B

Figures B and B1 represent a stage of movement in which the pelvis is tilted posteriorly before beginning to raise the trunk. (Note the 10° posterior pelvic tilt.) In testing, this movement often is performed as a separate stage to ensure lumbar spine flexion.

When the posterior tilt is not done as a separate movement, as shown in Figures B and B1, it occurs simultaneously with the beginning phase of trunk raising (i.e., the trunk-curl phase), unless the abdominal muscles are extremely weak or the hip flexors are so short that they prevent posterior tilt when the subject is supine with the legs extended.

In Figure B, the hip flexors have lengthened, and the one-joint hip flexors (chiefly the iliacus) have reached the limit of length permitted by the hip joint extension. At this length, they help to stabilize the pelvis by restraining further posterior pelvic tilt.

In Figure B1, the hip flexor length is slightly more than that in Figure A1, because the pelvis has tilted posteriorly 10° away from the femur. Posterior pelvic tilt exercises are frequently used with the intention of strengthening the abdominal muscles. Too often, however, the tilt is done without any benefit to the abdominals. The subject performs the movement by contracting the buttocks muscles (i.e., the hip extensors) and, in the case of the knee-bent position, by pushing with the feet to help "rock" the pelvis back into posterior tilt.

To ensure that the pelvic tilt is performed by the abdominal muscles, there must be an upward and inward pull by these muscles, with the front and sides of the lower abdomen becoming very firm. (See p. 215.)

It is necessary to discourage use of the buttocks muscles to force action by the abdominals when performing a posterior pelvic tilt.

SPINE FLEXION PHASE (TRUNK-CURL) COMPLETED

In Figures C and C1, the neck (i.e., cervical spine), upper back (i.e., thoracic spine), and low back (i.e., lumbar spine) are flexed. The low back remained in the same degree of flexion as shown in Figures B and B', where it reached maximum flexion for this subject.

In Figures C and C1, the abdominal muscles have shortened to their fullest extent with the completion of spine flexion. In Figure C, the hip flexors have remained lengthened to the same extent as shown in Figure B.

In Figure C\ the one-joint hip flexors have not reached the limit of their length and, therefore, do not act passively to restrain posterior tilt. The hip flexors contract to stabilize the pelvis, and palpation of the superficial hip flexors provides evidence of firm contraction as the subject begins to lift the head and shoulders from the table.

HIP FLEXION PHASE (SIT-UP) INITIATED

With flexion of the spine complete (as shown in Figures C, C , D, and D1), no further movement in the direction of coming to a sitting position can occur except by flexion of the hip joints.

Because the abdominal muscles do not cross the hip joint, these muscles cannot assist in hip flexion.

From a supine position, hip flexion can be performed only by the hip flexors acting to bring the pelvis in flexion toward the thighs.

Figures D and D1 represent the beginning of the sit-up phase as well as the end of the trunk-curl phase.

HIP FLEXION PHASE (SIT-UP) CONTINUED

between the completed trunk curl (as shown in Figures table.

C, C1, D, and D1) and the full sit-up. The abdominal When necessary, the feet may be held down at the ini-

muscles maintain the trunk in flexion, and the hip flex- nation of and during the hip flexion phase. (See p. 208.) Be-

ors have lifted the flexed trunk upward toward the sitting fore the hip flexion phase, the feet must not be held down.

between the completed trunk curl (as shown in Figures table.

C, C1, D, and D1) and the full sit-up. The abdominal When necessary, the feet may be held down at the ini-

muscles maintain the trunk in flexion, and the hip flex- nation of and during the hip flexion phase. (See p. 208.) Be-

ors have lifted the flexed trunk upward toward the sitting fore the hip flexion phase, the feet must not be held down.

In Figures F and F1, as the subjects reach the sitting position, the cervical and thoracic spines are no longer fully flexed, and the abdominal muscles relax to some extent.

In Figure F, the hip flexors have moved the pelvis in flexion toward the thigh, completing an arc of approximately 80° from the table. In this position, with the knees extended and the lumbar spine flexed, the hip joint is as fully flexed as the range of normal hamstring length permits. The lumbar spine remains flexed, because moving from the flexed position of the low back to the zero position (i.e., normal anterior curve) would require that the pelvis tilt 10° more in flexion toward the thigh, which the hamstring length does not permit

In Figure F', the hip flexors have moved the pelvis in flexion toward the thigh through an arc of approximately 75° from the table. The lumbar spine remains in flexion, because the hip joint has already reached the 125° of full flexion. Further flexion of the hip joints by tilting the pelvis forward (and bringing the low back into a normal anterior curve) could be done only if the flexion of the thigh were decreased by moving the heels farther from the buttocks in this sitting position.

Trunk curl refers to flexion of the spine only (i.e., the upper back curves convexly backward and the low back straightens). When abdominal muscles are strong and the hip flexor muscles very weak, only the trunk curl can be completed when attempting to do a sit-up. (See p. 205.)

Sitting position is one in which the trunk is upright and the hips are flexed. To sit down means to move from an upright to a sitting position by flexing at the hip joints; however, this movement may not require action of the hip flexor muscles. To sit up means to move from a reclining to a sitting position by flexing at the hip joints. When done unassisted, this movement can be performed only by hip flexor muscles. Whether alone or in combination, the word sit should be used only in connection with movement that involves hip joint flexion.

The sit-up exercise, therefore, is the movement of coming from a supine to a sitting position by flexing the hip joints, and it is performed by the hip flexors. It may be combined correctly with trunk and leg positions as illustrated below, or incorrectly as illustrated on page 210.

A curled-trunk sit-up with the legs extended consists of flexion of the spine (i.e., trunk curl) performed by ab-

dominal muscles followed by flexion of the hip joints (i.e., sit-up) performed by hip flexors (2-4).

A curled-trunk sit-up with the hips and knees flexed (i.e., knee-bent sit-up) starts from a position of hip flexion (i.e., flexion of the thigh toward the pelvis) and consists of flexion of the spine (i.e., trunk curl) performed by the abdominal muscles followed by further flexion of the hip joints (by flexion of the pelvis toward the thigh) performed by the hip flexors (2,3).

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