Doctor of Philosophy
Dr. Anthony A. Vandervoort
Unaccustomed lengthening contractions have been shown to impair muscle function - however little is known regarding this impairment on muscle power - specifically, the velocity component of power during voluntary contractions in humans. The four studies presented in my thesis investigated power-loss following lengthening contractions in healthy young and old women and young men.
The purpose of Study 1 was to determine reliability of velocity-dependent power of the dorsiflexors using the isotonic mode of the Biodex Dynamometer. I determined the isotonic mode is reliable and can be used to track changes in velocity and power following fatigue and lengthening contractions.
The purpose of Study 2 was to investigate changes in neuromuscular properties of the ankle dorsiflexors during and following repetitive lengthening contractions and throughout recovery in 21 (10 men, and, 11 women) recreationally active young adults (25.8 ± 2.3 y). The protocol for the following 3 studies involves subjects performing 5 sets of 30 lengthening contractions, with neuromuscular measures (i.e., electrically evoked twitch, tetanus, voluntary activation, voluntary contractions) recorded at baseline, during the task, and throughout recovery. Exercise induced muscle damage ultimately led to velocity-dependent (i.e., isotonic) power loss at a moderate load (i.e., 20% maximum voluntary strength).
Compared with isometric and isokinetic tasks, less is known regarding velocity-dependent muscle power and recovery in older adults following repeated lengthening contractions. In Study 3 we tested 9 old (68.3 ± 6.1 y) and 9 young women (25.1 ± 1.3 y). Old were more impaired following the task than young as shown by greater low-frequency torque depression at task termination leading to a more pronounced initial loss of power than young. However, power remained reduced in both groups during the 30 min recovery period. Older women were more susceptible to power loss than young following lengthening contractions likely owing to a greater fatigue response.
In Study 4, power curves were constructed [8 men (27 ± 3 y), 8 women (26 ± 4 y)] using various isotonic loads before and following task termination. There was a preferential loss of power at higher loads, with a relative maintenance of maximal shortening velocity shifting the power curve down and leftward. When stressed with heavier loads during dynamic contractions, force modulators arranged in parallel seem to be affected more by damage than those organized in series (velocity), which was highlighted by the attenuation of power at higher versus lower resistances.
The main findings of my thesis are that repetitive lengthening contractions fatigued and temporarily weakened the dorsiflexors, thus impairing their power producing ability immediately (i.e., fatigue + weakness) and longer term (i.e., weakness) owing to an inability to generate torque rapidly.
Power, Geoffrey A., "Neuromuscular Function Following Lengthening Contractions" (2012). Electronic Thesis and Dissertation Repository. 962.