Cumulative trauma may also be a combination of major and micro traumas. A large trauma may weaken the body so that micro trauma does more damage. Or the opposite may occur where micro traumas has weakened the body to a point where a larger trauma does more damage than it would have on a healthy body.
Trauma occurs when the body part is called on to work harder, stretch farther, impact more directly or otherwise function at a greater level then it is prepared for. The immediate impact may be minute, but when it occurs repeatedly the constant trauma cause damage.
External factors may apply cumulative trauma as well. Irritation, inflammation, chemical exposure and other environmental factors can cause major and micro traumas.