Dr David McGrath

Spine Physician

MB BS (Hons) FAFOM, RACP, FAFMM
Master of Pain Medicine

Muscles and Exercise

Muscles are chemical to mechanical energy transformers. In doing so they shorten against load. When we exercise there are many variables or boundary conditions which the muscles must overcome. This is worthy of some thought. Here are some typical situations.

1. Iso-metric exercise occurs when the joints are preventing from moving. This can useful when the pain free dynamic range of the involved joints is virtually zero. Given that the muscular force must be balanced by an equal opposite force (or torque about a rotation axis) the structures involved are compressed. Potential benefits include an increase in endurance capacity, better blood flow and strength if the load is increased. Potential harm can come from the compressive load of the balanced torque, possible ischemia (blood flow insufficiency) or inability to detect strain under a static neurological condition. (the nervous system tends to ignore a constant )

2. Isokinetic exercise in one in which the speed of movement is kept constant.

3. Isodynamic implies work against a constant force or torque. The work performed is equal to the angular displacement times the opposing torque. Given a constant torque, energy expenditure is proportional to the angular displacement, or size of movement. An isokinetic and isodynamic movement would use a constant energy expenditure per unit of time. ie A constant power output

4. If the muscle force exceeds the resistive torque, the limb will accelerate. (increase speed). If the torque exceeds muscle force, the limb will slow down. In general most movements have variable speed.

5. Torque generated by gravity will in general vary as the joint angle changes.This is because gravity remains in the same direction while the opposing limb changes direction. The torque generated by the muscles will also alter with joint angle, being maximum at 90 degrees.

6. Most movements are against a variable torque with variable angular speed through range. If the torque decreased as speed increased the power output could still be constant over range.

7. An isokinetic movement is truly a marvel given the constantly changing opposing torque and changing mechanical efficiency of the muscles. The brain has to constantly change the muscle force to allow an exact torque balance, leading to no change in velocity, at some point following an acceleration.

8. Given most movements are against variable torque (ie not isodynamic) an isokinetic movement would in general create an uneven power profile.

9. There is another consideration. Muscles can generate force and power as the muscle is shortening (concentric) or lengthening (eccentric). Lowering a load to the floor is eccentric while lifting is concentric.

10. The physiological effects of common functional environments are difficult to gauge, and leaves uncertain the idea of an optimal exercise, for the health of the muscles and the nervous system that controls them.

(to be continued)