One should exercise at various angles in the ROM because of the limited physiologic overflow. If we
apply isometrics only at one point, there is approximately a 10° overflow on each side. How then would
we apply isometrics when there is a painful point in an arc of motion? We would exercise at every 20°
using the Rule of Tens throughout the range of motion, beginning at 10° on each side of the painful
position. Thus we would get a physiologic overflow into the painful deformation and achieve our goal to
increase strength at that point and decrease related pain.
1.2.4 Isotonic Mode
Isotonic exercise involves work in a physical sense. This is called dynamic muscular work. For
example, when the biceps contracts and shortens and the lower arm is bent or a weight is lifted,
movement is accomplished. The physical formula of work = force Χ distance is fulfilled.
Dynamic muscular work does not involve lengthy contractions but instead is distinguished by the
alternation between contraction and relaxation. In the concentric contraction phase, individual muscle
fibers shorten, and their origin and insertion approximate. In the eccentric relaxing phase, individual
muscle fibers go through a lengthening process with their origin and insertion moving apart. In each
motion, the agonist and antagonist muscle groups are involved. In isotonic exercises, the prime mover
(agonist) produces a concentric muscular contraction (e.g., quadriceps producing knee extension). This
is followed by an eccentric contraction of the same muscle group (i.e., quadriceps slowly lowering the
leg toward flexion). When multiple repetitions are performed by the same muscle group concentrically
and then eccentrically, a transient muscle ischemia is produced that compromises blood flow.
From two to three times more force can be generated with eccentric contractions. The clinical
implications of this are evident when dealing with a patient who cannot initiate a concentric contraction.
An example may be the use of eccentric straight-leg raises in a knee rehabilitation program.
Immediately post surgery, after the patient can perform quadriceps isometric sets, immediate
progression to eccentric straight-leg raises is often effective using the iliopsoas. The clinician passively
assists with hip flexion, or a sling mechanism can be developed to allow the patient to work
independently after which active eccentric straight-leg lowering is performed by the patient. (Note: The
quadriceps can isometrically contract while the iliopsoas muscles eccentrically contracts.)
Although eccentric contractions are useful in early rehabilitation programs or in gaining muscle mass
and strength, there is a distinct disadvantage involving residual muscle soreness that may cause
decreased performance due to pain and biochemical changes in the involved muscle. Eccentric isotonic
exercises are thus used at the two extremes of a rehabilitation program. They generate more tension
early in rehabilitation and can perhaps help prevent a reflex disassociation by maintaining a
neurophysiologic pathway for muscle contraction. They are then used near the terminal stage of
rehabilitation to maximize the eccentric joint strength needed during daily activities.
Regarding circulation and isotonic exercise at the moment of contraction, the intramuscular pressure
increases. This forces blood into the veins and can be accomplished with only one-fifth of maximal
contraction. During relaxation, the increase in the capillary bed is then so extensive that the circulation
is 15—20 times greater than when the muscle is at rest. In this way, the circulation can supply the
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