Isometric knee-extension exercise

The quadriceps is the strongest muscle in the body. This can be demonstrated by performing an isometric knee-extension exercise. Here, the subject is seated comfortably in a Cybex or Biodex dynamometer with the torso and thigh strapped firmly to the seat. The hip is flexed to 60°, and the leg is strapped to the arm of the machine, which can be either fixed or allowed to rotate at a constant angular velocity (see Fig. 6.24). Locking the machine arm in place allows the muscles crossing the... 

FIGURE 6.27 Resultant forces acting between the femur and patella (PF) and between the femur and tibia (TF) during maximum isometric knee-extension exercise. See Fig. 6.25 for details. 

FIGURE 6.26 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 in press). 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 at. (in press. ]... 

Subject A exercised his lower extremity muscles at home using a PC computer to control the implanted stimulator. In lanuary 1997, he was provided with a battery-operated external portable conditioning system (19x11x6 cm ), which he uses at home and at work sitting in his wheelchair. The exercise protocol stimulates the right and left knee extensors and ankle plantar/dorsi flexors alternately 4 sec ON/4 sec OFF, for a total of 20 min. After the muscles have been conditioned, dynamometric testing (isometric mode) has shown that implanted FES stimulation produces bilateral knee extension torque of 45 to 55 Nm at 30° and 65 Nm at 60° of knee flexion. Subject A exercised at least 3 days a week, and found if he did not do so the spasticity in the lower extremities increased. 

The forces exerted between the femur and patella and between femur and tibia depend mainly on the geometry of the muscles that cross the loiee. For maximum isometric extension peak forces transmitted 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. 6.27). As the knee moves faster during isokinetic extension exercise, joint-contact forces decrease in dire proportion to the drop in quadriceps force (T. Yanagawa and M. G. Pandy, unpublished results). 

Data suggest that extensive physical exercise may increase blood plasma TAC. Long-term effects of systematic physical exercise are, however, controversial. Sub-maximal exercise (30 min) was reported not to alter blood plasma TAC significantly (A7). TAC of blood plasma was reported to increase immediately after a marathon run (by 25%) and this increase persisted 4 days later (by 12%) (L19). Similar effects (increase by 19%) were noted after a half-marathon (C29). Another study reported an increase in blood serum TAC by 22% during a 31-km run and by 16% during a marathon (V10). TAC of blood plasma was increased by 25% after a maximum aerobic exercise test and by 9% after a nonaerobic isometric exercise test (A8). Eccentric muscle exercise (70 maximal voluntary eccentric muscle actions on an isokinetic dynamometer, using the knee extensors of a single leg) did not affect blood serum TAC (C27). In another study, TAC increased after exhaustive aerobic (by 25%) and nonaerobic isometric exercise (by 9%) (A8). 

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