and why can't it break a tendon or a ligament?
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Most all muscle contractions begin with a signal called an action potential along a motor neuron that crosses the synaptic gap at the neuromuscluar junction and then gets propogated along the sarcoplasmic reticulum and the T-tubules inside of a muscle cell. This releases calcium so it can attach to the actin by opening the troponin-tropomyosin attachment sites for the myosin heads. With the help of ATP, the myosin heads form cross-bridges to the actin myofilaments and pull the Z-lines closer together which makes the sarcomeres shorter, thereby contracting the muscle cells.
Read more: http://wiki.answers.com/Q/What_events_occur_leading_to_muscle_contraction#ixzz1adD56Qjt
Tendons are the elastic, bungee cord-like structures made up of interlocking crimped fibers that attach muscle to bone, stabilize joints and act as shock absorbers. They are major players in the active motion of a horse's leg. By contrast, ligaments are strap-like stabilizers. Although ligaments don't participate in the active muscle movement, they, too, are comprised of interlocking crimped collagenous fibers, but they are non-elastic and tighten when under stress, rather than stretch. Ligaments hold and stabilize bones, joints, muscles and tendons from excessive movement and stretching.
The ankle and the foot are held together by both ligaments and tendons. The ligaments on both sides of the ankle are tightly attached to the bones. On the outside (lateral) aspect of the ankle there are 3 major ligaments. On the inside (medial) aspect of the ankle there are a complex network of several ligaments, much stronger than the lateral ligaments. They help to restrict the motion of the ankle joint. The tendons that cross the joint are attached to the muscles in the leg and into the bones of the foot. When the muscle contracts it moves the tendon that then moves the foot at the ankle joint. The ankle joint moves primarily in one direction or plane; upwards (dorsiflexion) and downwards (plantarflexion). The normal joint does not move front to back nor does it tilt on either side. When there is an injury to the ligaments they can be stretched out or torn, leading to a sprain of the ligament, weakening them. In some cases, when the rotational forces are strong enough, the ankle fractures as well. There can also be damage done to the cartilage of the ankle joint leading to a defect of the cartilage (osteochondral defect) and ankle arthritis. The tendons that cross the ankle can be injured as well, leading to a tendonitis or tendon tears. In cases that are left untreated, or in cases after many sprains in a short period of time, there will weakening of the ligaments leading to instability of the ankle. This is called lateral ankle instability. This can lead to chronic pain and loss of function, which usually will need surgical correction.
Read more: http://wiki.answers.com/Q/What_events_occur_leading_to_muscle_contraction#ixzz1adD56Qjt
Tendons are the elastic, bungee cord-like structures made up of interlocking crimped fibers that attach muscle to bone, stabilize joints and act as shock absorbers. They are major players in the active motion of a horse's leg. By contrast, ligaments are strap-like stabilizers. Although ligaments don't participate in the active muscle movement, they, too, are comprised of interlocking crimped collagenous fibers, but they are non-elastic and tighten when under stress, rather than stretch. Ligaments hold and stabilize bones, joints, muscles and tendons from excessive movement and stretching.
The ankle and the foot are held together by both ligaments and tendons. The ligaments on both sides of the ankle are tightly attached to the bones. On the outside (lateral) aspect of the ankle there are 3 major ligaments. On the inside (medial) aspect of the ankle there are a complex network of several ligaments, much stronger than the lateral ligaments. They help to restrict the motion of the ankle joint. The tendons that cross the joint are attached to the muscles in the leg and into the bones of the foot. When the muscle contracts it moves the tendon that then moves the foot at the ankle joint. The ankle joint moves primarily in one direction or plane; upwards (dorsiflexion) and downwards (plantarflexion). The normal joint does not move front to back nor does it tilt on either side. When there is an injury to the ligaments they can be stretched out or torn, leading to a sprain of the ligament, weakening them. In some cases, when the rotational forces are strong enough, the ankle fractures as well. There can also be damage done to the cartilage of the ankle joint leading to a defect of the cartilage (osteochondral defect) and ankle arthritis. The tendons that cross the ankle can be injured as well, leading to a tendonitis or tendon tears. In cases that are left untreated, or in cases after many sprains in a short period of time, there will weakening of the ligaments leading to instability of the ankle. This is called lateral ankle instability. This can lead to chronic pain and loss of function, which usually will need surgical correction.