A musculoskeletal shoulder model based on pseudo-inverse and null-space optimization.

The goal of the present work was assess the feasibility of using a pseudo-inverse and null-space optimization approach in the modeling of the shoulder biomechanics. The method was applied to a simplified musculoskeletal shoulder model. The mechanical system consisted in the arm, and the external forces were the arm weight, 6 scapulo-humeral muscles and the reaction at the glenohumeral joint, which was considered as a spherical joint. The muscle wrapping was considered around the humeral head assumed spherical. The dynamical equations were solved in a Lagrangian approach. The mathematical redundancy of the mechanical system was solved in two steps: a pseudo-inverse optimization to minimize the square of the muscle stress and a null-space optimization to restrict the muscle force to physiological limits. Several movements were simulated. The mathematical and numerical aspects of the constrained redundancy problem were efficiently solved by the proposed method. The prediction of muscle moment arms was consistent with cadaveric measurements and the joint reaction force was consistent with in vivo measurements. This preliminary work demonstrated that the developed algorithm has a great potential for more complex musculoskeletal modeling of the shoulder joint. In particular it could be further applied to a non-spherical joint model, allowing for the natural translation of the humeral head in the glenoid fossa.

Alcohol consumption in late adolescence and early adulthood - where is the problem?

Risky single-occasion drinking (RSOD) is more common in late adolescence and early adulthood (approximately between the ages of 16 and 30) than in any other period in life. This is also the age when young people in Switzerland and many other European countries are legally allowed to buy and drink alcohol, but they usually do not yet have adult responsibilities. This paper reviews evidence from the international literature and provides examples of studies conducted in Switzerland demonstrating that (a) RSOD is by far most prevalent on Saturday evenings followed by Friday evenings, usually because young people go out and do not have any work or study responsibilities the next day; (b) RSOD results from drinking in private before going out ("predrinking") and accelerating the pace of drinking (i.e. increasing the number of drinks consumed per hour); (c) RSOD is often not accidental but purposeful,. to seek excitement, to have fun and to feel the effects of alcohol; (d) RSOD occurs predominantly outside the home, mostly in bars, pubs, discos or at special events and festivals; (e) RSOD often results in intended and unintended injuries and other acute consequences, which are leading risk factors for mortality and morbidity in this age group. Effective prevention strategies should include attempts to reduce opportunities to engage in heavy drinking as well as strategies to reduce its harmful consequences.