Weakness: Upper Body/core Strength
A strong upper body, including all of the torso, is critical for a strong swim stroke. A strong core is also the foundation for holding aero position comfortably during an full-distance race and maintaining tall body posture late in the marathon. Riding a mountain bike improves both upper body and core strength to address these weaknesses.
Mountain biking requires a lot of body weight transfer on the bike to steer through technical terrain. Maintaining traction, while cornering and descending, is the result of centring weight over the bike so the tire tread has weight into the ground. This is accomplished by counterbalancing through the body. This balance and coordination builds core and upper body strength. The arms also function as shock absorbers when riding off-road. This ends up acting like a constant push-up workout during a mountain bike session. Athletes can find swimming more difficult the day after a hard mountain bike ride.
Using balance skills and upper body strength to manoeuvre the bike through technical terrain builds upper body and core strength. This creates a solid platform for triathlonspecific activities.