Other than endurance, leg power (limb strengthening) and mental conditioning – the often neglected physical training aspect is core strengthening.
Core muscles are mostly muscles around the abdomen (6pack), lumbar, pelvis area. There are complicated and hard to remember muscle names in the inner and outer core – but suffice it to say that this group of body muscles help stabilize/ protect one’s spine, mid-body section; and allows transfer of stress and energy between upper and lower limbs – like a starting and connecting middle engine gear. A weak core could easily spell back/spinal injury – something not very uncommon given heavy load lifting demand of mountaineering.
Combining endurance (cardio-vascular), general strengthening and core muscle trainings will holistically improve body condition and eventual performance. While some muscle strengthening already benefits the core – a few more extra routines might just make you climb that next summit. Or if not, at least your body can tolerate more brutal body punches and kicks the next time you get drunk and got involved in a bar brawl.
I started learning some basic calisthenics/ non-weighted routines when I joined a COCC (cadet officer training) way back in high school. Still the simplest routine out there with no gym fees and hassle-to-mingle-with-strangers environment. I still remember doing 100 plus full push ups, 150-200 sit-ups, and some more. You may include other simple/ non-machine oriented routines like pull-ups (chin ups) including ‘halfway holds’, dips (using arm chairs is an option), or bar gymnastic moves. For women, I heard a ‘rumor’ that over-doing some of these may decrease their ‘bumper size’ (i.e. fats to muscle) - just my word of caution.
Today, part of my pre-expedition weighted training (more for cardio and leg muscle) somehow benefits the core. If you have 20kg of weight, lifting them often (after rest/pause) and generally walking with them already benefits the core muscle (provided your posture is good). Overdoing it (i.e. over-loading without pre-weight training) however may wreck your back. I once did a 30li-water weight training, walking 11 kilometers on a highway – but I concluded that it was an overload (given my mere 68kg weight). If one prefers gym, weighted core exercise (squats, lifts) will definitely help – but get a good coach if you don’t know where to start.
Other beneficial routines may include kick-boxing, yoga, relevant dancing routines, and other body-flexibility exercise. No harm mixing them with other routines. You don’t need to have a visible 6-pack abs to ‘know’ your core is good. Performance improvement (and being injury-free) is still the best measure. Of course if you have an extra bulge (we call ‘AB’, aka round belly) – it helps to reduce/eliminate and transform them as useful set of muscle. Plus for guys, you’ll be able to (or better) see your.... toes. :)