Page 206 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMECHANICS OF THE MUSCULOSKELETAL SYSTEM 183

800
Isometric

600 30 deg/s
ACL force 400 180 90

300
200 Increasing isokinetic
speed
0
0 30 60 90
Knee flexion (deg)
FIGURE 7.26 Resultant force in the ACL for isometric (thick line) and isokinetic
(30, 90, 180, and 300 deg/sec) knee-extension exercises. The results were obtained
from a two-dimensional model of the knee joint, assuming the quadriceps are fully
activated and there is no cocontraction in the flexor muscles of the knee (Serpas et al.,
2002). The model results show that exercises in the studied speed range can reduce the
force in the ACL by as much as one-half. [Modified from Serpas et al. (2002).]

The forces exerted between the femur and patella and between femur and tibia depend mainly on
the geometry of the muscles that cross the knee. For maximum isometric extension, peak forces trans-
mitted to the patellofemoral and tibiofemoral joints are around 11,000 N and 6500 N, respectively
(i.e., 15.7 and 9.3 times body weight, respectively) (Fig. 7.27). As the knee moves faster during
isokinetic extension exercise, joint-contact forces decrease in direct proportion to the drop in
quadriceps force (Yanagawa et al., 2002).

12000
PF
10000
8000 TF
Force (N) 6000

4000

2000

0
0 30 60 90
Knee flexion (deg)
FIGURE 7.27 Resultant forces acting between the femur and patella (PF) and
between the femur and tibia (TF) during maximum isometric knee-extension exer-
cise. See Fig. 7.25 for details.