What happens when our muscle tense?
Muscles are designed to contract i.e., when they tense they get shorter. Their stretch is a passive process, typically facilitated by antagonist muscles in case of skeletal (striated) muscles. Consider the biceps and triceps antagonists in the upper arm in Fig. 1:


Fig. 1. Left: The biceps actively contracts to bend the arm and is called the agonist. The triceps is inactive and is passively stretched by the biceps, called the antagonist. Right: To stretch the arm, the reverse happens, i.e. the triceps contracts and is now called the agonist. The biceps is now inactive and is called the antagonist and it is passively stretched by the triceps. Source: Pilates Studio

At the molecular level, the tensing of muscles can be readily seen as contraction, as opposed to stretching (Fig. 2):


Fig. 2. Muscle fibers contract actively and stretch passively. The passive stretched state is depicted in the upper panel. Under the influence of ATP (active part) the muscle fibers condense and the muscles contracts. source: Emaze Presentations.  

Further Reading
- Do both ends of a muscle contract?