Final answer:
The contraction of the biceps resulting in the arm raising towards the shoulder is an example of isotonic contraction. If the triceps contracts, it extends the arm. Joints with greater mobility tend to be less stable than those with less range of motion.
Step-by-step explanation:
Understanding Muscle Contraction and Joint Mobility
When the biceps contract and become shorter, this produces a flexion motion in the arm, causing it to lift towards the shoulder. This situation is more likely to be an isotonic contraction because the muscle length is changing as the arm moves, which is not the case in isometric contractions where the muscle length remains the same while tension is generated.
If the triceps were to contract, the arm would extend, moving in the opposite direction of the biceps contraction. This would result in the arm straightening at the elbow joint.
Regarding joint stability and mobility, it is generally true that the more mobile a joint, the weaker the joint. The shoulder joint allows the greatest range of motion and is less stable compared to the carpal bones in the wrist that provide stability with less mobility.