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 6.24 Photograph and schematic diagram showing the arrangement commonly used when people perform a knee-extension exercise on a Biodex or Cybex dynamometer. Notice that the strap fixed on the machine arm is attached distally (near the ankle) on the subject s leg. 

Quadriceps force increases monotonically from full extension and 90° of flexion, but the forces borne by the cruciate ligaments of the knee do not (Fig. 6.25, ACL). Calculations obtained from a mathematical model of the knee (Shelburne and Pandy, 1997 Pandy and Shelburne, 1997 Pandy and Sasaki, 1997) indicate that the ACL is loaded from full extension to 80° of flexion during knee-extension exercise. The model calculations also show that the resultant force in the ACL reaches 500 N at 20° of flexion, which is lower than the maximum strength of the human ACL (2000 N) (Noyes and Grood, 1976). 

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. ]... 

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. 

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,... 

We hypothesize that applying these strategies can improve muscle torque production during intensive knee extension exercises. The purpose of this study, therefore, is to quantify and compare Resultant Muscle Torque (RMT) production within performing 8-RM seated knee extension in contribution of two types of ER training. 

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. 

Because muscle, ligament, and joint-contact forces cannot be measured noninvasively in vivo, estimates of these quantities have been obtained by combining mathematical models with either the inverse-dynamics or the forward-dynamics approach (Sec. 6.6). Below we review the levels of musculoskeletal loading incurred in the lower-limb during rehabilitation exercises, such as isokinetic knee extension, as well as during daily activity such as gait. 

For isokinetic exercise, in which the knee is made to move at a constant angular velocity, quadriceps force decreases as knee-extension speed increases. As the knee extends more quickly, quadriceps force decreases because the muscle shortens more quickly, and, from the force-velocity property, an increase in shortening velocity leads to less muscle force (see Fig. 6.4). As a result, ACL force also decreases as knee-extension speed increases (Figure 6.26), because of the drop in shear force applied to the leg by the quadriceps (via the patellar tendon) (Serpas et al., in press). 

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). 

Roberts et al. studied the knee proprioception of 36 patients with ACL deficiency by measuring TTDPM before and after a short period of exercise on an ergometer bicycle [47]. They found trends of enhanced proprioception towards extension in the patient group after cycling, but not in the age-matched control group. 

Postoperatively, the knee is immobilized at 10° flexion with a brace for 2 weeks, followed by ROM exercise. Partial weight-bearing was allowed at 3 weeks, followed by full weight-bearing at 4 weeks. Full extension or flexion exceeding 130° is not allowed until 5 weeks. Jogging was allowed at 3 months and running was permitted at 4 months, followed by return to strenuous sports activity at 8-10 months. 

The knee is splint immobilized at 10 ° flexion for 1 week, followed by passive and active ROM exercises. Partial weight bearing is allowed at 2 weeks followed by full weight bearing at 4-5 weeks. Full extension or flexion exceeding 130 ° is not permitted until 5 weeks. Jogging is recommended at 3 months. Return to strenuous activity is not allowed imtil 6 months. 

Quadriceps tendinopathy is far less conunon than patellar tendinopathy and usually relates to sporting activities or strenuous exercise. Clinically, this condition is characterized by focal pain over the distal portion of the tendon, exacerbated by resisted extension of the knee or firm pressure over it. The skin is normal and there is no evidence of intra-articular effusion at physical examination. Similar to other tendinopathies, the involvement of the quadriceps tendon mainly relates to degeneration and fibro-myxoid changes. In this setting, the main value of... 

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