Moving upwards or from ground up in our gait consideration of body segments and their role while walking, this week calls for detailed look of the trunk. The trunk appears very stable with relatively less movement during the gait cycle, and so it appears that the trunk is less active, however this is not the case. The noteworthy activity in the major trunk muscles in harmony with the lower extremity muscles is responsible for the appearance of subtle movement and evident stability.
The last post mentioned that the pelvic bone is made up of three bones which are fused together. Namely the ilium, ischium and pubis bones. In the channel of weight transfer, the spine is closely linked to the pelvis for continuity. The sacrum which is the lower part of the spine is made of five fused vertebrae and it forms a very stable and well supported joint (supported by muscles and ligaments) with the pelvis(ilium) and hence is known as the sacro-iliac joint or SI joint. The weight transfers from the spine to the pelvis via sacrum to ilium and from pelvis to femur (lower extremity) via the hip joint.
Here it is important to mention two regions of the skeletal system-namely the lumbo-pelvic region (lumbar spine is highly mobile) and the pelvic-hip region (femur at hip joint is highly mobile). The strong muscular and ligamentous support in these regions are key to stability during weight transfer in the gait cycle. The former allows the weight transfer from a stable spine to the pelvis and the latter from the pelvis to the stable lower extremities. When you go to the physical therapist and are working on your core, it mainly concerns the stability in these two regions through the integrity of the supporting ligaments and strength of the surrounding musculature. See how it is all connected so immaculately?
We have considered the major muscles of the trunk and hip in earlier posts and I will refresh those briefly:
1) Abdominals- rectus abdominus, internal and external obliques and transversus abdominus
2) Back Extensors- multifidi and semispinalis and extensor spinae
3) The hip extensors- 3 gluteii which extend and rotate the hip and proximal hamstrings
4) Hip flexors- iliacus, psoas, rectus femoris, sartorius, gracilis, pectineus
5) Hip adductors- adductor magnus, adductor longus and adductor brevis
6) Hip abductors- Gluteus maximus, gluteus medius and gluteus minimus and Tensor Fascia Lata
There is motion of the trunk and pelvis in 3 major planes which I have considered in detail in an earlier post. They are:
1) Transverse plane which is parallel to the ground (forward and backward rotation of pelvis happen in this plane along a vertical axis)
2) Sagittal plane which divides the body into left and right halves (anterior and posterior pelvic tilt happens in this plane) and
3) Coronal plane which divides the body into front and back (lateral tilting of pelvis happens in this plane)
Now let us consider the pelvic and trunk movement and muscle work during the gait cycle.
• In the transverse plane: at right heel strike, the ipsilateral (same-sided) pelvis is forward in the transverse plane along a vertical axis in the direction of progression. The trunk, however, in order to conserve energy and for maximal efficiency, must stay facing in direction of progression and not follow the right leg. That is the trunk rotates to the left or swing side during single leg support and then comes to neutral at right leg mid-stance, then it rotates to right side, till the next right single leg support. That is, muscle contraction from right(ipsilateral) back extensors keep the spine upright and in direction of progression while the bilateral obliques co-contract as do the transverse abdominus and multifidi for stability. The passive left arm swing occurs in response to momentum generated and it contributes to keep the upper trunk from rotating towards the right (weight bearing) limb.
• In the sagittal plane: The gluteus maximus and ilio-psoas maintain pelvic stability, as the hip flexes and extends at heel strike and push-off, respectively. The pelvis posteriorly rotates at heel strike, then goes to anterior tilt quickly and slowly returns to posterior tilt again for next heel strike.
• In the Coronal plane: at right heel strike, with impact, there is lateral trunk flexion to the right. The gluteus medius on right contracts as left leg goes into swing phase, thus preventing the pelvis from dropping excessively laterally towards the left side when the left foot is off the ground (swing phase). Although there is a mild list of pelvis to swing leg in order to keep the COG from rising excessively.
As we know, the center of gravity moves in a figure of eight pattern “∞.” This means that it rises and drops vertically as it progresses forward. Gait is efficient with least movement of the COG. At early and late stance, it drops towards the ground and is at the highest during the mid-stance phase of each leg. All this muscle work just ensures, the vertebral column stays stable in as near neutral as possible and gets a ride over the mobile lower extremities, thus conserving energy.
Ami Gandhi is a licensed physical therapist in the state of California. She is the owner of StableMovement Physical Therapy, a small boutique practice in San Jose that offers patient centered, one-on-one, hands-on physical therapy.