Study Design: Repeated measures analysis o i joint angle e i i~t s on hip and k n w muscle cltrtromyographic IEMGI activitv. Objectives: To simultan~~usly determine angle-dependent changes in maximal voluntary isometric contraction IMVICI torque and EMG activity during hip extension and knee ilexion. Background: Procedures ior normalizing EMG data and ior determining torque-angle relationships for various joint motions both entail asking suhjects to exert an MVIC. The implicit assumption in t h r w paradigms is that magnitude of the EMG response is at a constant, m.iximum level so that ohservcd angle-tlepcndmt variations in torque are due to mechanical iactors, such as muscle length and muscle moment arm.Methods and Measures: Fifty suhjects (25 men and 25 women1 particip.itcd in this study (age, 23.5 + 4.6 y; range, 18-38 y). Subjects periormed maximal isometric knee ilexion at 4 kncv angles and maximal isometric h~p extension at 4 hip angles. The dependent variahlrr were nnrmalizrd root-mean-square EMG and torque. The process ior normalizing EMG and torque data consisted o i determining the largc3t mean value ior each suhject across tcrting positions for the muscle o i interest. That value was designated as corrcqmnding to 100% MVIC, and all other data ior that muscle were expressed as a percentage of the MVIC value. Repeated measures was used to determine angle-dependent changes in normalized MVICtorque and MVIC-EMG values ior each muscle group. Results: Mean torque-angle relationships were generally consistent with previous r e p m , though ~~n s i d e r d~l e intersuhject variability was observed. There were signiiicant .inglc-dependent diiierenccr in maximal EMG ior Ix)th the hamstring and gluteus maximus muscles. Worrell, Duke Universitv Medical Center, Deptartment oi Physical and Occupational Therapy PO 3965, Durham, NC 27710. E-mail: ~vorreOO6@mc.duke.edu xperimental approache.5 to the study of neuromuscular function frequently entail asking subjects to perform a maximal voluntary isornetric contraction (MVIC) of selected muscle groups. This approach is commonly used for amplitude normalization of electroinyographic (EMG) activity in order to determine the potential efficacv of various strengthening exercises by comparing the relative activity elicited by each e x e r c i~e . " . ' . ' .~"~ Maximal voluntary isometric contraction has also been used to determine in vivo torque-angle relationships of various joint motions by having subjects exert an MVIC at multiple points in the range of lnotion.~.~ t.21.22 The implicit assumption in these paradigms is that the instruction to perform an MVIC elicits a constant, rnaxi~nal level of motor iunit activity in the muscle of interest.The question of whether performance of an MVIC results in a

These findings contradict a previous report of differences in reflex timing related to PFPS. Differences in the relative timing of onset of EMG activity of the VMO and VL during voluntary knee extension were not significant between SYMP and ASYMP groups, and were not related to the relative timing differences observed during reflex testing.

COP measures show clear differences when comparing healthy adults with minimally impaired adults with MS. The lack of between-group differences when COP displacement during reaching was expressed as a percentage of the maximum COP displacement during leaning suggests that the subjects with MS adopt a reaching strategy that allows them to stay within their reduced limits of stability. COP measures during standing tasks appear well-suited to quantifying changes in postural control over time or in response to intervention for minimally impaired persons with MS.

1. We studied electromyographic (EMG) and kinematic features of self-paced human arm movements involving rotations about the shoulder and elbow joints. Movements were initiated from various positions and covered much of the reachable work space in the horizontal plane. The attempt was to characterize robust features of the relative timing and magnitude of the EMG activity at the two joints, and to correlate them with variables related to the initial and final positions. 2. The pattern of muscle activity at each joint was typically characterized by bursts of alternating agonist and antagonist activity, comparable with the three-burst pattern associated with single-joint movements. As the spatial direction of the target was altered, the magnitude of each burst was modulated over a continuous range. Modulation down to zero activity was observed, not only for later bursts, as has been shown in some cases of single-joint movements, but for the first agonist burst as well. 3. In the preceding paper we showed that the choice of agonists (i.e., flexors or extensors) at each joint is predictable on the basis of the target direction relative to the distal segment (psi). Here, we present quantitative analyses of initial agonist EMG activity at the shoulder and elbow, which reveal that the onset-time difference between agonists at the two joints also varied systematically with psi, and so did their relative magnitude. 4. For most target directions, initial EMG activity at the shoulder preceded that at the elbow by 5-40 ms. Exceptions were observed mainly for target directions near the transitions between initial flexor and initial extensor activity at the shoulder. In these cases the initial agonist activity at the shoulder was greatly reduced or, in some cases, appeared entirely suppressed, although the later bursts were present in their usual temporal alignment with the corresponding bursts at the elbow. 5. Antagonist onset at the elbow tended to precede antagonist onset at the shoulder, but the difference in timing did not vary consistently with psi. 6. Despite the consistency of initial agonist timing between the two joints, the agonist onset-time difference was poorly correlated with the apparent difference in the onset times of shoulder and elbow joint rotations. The latter difference, which is affected by mechanics, cannot therefore be imputed directly to the CNS.(ABSTRACT TRUNCATED AT 400 WORDS)

These findings do not support the notion that concurrent use of the hip adductors during knee extensor exercises results in preferential strengthening of the VMO.