Musculo-Skeletal Interaction: What Limits Force Generation?

Abstract

Several strings of evidence suggest that skeletal muscles are intrinsically stronger than one would expect on grounds of the forces applied during habitual motor activities. Firstly, vertebral fractures, without any trauma, are a known complication of tetanus1. Of note, tetanus is causes spinal disinhibition that leads to forceful muscle contractions, which are thought to be responsible for these fractures that are not occurring with an un-impaired nervous system. Secondly, non-traumatic fractures of the spine2 as well as in the appendicular skeleton3 can occur during epileptic seizures. Again, forceful muscle contractions offer the most likely explanation. Thirdly, non-traumatic fractures have likewise been a known complication in the early days of electroconvulsive therapy4, so that the usage of muscle-relaxing medication has become mandatory in the modern form of electroconvulsive therapy4. In this intervention, a seizure-like state is induced by a strong current between two electrodes on the skull stimulating the brain. It is common to all three examples mentioned that the motor neurones are activated outside their physiological pathways, but that muscle activation occurs in a physiological way. Importantly, it is currently thought that habitual loads involve bone deformation up to 2,000 or 3,000 µstrain, and that bones fracture when deformed by more than 20,000 µstrain. That would mean that the fractures under discussion here must engender muscle forces that are almost 10 times greater than during habitual activity. The fourth string of evidence emerges from a study that applied electrical stimulation directly to the muscle and then measured the recruitment pattern with T2-weigted magnetic resonance tomography5. The outcome of that study suggests that muscles have the capability to contract at least 25% stronger than can be done by volition, and that this surplus might even be hampered by muscle the fatigue that was likely to be present in that study. The fifth line of evidence suggests that bone strength is rapidly re-gained after experimental bed rest6, and that it cannot be increased by large amounts through exercise7. Taken together, this indicates a very tight control of bone strength around an individually set value that is retained over large parts of life. Given that bones adapt to the forces they are exposed to8, it follows that the limit to force generation is a ‘hard-wired’ personal trait. Finding out what constitutes the postulated limit to force generation is highly relevant to our understanding of the muscukloskeletal system. The mechanism behind it will not only be important to exercise and sport science, but also for the emergence of osteoarthritis, and potentially to other clinical entities currently classified as ‘over-use’ injuries.