Professor David L Morgan

Research Interests:

The following summary introduces the paper listed below. Links are provided to web presentations at the end of each section. Further details are provided under the Monash University Centre for Biomedical Engineering page.

Effect of whole body vibration

Muscle mechanics: non-uniform sarcomeres.

I have a long involvement with the mechanical properties of muscle, at the whole animal, isolated muscle and single fibre level. A common thread over a long time has been the effects of non-uniformities between the lengths of the sarcomeres in a muscle fibre or fibril. The sarcomeres are the units of contractile material, structures of interdigitating protein filaments that slide past each other as the muscle changes length. Most models of muscle assume that all the sarcomeres change length together, so that a whole muscle can be considered as a scaled sarcomere. There is now good evidence to suggest that that is not true in a number of important situations.

Work in recent years has concentrated on the non-uniformities of sarcomere lengths during stretch of active muscle at lengths beyond the optimum for tension generation. The popping sarcomere hypothesis postulates that lengthening of muscle under these conditions does not take place by uniform lengthening of sarcomeres, but more nearly by popping, or rapid unstable lengthening, of sarcomeres one at a time in order from the weakest to the strongest. This is similar to stretching a flexible straw, where it is impossible to lengthen the folds uniformly.

Eccentric exercise

A branch of the study of lengthening muscle has been interest in eccentric exercise damage. This is the pain and tenderness in muscles typically felt one to two days after unaccustomed exercise involving lengthening of muscle while generating tension, known as eccentric exercise. Examples include climbing down mountains, running down hill, horse-riding and skiing, but not cycling, rowing or swimming, where active muscles are always shortening.

Such exercise is known to cause damage to cells, and the pain and recovery are resonably well understood. The mysteries have been why the damage occurs, and what changes with training to reduce the damage. I have suggested that it is the sarcomere non-uniformities that initiate the damage, and that training consists of growing extra sarcomeres in order to avoid working at long sarcomere length. Recent work has shown that decline exercised rats to have more sarcomeres in their fibres, and do generate optimum tension at longer muscle length. (Paper in review) Other work has shown extra series compliance after a series of stretches that is predicted by the theory in humans, whole muscles and single fibres, and shown the importance of muscle length in determining the extent of the damage.

The sensory consequences of ecentric exercise in terms of position and force sense have also been investigated.

Idiopathic toe walking

Some children walk on the balls of their feet, without heel contact. Some of these have identifiable neurological problems, such as cerebral palsy, both others do not. These are known as idiopathic toe walkers. Walking with contracted calf muscles leads to the inability to put heels on the ground. They are treaated by various interventions, ranging from waiting for them to grow out of it (which they usually do during puberty, though often with residual back and other problems) to surgery.

Our work predicts that eccentric exercise makes muscle fibres incorporate extra sarcomeres, and so grow longer. We are experimenting with eccentric exercise as a treatment, with very promising results. Many children can avoid more intrusive interventions, if they do their eccentric exercises regularly.

Hamstring injuries

It is noticable that hamstring tear injuries also occur during eccentric exercise at long muscle length, when the hamstring muscles are used to brake the forward swing of the leg during sprinting. This caused us to speculate that the microscopic damage associated with delayed onset muscle soreness may initiate a gross muscle tear under certain conditions. This raises the possibility that eccentric exercise of the hamstring muscles may reduce hamstring injury rates, and that susceptable players may be identifiable from their hamstring angle torque curves.

This has been tested primarily with St Kilda Football club, who have seen their hamstring injury rates fall from 16 in 2001, to 4 in 2002, and 2 in 2003. Other clubs have now joined the testing program.

Extended outline of work

IOC presentation on muscle length

IOC presentation on fibre type

Submaximal activation of muscle

Muscles are normally activated maximally in the laboratory and submaximally in the body. This is because the intact animal has many inputs to different parts of a single muscle (hundreds of motor units) where the experimenter usually only has one. One particular preparation uses 5-7 inputs to simulate physiological activation, but requires closely matched inputs, as they are stimulated in sequence to produce a smooth tension. (Old papers not in list.) I have proposed a modification of that process to generate smooth tension from a small number of unequal parts, by adjusting the time intervals in the stimulations sequence. Investigation of the proposal is proceeding, with simulation being complete, several different implementations of the stimulator having been built, preliminary experiments showing excellent results, and further experiments proceeding. Potential applications include functional electrical stimulation to return movement to paralysed limbs, as well as in the study of the properties of submaximally activated muscle.

Sensory organs in muscle

Collaborative work with the Physiology department concerns the study of nerve pulse generation and of the effects of muscle properties on sensory organ signals.

Role of calcium in activation of muscle

Calcium ions are the immediate messenger for activating muscle. It can be monitored by the use of fluorescent dyes that change the fluorescence when bound to a calcium ion. Work with colleagues at Harvard uses this extensively, particulary to determine the steady state relation between calcium and tension.

Fundamental mechanism of contraction

Models of muscle at the lowest level deal with the distribution of molecular bonds between the protein filament. I have a long standing interest in such models and the data that they need to fit.