The Complete Guide to Stretching

4th edition

Christopher M. Norris

• Publisher: Bloomsbury Sport
• Year: 2015
• ISBN:
  9781472908407
• Edition: 4
• Topic: Biological Sciences
• Subtopic: Sport & Exercise Science
• Index: Biological Sciences

A reasonable level of flexibility is essential to the healthy functioning of joints and muscles, which in turn facilitates performance and reduces the risk of injury. Now in its 4th edition, The Complete Guide to Stretching provides an accessible overview of the scientific principles that underpin this form of training and offers more than 70 exercises designed to safely increase range of motion right across the body. The Complete Guide to Stretching is the definitive practical handbook for:
- Sports participants and recreational exercisers who are keen to achieve a level of flexibility that will enhance their performance
- Sports coaches and fitness instructors who are seeking a thorough understanding of the principles and practice of this often neglected component of physical fitness
- Sport and exercise therapists who use stretching as an important part of a balanced rehabilitation programme Full colour photographs demonstrate the stretches throughout.

1

PART ONE

THE SCIENTIFIC PRINCIPLES BEHIND STRETCHING

1 BIOMECHANICAL FACTORS IN STRETCHING

The study of the effect of mechanical forces on biological materials is known as biomechanics. Biomechanical principles are important to all aspects of sports training, but especially to stretching. To be effective, and to prevent injury, stretching exercises must be applied on a foundation of good biomechanical principles.

LEVERAGE

The limbs and spine act as levers when we move. A lever is simply a rigid bar that moves around a fixed point called the pivot or fulcrum. Two forces act on the lever, effort and resistance. The effort attempts to move the lever, while the resistance tries to stop movement. In the body, the effort is supplied by muscle contraction, while the resistance is weight. The weight is a combination of the weight of the moving limb and the weight of any object lifted. Take as an example the arm lifting from the side of the body (see fig 1.1). The fulcrum is the shoulder joint, the effort is supplied by the deltoid muscle, which contracts and abducts the arm, and the resistance is the weight of the arm.

HOW DO YOU CALCULATE THE AMOUNT OF LEVERAGE?

The amount of leverage produced in any exercise can be calculated by multiplying the weight of the resistance by the horizontal distance between the point where the resistance or effort acts and the fulcrum. Figure 1.2 illustrates a simple example of a lever. A resistance of 6 kg is placed 3 m away from the fulcrum. Multiplying these together gives a leverage force of 18 units. To balance this out, the effort has to be of the same magnitude. So, the 9 kg weight has to be placed only 2 m from the fulcrum for the lever to balance.

LEVERAGE IN STRETCHING EXERCISES

It is important to note that in the example given in figure 1.2, the horizontal distance between the fulcrum and effort or resistance is used, rather than simply the distance along the lever. This means that leverage will be increased as a body-part is moved into a horizontal position, and will reduce as the body part moves away from the horizontal position (see fig 1.3). This fact must always be borne in mind when choosing starting positions for stretching exercises, especially with regard to injury to the spine.

Take as an example a simple toe-touching movement. Performed from long sitting, the leverage on the spine is minimal (see fig 1.4(a)); however, the same body movement performed from standing (see fig 1.4(b)) places a considerable stress on the spine through leverage forces acting on the lumbar region.

Figure 1.1 Leverage as the arm is abducted

Figure 1.2 Calculating leverage

Figure 1.3 Leverage in weight training

Figure 1.4 Reducing leverage by altering the starting position

Key point:

When calculating leverage, it is the horizontal distance from the fulcrum which is used.

This example illustrates an important safety factor with regard to leverage: exercises that involve moving the spine into a horizontal position will place great amounts of leverage on the spine and should be used with caution. Often, simply altering the starting position will move the spine away from the horizontal and reduce the stress on the lower back. When a horizontal position must be used, the spine should be supported. In the examples in figure 1.4, the athlete is stretching the hamstrings by bending forwards. This action places an excessive leverage stress on the spine. Simply by putting one hand down on the knee, the spine is supported and the stress reduced (see fig 1.4(c)).

Considering the effect of gravity is also important. In figure 1.5 the athlete is performing the splits. The leverage on the leg is excessive, tending to force the knee downwards, which opens the joint. This action can severely stress the medial ligament on the inside of the knee. Performing a similar action sitting down takes the weight away from the knee and, although the lever length is the same, the effect on the knee ligaments is considerably reduced, making the exercise far safer.

Figure 1.5 Leverage and the splits (a) knees unsupported; (b) knees supported

LEVERAGE AND POSTURE

Leverage is also important with regard to posture. A good posture (see Posture) is one that exerts minimal force on the joints and requires little work from the muscles to maintain it. Anything that increases either joint loading forces or muscle work can increase the risk of injury and pain, and will certainly make the posture harder to maintain over a period of time. The human body, like any other object, has a centre of gravity, and the extension of this down to the floor is known as the line of gravity (see here). This can be thought of as the balance point of the body’s levers. When a body segment rests on this line its leverage force is minimised, and when it moves away from the line the leverage force is increased. Take as an example the head resting on the neck (fig 1.6). In a good posture, the chin is held in and the centre of the head lies directly over the neck and the gravity line. However, if the head moves forwards – as it so often does when we are looking at a computer for a long time, for example – the leverage force is increased. Using the principle of calculating leverage we can see that, if the head moves forwards, it is as though it actually weighs more. A 10 kg head resting directly over the gravity line will only weigh 10 kg, but the same head resting 20 cm forward of this line will weigh 10 x 20, or 200, units of leverage. This increased weight dramatically increases the forces on the neck and requires a significantly greater amount of muscle work to maintain it – so much so, in fact, that the muscles become tired and tight and feel knotted. The answer to this pain is simply to draw the head back on to the posture line by tucking the chin in slightly and thus reducing the leverage forces acting on the region.

Key point:

When devising a stretching programme, consider leverage and the effects of gravity. Are the exercises you choose using these elements to work with the body to enhance training or to work against it to increase the likelihood of injury?

Figure 1.6 Leverage and posture

Key point:

Leverage can multiply the effect of postural changes significantly.

CENTRE OF GRAVITY AND STABILITY

The centre of gravity of an object is its balance point, where all the weight of the object is focused. The centre of gravity of a symmetrical object, such as a brick, will be at its centre. However, in the case of asymmetrical objects, such as the human body, the centre of gravity will be nearer to the larger, and heavier, end.

WHERE IS THE BODY’S CENTRE OF GRAVITY?

Because the legs are heavier than the arms, when a person is standing their centre of gravity is not in the middle of the body at the navel, but lower down within the sacrum. As the body moves away from the standard upright position, the centre of gravity also moves. Lifting the arms overhead, for example, moves the centre of gravity upwards, while carrying something moves the centre of gravity towards the object being carried. In addition to the centre of gravity of the body as a whole, each limb also has a centre of gravity. For example, the weight of the arm will act through its own centre of gravity, which, rather than being in the middle of the arm at the elbow, is actually closer to the shoulder because the upper arm is heavier than the forearm.

WHAT AFFECTS STABILITY?

Extending the centre of gravity downwards towards the floor gives us the object’s line of gravity. Where the centre of gravity is the balance point of an object, the line of gravity can be imagined as a plumb-line hanging down from this point. For an object to remain in balance, its line of gravity must pass through its base of support. If the line of gravity moves outside the base of support, the object becomes unstable and will topple over.

Key point:

To remain stable, the gravity line of an object must pass through its base of support.

To compensate for this, the body position will change when something is carried. In figure 1.7(a), the centre of gravity of the body is within the sacrum. In figure 1.7(b), the suitcase carried in the right hand moves the centre of gravity of the body and case combined to the right. This would move the line of gravity outside the person’s base of support, making him unstable. To compensate for this, the body position is changed by leaning over to the left to pull the line of gravity back within the base of support (see fig 1.7(c)).

Stability is an important safety factor when performing stretching exercises. An unstable position can cause an athlete to wobble or fall, unintentionally increasing a stretch and pulling muscles or spraining joints. When discussing stability, there are two factors to consider: first, the position of the object’s centre of gravity; and second, the size of the objec...