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PubMed1999

1996 Remington lecture: modeling multivariate longitudinal data that are incomplete.

PURPOSE: We describe the impact that missing data may have on model selection for longitudinal multivariate data. METHODS: Maximum likelihood was used to fit several models to ultrasonographic measurements from the Asymptomatic Carotid Artery Progression Study (ACAPS). Graphical techniques were used to examine evidence concerning the underlying missing data mechanisms associated with each model. RESULTS: Using statistical methodology that addressed missing data substantially increased the statistical efficiency of our analysis of ultrasonographic data. Only complex models that included segment-specific parameterizations for longitudinal correlations appeared to allow missing data to be assumed to occur at random. CONCLUSION: Ignoring the nature of missing data in conducting statistical analyses can have serious consequences when missingness is not rare. It may be necessary to fit models of high dimension with maximum likelihood techniques to address missing data appropriately, however these approaches may improve statistical efficiency.

Carotid Stenosis↗
PubMed1998

1998 ISEK Congress Keynote Lecture: The use of electromyography in applied physiology. International Society of Electrophysiology and Kinesiology.

Electromyogram (EMG) analyses (surface, intramuscular and evoked potentials) in studies of muscle function have attracted increasing attention during recent years and have been applied to assess muscle endurance capacity, anaerobic and lactate thresholds, muscle biomechanics, motor learning, neuromuscular relaxation, optimal walking and pedalling speeds, muscle soreness, neuromuscular diseases, motor unit (MU) activities (MU recruitment and rate coding), and skeletal muscle fatigue. This paper deals with the use of EMG analyses employed in the area of applied physiology and is divided into three sections: surface EMG analyses; intramuscular EMG analyses; and evoked potential analyses.

Adaptation, Physiological↗
PubMed1998

1998 ISEK Congress Keynote Lecture: Multi-muscle control in human movements. International Society of Electrophysiology and Kinesiology.

It has been suggested that the coordination of the activity of multiple muscles results from the comparison of the actual configuration of the body with a referent configuration specified by the nervous system so that the recruitment and gradation of the activity of each skeletal muscle depend on the difference between these two configurations. Active movements may be produced by the modification of the referent configuration. The hypothesis predicts the existence of a global minimum in electromyographic (EMG) activity of multiple muscles during movements involving reversals in direction. This prediction was tested in five subjects by analysing movements resembling the act of reaching for an object placed beyond one's reach from a sitting position. In such movements, initially sitting subjects raise their body to a semi-standing position and then return to sitting. Consistent with the hypothesis is the observation of a global minimum in the surface EMG activity of 16 muscles of the arm, trunk and leg at a specific phase of the movement. When the minimum occurred, EMG activity of each muscle did not exceed 2-7% of its maximal activity during the movement. As predicted, global EMG minima occurred at the phase corresponding to the reversal in movement direction, that is, during the transition from raising to lowering of the body. The global EMG minimum may represent the point at which temporal matching occurs between the actual and the referent body configurations. This study implies a specific link between motor behavior and the geometric shape of the body modified by the brain according to the desired action.

Abdominal Muscles↗
PubMed1998

1998 ISEK Congress Keynote Lecture: Motor units: how many, how large, what kind? International Society of Electrophysiology and Kinesiology.

There are now at least nine methods for motor unit number estimation (MUNE) in living human muscles. All methods are based on the comparison of an average single motor unit potential (or twitch) with the response of the whole muscle. Such estimations have been performed for proximal and distal muscles of the arm and leg in healthy subjects and in patients with various neuromuscular disorders. In healthy subjects there is a loss of motor units which is most evident in distal muscles and after the age of 60 years. Substantial losses of motor units have been measured in patients with ALS, post-polio symptoms, and diabetic peripheral neuropathies. In contrast, normal MUNEs have been found in approximately half of patients with persisting obstetric brachial palsies. The sizes of motor units show considerable variations within the same muscle and also between muscles; very large units are usually present in severe partial denervation. Although many motor unit properties are largely governed by motoneurons, some exhibit less plasticity in humans than in other mammals.

Adolescent↗
PubMed2000

1999 Costenbader Lecture. Outcome study in amblyopia: treatment and practice pattern variations.

This study retrospectively evaluates the results of amblyopia therapy and suggests hypotheses for future study. We address the various methods of treatment and evaluate the results from the most common therapy techniques. Practice pattern variations were analyzed in addition to the analysis of overall results. For ophthalmologists, there is a need to determine whether actual medical practice approaches the established standard of care, if it exists at all. How often are medical procedures, thought to be appropriate, based on anecdotal observation (case reports) rather than hard data (clinical trials)? The 3 types of vision loss evaluated were strabismic, anisometropic, and deprivation amblyopia. The methods of treatment studied were full-time patching part-time occlusion, penalization, and occlusion of the contact lens. Nine centers, thought to have private as well as indigent patients, were recruited to participate in this study. The centers responded by filling out an extensive questionnaire and sending the information through the World Wide Web for inclusion in a spreadsheet. This information was then collated, and various statistical programs tabulated the results. Although trends, as a consequence of therapy, are suggested from our retrospective analysis, concrete results can only arise from a randomized prospective study. The study included 279 patients. There were a similar number of male and female patients. Only 77% of the patients without fusion before treatment had either single binocular vision or peripheral fusion at the conclusion of therapy. The log improvement of vision was significant in each group. Factors that potentially influenced the results were severity of distance acuity in the amblyopic eye before treatment, duration of treatment, and length of daily patching. The paper suggests that worse vision, not better vision, at the beginning, predicts better outcome in terms of improvement of visual acuity. For example, visual acuity less than 20/70 at the initiation of treatment led to better visual results of geometric log improvement. Surprisingly, among the 9 centers studied, there was a statistically significant difference in many of the areas related to practice patterns. Patient compliance, which directly affects outcome, was highly variable and is a factor that may be readily influenced by the treating physician.

Amblyopia↗