Biomechanics: muscles and levers

When you hold something in your hand with your elbow bent:
the elbow joint is the pivot;
the biceps pulls upwards on the forearm, close to the elbow;
the load (the weight in your hand) pulls downwards, far from the elbow.
The bones act as levers, the joint as the pivot and the muscle provides the effort.

moment = force × perpendicular distance from the pivot
If the arm is held still, the moments balance:
biceps force × biceps distance = load × load distance
The biceps is attached very close to the elbow, so its force must be much bigger than the load.

Because the biceps is attached near the elbow, a small shortening of the muscle moves the hand a long way. So we trade force for movement: the muscle pulls hard, the hand moves far and fast.
The further the load is from the elbow, the bigger the force the biceps must make. That is why it is harder to hold a heavy bag with your arm stretched out.
Muscle strength can be measured, for example by pulling on a newton meter or squeezing a grip meter.