I am pretty new to mountain biking and I have noticed on longer rides my hands will start to cramp/ache to the point where it is not comfortable to wrap my thumb around the handle bars. Obviously putting my thumb on top on the grip is less safe because I have less of a grip on the bike so is there anything I can do to lessen the cramps/aching? I have just standard grips on the bike and would be open to trying out some new ones if that would do the trick.
Your question suggests that you might hold your handle bars too tight.
- A tense grip over a long tube leads inevitably to cramps.
If you are not going over technical terrain or going very fast it is usually safe to change grips and hold bars much more loosely. With experience you will develop a firm but not too tight grip also in technical sections.
- Pressure on the palms of your hands may also cause numbness and/or pain.
The reason therefore is described in detail in Rich Manson's answer. Unergonomic hand and arm position on flat handlebars exacerbates this. Some grips, mitts, bars or a changed position on the bike might help. The latter is the easiest to fix: check if your position on the bike is good. See Craig Hicks' answer on how to improve your position on your bike.
Lastly, unless you are in a race, simply take a break from time to time. This helps immensely with many other forms of cycling discomfort caused by prolonged restrictions of movement.
A few things I have done that seem to help. Swap your grips to something with an ergo design. This type has a paddle like shape. The larger width spreads the weight to a larger contact area. Another advantage is these grips tend to have a larger diameter. The larger size makes it easier to grip. Check your fork settings. Unless you are currently bottoming the fork out, set it to a softer setting. This will let the shock do it job of absorbing vibration.
The hand receives its peripheral nerve supply from well-defined nerves such as the radial and median nerve which are involved with sensation to the thumb.
The median nerve often gets pinched, and the result is numbness. Sometimes, weakness can also result, particularly of the muscles that bend the thumb towards the little finger.
The most common place for the median nerve to become pinched is in the carpal tunnel. This narrow passage in the wrist where the median nerve travels along several tendons to the fingers. If the tendons become inflamed, the swelling in the narrow tunnel can lead to a pinched nerve, diminished electrical signal transmission and numbness.
Things that may help:
Looser grip where possible
Constant position changing so that the nerves mentioned above do not get pinch keeping inflation to a minimum.
Try not to put to much weight on your wrists if your wrist are making contact with the bars. Cutting off circulation to the carpal tunnel are is a big reason thumb numbness sets in.
Use a good pair of gloves with a good gel palm pad.
Better ergonomic grips
Better grip position on bars and proper handlebar position
Shaking hands periodically for a few seconds to get circulation going again. Try alternating a fist, then a shake a couple times for each hand when you are not in a technical area of the trail
Limiting resting on the area of the carpal tunnel, the are between the palm and wrist, helps the most.
Note: I have noticed that I have the most problem when my fitness has suffered coming out of winter months and not riding as often. Seams as you get more fit, your body adjusts and carries your body weight better on the bike. New riders also seam to have initial problems more often when they start into bike riding. this is not considered by experts to be a serious medical issue.
Besides grips and handle sweep (affecting wrist angle), you might want to consider bike fit (esp. seat fore-aft position, handlebar height, and length of stem) which greatly affect how much weight you put on your hands. Generally speaking, seat further back, handlebars higher, and stem shorter will have you putting less weight on your hands. However, beware, as this may put more weight on your ass which may then get very sore.
Imagine you are pedaling so hard that the downward force of your feet is strong enough to lift your whole weight - then if your center of mass is balanced over the pedals you will be touching your seat and the handles just enough to keep you balanced, not using either to carry your weight.
The problem is, unless you are a really fast racer going out for a 1 or 2 hour spin, you can't pedal that hard for long so your ass and hands have to carry the weight. This is why long distance riders tend to put their handlebars a little higher and get a leather saddle.
I do some long distance mixed terrain MTB riding. I use a Brooks leather saddle and have put aerobars on my MTB. The point of the aerobars is not to go faster (although that helps a little on the road sections) but to have an alternate position. Once any part of your posture becomes too tired or painful, it really starts to slow you down.