George is lifting weights. he starts by doing biceps curls which involves his elbows. his elbow is able to bend because the humero-ulnar joint is a joint. sam is doing leg presses, his knees as he pushes against the weights. the knee joint is primarily a joint, though it can act as a condylar joint when flexed. meanwhile, jane is doing "lat pulls" by pulling downward on a weighted bar. this movement involves of her arms. her shoulder joint is a joint, allowing a wide range of movements.

George is lifting weights. He starts by doing biceps curls which involves flexing his elbows.  His elbow is able to bend because the humero-ulnar joint is a hinge joint.  Sam is doing leg presses, extending his knees as he pushes against the weights. Meanwhile, Jane is doing "lat pulls" by pulling downward on a weighted bar. This movement involves adduction of her arms.  Her shoulder joint is a  ball-and-socket joint, allowing a wide range of movements. 

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.