Page 212 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMECHANICS OF THE MUSCULOSKELETAL SYSTEM 189

of the quadriceps muscles, and their resulting pull on the patellar tendon. In later stance, anterior
shear forces produced by the quadriceps and gastrocnemius muscles are balanced by a posterior
shear force produced by the ground reaction force (Fig. 7.33), resulting in a relatively small ACL
force for this period of the stride.
Injuries to the ACL are common and often result in rupture of the ligament, producing an ACL-
deficient (ACLD) knee. In the ACLD knee, the total anterior shear force acting on the tibia is
reduced, from a peak of 262 N in the intact knee, to only 128 N in an ACLD knee (Shelburne et al.,
2004b). This reduction in total anterior shear arises mainly from a reduction in the shear force
induced by the quadriceps muscles acting via the patellar tendon. In the ACLD knee, the ACL no
longer acts to resist anterior translation of the tibia, and so tension in the patellar tendon can trans-
late the tibia to a more anterior location, relative to the femur, than it occupies in the intact knee. This
anterior translation reduces the angle between the tibia and the patellar ligament, and causes a con-
comitant reduction in quadriceps shear force (Shelburne et al., 2004b; Fig. 7.34). The smaller ante-
rior shear force in the ACLD knee is then supported by the other, remaining ligaments, with the vast
majority borne by the medial collateral ligament.

7.7.3 Landing from a Jump
One possible cause of ACL injury is landing from a jump, which can produce a peak ground reaction
force 4 times larger than that experienced during walking. However, simulation of landing from a jump
with bent knees has revealed that the peak ACL force experienced in such a configuration is only 253 N
(Pflum et al., 2004), which is comparable with the value of 303 N experienced during normal walking.
This result is somewhat surprising, because increasing knee flexion increases the angle of the patellar
ligament relative to the tibia (Fig. 7.34), and thus increases the anterior shear force exerted by the quadri-
ceps muscles. However, when landing with bent knees, the anterior shear forces are counteracted by a
large posterior shear force generated by the ground reaction force (Fig. 7.35). Thus, the potential for ACL
injury when landing from a jump is not mediated by quadriceps force alone and the action of the ground
reaction force must also be considered in this case.

Total patellar
tendon force

Shear force
component Anterior
tibial
translation

A B
FIGURE 7.34 Illustration of the shear force generated by the patel-
lar tendon in the normal (a) and ACL-deficient (b) knee.