, stifle, and hock in a slightly extended posture when standing and had a subtle asymmetric hind limb gait. The stride of the right hind limb was slightly shortened with compensatory circumduction of the left hind limb during the swing phase of the stride.

The Body Context: Interpreting Early Nasca Decapitated Burials

One of the most enigmatic aspects of decapitation practices among south coastal Peruvian Paracas and Nasca cultures (900 B.C.-A.D. 750) is the near absence of headless bodies in the archaeological record. Drawing on a case study of an Early Nasca, Phase 3 headless burial at Site PV62D13 in the lower Ica Valley, together with examples of headless interments reported in the literature, I examine the nature and disposition of headless burials. Three burial patterns, or forms, are proposed. The burial at PV62D13 is distinguished for its extended posture and interment in a non-cemetery setting and is proposed to be a dedicatory burial. Burial forms for disembodied heads are also examined, and over time, a notable increase in cached heads, buried without grave offerings is observed. Painted ceramic images of Nasca trophy heads and headless bodies are reviewed diachronically and a marked increase in decapitation scenes is noted for Late Nasca. Attention is drawn to the limited number of headless body images overall, and possible solutions to reconcile the visual imagery with the archaeological data are offered.

The Political Economy of American Strategy

Recent discussions whether or not the United States is strategically “overextended” raise two important questions. First, to what extent can the fiscal and industrial difficulties of the last several decades be attributed to the comparatively high military budgets of the post-1945 period? Second, can the United States continue to maintain something resembling its postwar strategic posture without doing itself grievous economic harm? Although the issue remains open, defense spending would appear to bear only a small part of the responsibility for present U.S. economic problems. As to the future, the question is not so much whether the burden of an extended posture can be borne as whether it should be borne, and who, precisely, should bear it. These are political issues: they are conditioned but not determined by economic factors.

Dorsiflexion of the Human Ankle as it Relates to Ski Boot Design in Downhill Skiing

This study is part of an on-going series of studies that relate to Alpine or Downhill Ski Boot Design. In current Alpine skiing, the ski boot is an integral part of the ski boot-binding system. One of the roles of the ski boot is to protect the ankle from excessive dorsiflexion during forward falls, as the ski boot is levered out of the heel binding. A boot designer needs to know what the ranges of dorsiflexion are for human ankles so that the allowable forward flex built into the ski boot will not exceed some specified level. That specified level should be such that a large part of the population will not exceed a safe level of dorsiflexion. The stiffening of the ankle by voluntary contraction of the muscles that control the ankle joint cannot be relied upon since the reaction time to contract the muscles will be greater than the time available to the skier under many circumstances. This study looks at the maximum voluntary dorsiflexion of a group of people (n=64) similar to a skiing population. The anatomical and biomechanical posture of the subjects was intended to represent typical skiing situations, therefore the subjects were measured in a weight bearing, flexed knee, upright posture. The age, gender, height, weight and skiing experience of the subjects was recorded as independent variables. The maximum voluntary dorsiflexion of the ankle was the dependent variable. 10 subjects were measured while the knee was kept in a straight or extended posture. The analysis indicates that there is no statistically significant relationship between dorsiflexion and any of the independent variables. The mean dorsiflexion was 42.7 degrees, the 5th% value was 28.5 and the 95th% was 56.7 degrees. The straight knee posture reduces the effective dorsiflexion by 8.5 degrees. Current standards permit as much as 40 to 45 degrees dorsiflexion. The implications are that current standards are excessive, a reasonable limit would be something under 30 degrees. Such a limit, or less, is consistent with the maximum dorsiflexion found in most current ski boots.