注入10ml温开水，同时在胃部听到气过水声

抽吸出胃液

胃管末端放入水杯中无气体逸出

抽出液遇石蕊试纸变红色

注入10ml空气，同时在胃部听到气过水声

反向电压

正向电压

反向电压或正向电压

反向电压

正向电压

正向电压或反向电压

正向电压和反向电压

强度

活动

持续

对比

正确

错误

Athletes might also invent new sports to test themselves. The emergence of the triathlon in the 1970s was fuelled by runners, swimmers and cyclists looking for a new challenge; it made its debut as an Olympic event in 2000.

Once scientists have identified the genes that confer a genetic advantage in sport, athletes might also be screened to pick out the ones with most genetic potential. 'There are all sorts of people out there, and we don't know what they can do', says exercise and sports scientist Carl Foster of the University of Wisconsin in La Crosse. Because the rewards are growing and competition is becoming more intense, athletes are being driven more and more towards drugs to gain the edge. Experts predict that the next generation of champions will include many doped ones. They are particularly fearful of 'gene doping' in which athletes boost the performance of key genes.

Psychology is vital. Athletes need enormous focus and drive to win. Many people think that the main barrier to breaking the four-minute mile was a psychological one: once Roger Bannister did it in 1954, several others clocked sub-four-minute times shortly afterwards. Sometimes breaking a record involves taking a risk in an event, such as breaking from the pack with a full lap to go, and that takes a certain state.

Not every sport can be accurately measured, of course. Running and jumping can be quantified with stick or stopwatch, but football and tennis performances are much harder to gauge.

On top of this, however, training and technique are vital. They allow athletes to sculpt muscles, for example, so that they burn less energy while achieving the same speeds as others.

强度

对比

变化

活动

加反向电压

加正向电压

不加电压