课外阅读1：肌肉收缩的类型

Types of Muscle Contraction

Muscle Contractions can be divided into: 

Isotonic (meaning same tension) 

Isometric (meaning same distance or not moving) 

Isokinetic (meaning same speed) 

Isotonic Contractions

Isotonic contractions are those which cause the muscle to change length as it contracts and causes movement of a body part. There are two types of Isotonic contraction:

Concentric 

Concentric contractions are those which cause the muscle to shorten as it contracts. An example is bending the elbow from straight to fully flexed, causing a concentric contraction of the Biceps Brachii muscle. Concentric contractions are the most common type of muscle contraction and occur frequently in daily and sporting activities. 

Eccentric 

Eccentric contractions are the opposite of concentric and occur when the muscle lengthens as it contracts. This is less common and usually involves the control or deceleration of a movement being initiated by the eccentric muscles agonist.

For example, when kicking a football, the Quadriceps muscle contracts concentrically to straighten the knee and the Hamstrings contract eccentrically to decelerate the motion of the lower limb. This type on contraction puts a lot of strain through the muscle and is commonly involved in muscle injuries. 

Isometric Contractions 

Isometric contractions occur when there is no change in the length of the contracting muscle. This occurs when carrying an object in front of you as the weight of the object is pulling your arms down but your muscles are contracting to hold the object at the same level. Another example is when you grip something , such as a tennis racket. There is no movement in the joints of the hand, but the muscles are contracting to provide a force sufficient enough to keep a steady hold on the racket.

The amount of force a muscle is able to produce during an isometric contraction depends on the length of the muscle at the point of contraction. Each muscle has an optimum length at which the maximum isometric force can be produced. 

Isokinetic Contractions 

Isokinetic contractions are similar to isotonic in that the muscle changes length during the contraction, where they differ is that Isokinetic contractions produce movements of a constant speed. To measure this a special piece of equipment known as an Isokinetic Dynamometer is required. Examples of using isokinetic contractions in day-to-day and sporting activites are rare. The best is breast stroke in swimming, where the water provides a constant, even resistance to the movement of adduction. 

Now Test Yourself 

1. Isotonic contractions can be either concentric or?