Exercise is the only way for MS clients to retain the strength and motor control they need to avoid/delay being wheelchair-dependent. One of my personal training clients has had MS for over 20 years. It's been a serious battle for her for a long time, but she's still ambulatory. She wrote a short success story which can be read here: http://cstminnesota.com/5.html#susan
Since writing that story, Susan has returned to her writing and given some local readings - things she hadn't done for years. Though MS presents a number of challenges to remaining active, she continues to train with me, and is continuing to improve her balance, strength, and overall capabilities.
Since MS involves demyelinization of the nerves, the fatigue described by so many folks with MS is neurological rather than muscular in nature. Fine motor control and proprioception are early victims, while gross strength in simple movements can easily be maintained or even regained for many years. Balance is further challenged by increasingly poor posture.
I have found AIS stretching to be very safe and effective, and AIS has a wealth of strengthening exercises (see the book at http://www.stretchingusa.com) that work well. Occasionally, performing positional release techniques helps relax overactive muscles, enabling their aneforum.xxxonists to function more effectively by reducing reciprocal inhibition. PRT performed before stretching may make it easier to lengthen chronically tight tissues.
With advanced MS, it is also common to have a certain "mental fog" that impacts memory, cognition, etc. to some extent. Susan is a sharp lady, but she has found some mental tasks more challenging than they used to be. I don't know how much of this is due to neurological changes vs changes in level of daily mental and physical stimulation as personal activity levels and routines change. In any case, Susan has reviewed that her family and friends have commented on various perceived improvements in her memory, problem-solving, writing, comprehending, and other mental skills.
One thing I've found extraordinarily helpful for Susan: Intu-Flow (more info here: http://www.profcs.com/app/?Clk=2217683)
Intu-Flow is a simple follow-along program of light mobility exercises designed to help release excess tension and restore lost joint mobility from head to toe. It includes instruction in simple breathing drills and balance exercises that pretty much anyone can follow, and it's completely compatible with yoga, tai chi, and other mindful exercise practice. As Susan has worked with Intu-Flow, she has regained mobility throughout her body and become much more aware of her body. Though her proprioception is still impaired, she is developing improved sub-conscious awareness and control. At first, she was stumbling from one machine to another, and now she's becoming adept at using her mini-Clubbells to perform complex full-body exercises. It has been an amazing process!
MS clients need light, stimulating exercise that challenges their coordination. In working to master such exercises, they will see tremendous carryover to simpler tasks. Heavier, strength-oriented tasks should be simple and work a wide variety of muscle groups, with some light coordination drills practiced as well. Single sets of many reps tend to fatigue the CNS too quickly, while multiple sets of 5-8 reps (with short breaks between) permit greater CNS recovery and better endurance during an exercise session.
Since some MS clients have difficulty with sweating and may overheat easily, swimming and water aerobics are good options. It's important that trainers and clients be prepared to simply let things cool down now and then during a workout. This makes it very challenging to develop aerobic capacity, but it's possible.
Hope that helps!