Andy concentrates on each and every step. His feet, ankles and knees throb. Shoulders shudder under the 25-kilo weight of the rucksack. The nylon straps burn red lines across his collarbone.

• 马普安把精力集中在迈出的每一步上，他的脚、踝和膝盖都在发颤，肩膀在25公斤的背包下发抖，尼龙背带在锁骨上磨出了红红的印，就像烙上的一样。