THE IN VIVO EXPRESSION OF THE FORCE-LENGTH RELATIONSHIP AND ITS EFFECT ON SUSTAINED ISOMETRIC FORCE
Open Access
- Author:
- Winter, Samantha Lee
- Graduate Program:
- Kinesiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 21, 2007
- Committee Members:
- John Henry Challis, Committee Chair/Co-Chair
Philip Martin, Committee Member
Robert Barry Eckhardt, Committee Member
James Landis Rosenberger, Committee Member - Keywords:
- force-length relationship
biomechanics
steadiness - Abstract:
- A fundamental property of muscle is the relationship between muscle fiber length and the force it produces. In vivo this relationship is expressed when isometric muscle actions are produced. In this study two aspects of the force-length properties of muscle will be examined. Firstly the source of expressed section of the force-length curve in vivo will be examined, and secondly the fluctuations in force during isometric muscle actions will be examined. A generalized model of a mono-articular muscle-tendon complex was used to examine the effect of various model parameters on the section of the force-length relationship operated over for a 90 degree joint range of motion. Key architectural properties of muscle were identified and then were systematically varied in the model to examine their influence on the expressed section of the force-length curve. It was shown that the ratio of tendon resting length to muscle fiber optimum length was important in determining the amount of variability in the expressed section of the force-length curve. The effect of this ratio was modulated by the ratio of muscle fiber optimum length to average moment arm. These results indicate there is scope for inter-individual variation in the expressed section of the force-length curve. In groups of young (19 to 28 years old) and young-old females (66 to 71 years old) isometric knee extensions were performed at 25%, 50%, 75% and 100% of maximum effort. The fluctuations in the moment records were quantified using standard statistical techniques (e.g. standard deviation and coefficient of variation) and methods from statistical physics (e.g., signal complexity, and signal fractal properties). There was no difference between age groups or effort levels in the coefficient of variation. The young females demonstrated more complexity in the knee joint moment signal at all effort levels. There was also different fractal like scaling behavior over shorter and longer timescales, suggesting that two or more different processes may be responsible for the fluctuations in the joint moment during isometric contractions. Young-old females generally demonstrated scaling characteristics that indicated more slowly repeating processes. To investigate to what extent these differences were influenced by strength levels a similar group of young and young-old females participated in a ten week program of strength training. Strength training produced a small decrease in the coefficient of variation but not in the standard deviation of the moment record in both the young and the older age groups. The complexity of the joint moment record and the fractal like scaling remained unchanged due to training, this suggests that these measures reflect a training resistant factor that deteriorates with age, a possible candidate is the calcium kinetics at the level of the sarcoplasmic reticulum. In the final experiment a single muscle was examined, which was the first dorsal interosseus. By using this muscle the length and potentially the expressed section of the force-length curve could be manipulated. Subjects produced isometric contractions at long, medium and short muscle lengths and at 5%, 10%, 25%, 50%, 75% and 100% of maximum in three finger positions corresponding to short, medium, and long muscle lengths. The fractal like properties of the signal indicated that two or more different processes may be responsible for the fluctuations in the force produced during isometric contractions. The magnitude of the coefficient of variation was greatest at short muscle lengths, and the fractal like properties were also altered at these lengths; there was no indication of any influence of expressed section. Different motor unit firing characteristics required to achieve full activation at short muscle lengths may explain these results. The differences in the results for this muscle compared with the quadriceps muscle may be due to differences in the relative contributions of motor unit recruitment and rate coding to force gradation in these muscles.