For this study it was hypothesized that when participants intended to perform a maximum voluntary concentric (or eccentric) contraction but had an eccentric (or concentric) contraction imposed upon them, the initial EMG measured during the isometric phase preceding the onset of the dynamometer motion would reflect the intended contraction condition. The surface EMG of the vastus lateralis muscle was measured in 24 participants performing isokinetic concentric and eccentric maximum voluntary knee extensor contractions. The contractions were initiated from rest and from the same knee flexion angle and required the same level of external force to trigger the onset of dynamometer motion. Vastus lateralis EMG were quantified during the isometric phase preceding the onset of the dynamometer motion. When participants intended to perform a concentric contraction but had an eccentric contraction imposed upon them, the initial EMG resembled that of a concentric contraction. When they intended to perform an eccentric contraction but had a concentric contraction imposed upon them, the initial EMG resembled that of an eccentric contraction. Overall, the difference between concentric and eccentric contractions observed during the period of the initial muscle activation implies that descending signals include information that distinguishes between eccentric and concentric contractions.
Mark D. Grabiner and Tammy M. Owings
Masao Tomioka, Tammy M. Owings and Mark D. Grabiner
We previously reported that lower extremity muscular strength of older adults did not predict success of a balance recovery task. We propose that lower extremity coordination may limit performance independently of lower extremity strength. The present study was conducted to determine the extent to which knee extension strength and hip–knee coordination independently contribute to maximum vertical jump height. Maximum vertical jump height and isometric and isokinetic knee extension strength and power were determined in 13 young adults. Hip–knee coordination during the vertical jump was quantified using relative phase angles. Stepwise nonlinear multiple regression determined the variable set that best modeled the relationship between the dependent variable, maximum vertical jump height, and the independent variables of strength, power, and coordination. The quadratic terms of the normalized knee extension strength at 60 deg·s–1, and the average relative phase during the propulsion phase of the vertical jump, collectively accounted for more than 80% of the shared variance (p = .001). The standardized regression coefficients of the two terms, .59 and .52, respectively (p = .004 and .008), indicated the independence and significance of the contributions of knee extension strength and hip–knee coordination to maximum vertical jump height. Despite the pitfalls of extrapolating these results to older adults performing a balance recovery task, the results are interpreted as supporting the contention that while muscle strength confers a number of functional benefits, the ability to avoid falling as a result of a trip is not necessarily ensured. Increased muscle strength per se can occur in the absence of improved kinematic coordination.