Page 207 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

184 BIOMECHANICS OF THE HUMAN BODY

The results of Figs. 7.25 through 7.27 have significant implications for the design of exercise reg-
imens aimed at protecting injured or newly reconstructed knee ligaments. For maximum, isolated
contractions of the quadriceps, Fig. 7.25 shows that the ACL is loaded at all flexion angles less than
80°. Quadriceps-strengthening exercises should therefore be limited to flexion angles greater than
80° if the ACL is to be protected from load. In this region, however, very high contact forces can be
applied to the patella (Fig. 7.27), so limiting this exercise to large flexion angles may result in
patellofemoral pain. This scenario, known as the “paradox of exercise,” is the reason why surgeons
and physical therapists now prescribe so-called closed-chain exercises, such as squatting, for
strengthening the quadriceps muscles subsequent to ACL reconstruction.

7.7.2 Gait
Muscle and joint loading are much lower during gait than during knee-extension exercise.
Various studies have used inverse-dynamics or static optimization (Hardt, 1978; Crowninshield
and Brand, 1981; Glitsch and Baumann, 1997) and forward-dynamics or dynamic optimization
(Davy and Audu, 1987; Yamaguchi and Zajac, 1990; Anderson and Pandy, in press) to estimate
muscle forces during normal gait. There is general agreement in the results obtained from these
modeling studies.
Analysis of a dynamic optimization solution has shown that the hip, knee, and ankle extensors
are the prime movers of the lower limb during normal walking. Specifically, gluteus maximus and
vasti provide support against gravity during initial stance; gluteus medius provides the majority of
support during mid-stance; and soleus and gastrocnemius support and simultaneously propel the
body forward during terminal stance (also known as push-off) (Anderson, 1999; Anderson and
Pandy, 2001).
The plantarflexors generate the largest forces of all the muscles in the leg during normal gait.
Soleus and gastrocnemius, combined, produce peak forces of roughly 3000 N (or 4 times body
weight) during the push-off phase (Fig. 7.28, SOL and GAS prior to OHS). By comparison, the hip
extensors, gluteus maximus and gluteus medius combined, produce peak forces that are slightly less
(around 2500 N or 3.5 times body weight during initial stance) (Fig. 7.28, GMAXL, GMAXM,
GMEDP, and GMEDA near OTO). Finally, the quadriceps, vasti and rectus femoris combined,
develop peak forces that are barely 2 times body weight near the transition from double support to
single support (Fig. 7.28, VAS and RF at OTO).
Muscles dominate the forces transmitted to the bones for most activities of daily living. Thus,
the loading histories applied at the ankle, knee, and hip are in close correspondence with the pre-
dicted actions of the muscles that cross each of these joints (Fig. 7.29). For example, the peak
force transmitted by the ankle is considerably higher than the peak forces transmitted by the hip
or knee, which is consistent with the finding that the ankle plantarflexors develop the largest
forces during normal gait (compare joint-contact force at ankle in Fig. 7.29 with forces produced
by SOL and GAS in Fig. 7.28). Similarly, peak hip-contact force is around 4 times body weight
near OTO, which results from the actions of the hip extensors, gluteus maximus and gluteus
medius, at this time (compare joint-contact force at hip in Fig. 7.29 with forces developed by
GMAXL, GMAXM, GMEDP, and GMEDA in Fig. 7.28).
During walking, the peak articular contact force transmitted by the tibio-femoral joint at the
knee is less than 3 times body weight. This level of joint loading is much lower than that esti-
mated for knee-extension exercise, where forces approaching 10 times body weight have been
predicted to act between the femur and tibia (Fig. 7.27). During the single-support portion of
stance, where only a single foot is in contact with the ground, the center of mass of the body passes
medial to the center of pressure of the foot. This results in an adduction moment exerted at the
knee (Morrison, 1970). The magnitude of the knee adduction moment varies among individuals,
with a mean value of around 3.2 percent of body weight times height (Hurwitz et al., 1998). The
adduction moment is large enough that it would open the tibio-femoral joint on the lateral side, if
it were not resisted by some internally generated abduction moment (Schipplein and Andriacchi, 1991),