When you lift a weight, a complex set of actions takes place in your arms' muscles to allow movement and ensure no damage is done to the muscles. Let us consider what happens in the Triceps and Biceps.
How do muscles contract?
Muscle fibres are long, thin, tapered cylindrical cells full of the mechanisms required to convert chemical energy into movement. Fibres are arranged parallel to each other and usually lengthways. A sheath of collagen surrounds individual fibres. Bundles of fibres and the whole muscle are surrounded by more connective tissue. Blood vessels, motor neurons (the nerve that innervates muscle fibres) and other nerves wind in between the bundles.
The contractile apparatus in each muscle fibre is arranged in parallel long cylindrical strands, called myofibrils. Actin and myosin are the contractile protein polymers in myofibrils, and they, too, are long and lie parallel and lengthways. Using energy derived from ATP, the actin and myosin "filaments" attach via cross bridges and slide past each other in opposite directions, thus causing a contraction.
The "sliding filament" and cross-bridge theories explain how muscles shorten. Like an oar in a rowing boat, it reaches out from the myosin filament (or rowing boat), grabs onto the actin (or water), pulls the actin towards it, and then pushes it away. The cross-bridge oar is then recycled to hold onto another bit of actin (water) so the contraction continues.
When the Biceps (the agonistic muscle) contracts, a signal is sent to the Triceps (the antagonistic muscle) to relax, to allow movement.
Within the Triceps, and all muscles, a unique muscle fibre known as the annulo-spiral receptor. This receptor is sensitive to the rate and extent the Triceps are being stretched. As the Triceps lengthens, this receptor sends a signal proportional to the amount and rate of stretch to tell the Triceps to contract. It is a safety mechanism to prevent the Triceps from being overstretched.
Contained in the tendon of each muscle is the Golgi tendon receptor. This receptor is sensitive to the build-up of tension when a muscle is stretched or contracted. The receptor has a tension threshold that causes the tension to be released when it gets too high. As the Biceps contract and the threshold is exceeded, a signal is sent to the Biceps, causing them to relax. This mechanism prevents damage to the Biceps should the weight be too heavy or the movement is too fast.
As the Triceps lengthens, the combined effect of the stretching action and the stretch reflex contraction will cause a build-up of tension in the Triceps tendon. When the threshold is reached, the receptor will send a message to the Triceps muscle causing it to relax. It will allow the Triceps to be stretched even further.
Muscle soreness that occurs 24 to 48 hours after intense exercise involves eccentric contractions (Delayed Onset of Muscle Soreness). It causes increases in intracellular pressure that irritates the nerve endings, producing swelling and local pain.
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