Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action.

The purpose of this study was to estimate the efficiency of ATP utilization for concentric, eccentric, and isometric muscle action in the human tibialis anterior and extensor digitorum longus in vivo. A dynamometer was used to quantitate muscle work, or tension, while simultaneous 31P-nuclear magnetic resonance data were collected to monitor ATP, phosphocreatine, inorganic phosphate, and pH. The relative efficiency of the actions was estimated in two ways: steady-state effects on high-energy phosphates and a direct comparison of ATP synthesis rates with work. In the steady state, the cytosolic free energy dropped to the lowest value with concentric activity, followed by eccentric and isometric action for comparative muscle tensions. Estimates of ATP synthesis rates revealed a mechanochemical efficiency [i.e., ATP production rate/work (both in J/s)] of 15.0 +/- 1.3% in concentric and 34.7 +/- 6.1% in eccentric activity. The estimated maximum ATP production rate was highest in concentric action, suggesting an activation of energy metabolism under these conditions. By using direct measures of metabolic strain and ATP turnover, these data demonstrate a decreasing metabolic efficiency in human muscle action from isometric, to eccentric, to concentric action.

Normalized metabolic stress for 31P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume.

A critical requirement of submaximal exercise tests is the comparability of workload and associated metabolic stress between subjects. In this study, 31P-magnetic resonance spectroscopy was used to estimate metabolic strain in the soleus muscle during dynamic, submaximal plantar flexion in which target torque was 10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy, normally active adults, (PCr + Pi)/PCr, where PCr is phosphocreatine, was highly correlated with power output normalized to the volume of muscle in the plantar flexor compartment (r = 0.89, P < 0.001). The same variable was also correlated, although less strongly (r = 0.78, P < 0.001), with power normalized to plantar flexor cross-sectional area. These findings suggest that comparable levels of metabolic strain can be obtained in subjects of different size when the power output, or stress, for dynamic plantar flexion is selected as a function of plantar flexor muscle volume. In contrast, selecting power output as a function of MVC resulted in a positive linear relationship between (PCr + Pi)/PCr and the torque produced, indicating that metabolic strain was increasing rather than achieving constancy as a function of MVC. These findings provide new insight into the design of dynamic muscle contraction protocols aimed at detecting metabolic differences between subjects of different body size but having similar blood flow capacity and mitochondrial volume per unit of muscle.

A multimode dynamometer for in vivo MRS studies of human skeletal muscle.

The implementation of muscle ergometry during magnetic resonance spectroscopy and imaging is complicated by the restrictive dimensions of the magnet bore and the presence of a strong static magnetic field. We have developed a dynamometer that is compatible with these constraints. This device can provide resistance to voluntary muscle contraction during isometric, isokinetic concentric, and isokinetic eccentric muscle contractions. While controlling muscle contraction speed, the dynamometer simultaneously records muscle torque production at a 10-Hz sampling frequency to allow assessment of compliance and retrospective normalization of power output for the mass of active muscle. All parameters relevant to muscle contraction are selectable, including percentage of maximal voluntary contraction, velocity of muscle contraction, duty cycle, and range of motion for the contraction. This paper provides examples of 31P-magnetic resonance spectroscopic measurements during isokinetic concentric contractions of the ankle dorsiflexors, isokinetic eccentric contractions of the soleus, and isometric contractions of the soleus. Operation of the dynamometer has no adverse effects on the integrity of the 31P-magnetic resonance spectra at 4 T, permitting temporal resolution of the phosphocreatine resynthesis rate of approximately 1 spectrum/s. The unique capabilities of this dynamometer will facilitate studies into the metabolic response of working muscle in healthy and diseased populations.

Scholarship, consensus and advocacy: a comment on Cameron.

While Cameron is right about the need for Christians to contribute more visibly to medical ethical debates, he is wrong in his analysis of bioethics. Marsha D.M. Fowler and Andrew Jameton find specific problems in several points of Cameron's argument, particularly where the Hippocratic Oath and patient autonomy are concerned.

Professional associations, ethics, and society.

This article examines the role of professional associations, with particular reference to the Oncology Nursing Society (ONS), in relation to professional ethics. Also explored are the relationship between power structures and meaning and value structures, the relationship between the ideal of a profession and the reality of clinical practice, and the three functions of social ethics. Aspects of professional associations, ethics, and society are examined in terms of their reflection in the current ethical standards outlined by ONS in order to provide recommendations for future directions in ethics standards and oncology nursing practice.

Ethical decision making in clinical practice.

Contemporary nursing ethics education focuses on the use of an analytical model of ethical decision making for both its process and its content. Perhaps this is the case because it bears some resemblance to the nursing process, which is taught in a similar fashion. Thus, a deductivist method of ethical decision making fits within the same general schema of the hypotheticodeductive method of decision making that is taught for nursing diagnosis. Ethics requires that nurses respect persons, inform patients and secure their consent, not inflict harm, preserve the patient's quality of life, prevent harm and remove harmful conditions, do good for patients, and minimize risk to themselves. These are among the norms of obligation that guide ethical analysis and judgment in nursing practice and are the substance of the analytical model of ethical decision making. Nursing's ethics has established high ideals and strong demands for nurses. These are demands which nurses have met and ideals which have often been realized. Whatever the strength of our science, nursing is an inherently moral endeavor and is only as strong as its commitment to its ethical obligations and values. Between the grinding edges of the forces that affect it, nursing must establish its priorities among the aspects of its environment that it attempts to control. Ethics must be chief among those priorities.

Ethical inquiry and the concept of research.

The place of scholarly research in nursing has not been well studied. Ethical inquiry, as scholarly research, has been included in this neglect. The concept of ethics research is defended and new research approaches to ethics are outlined. Forms of ethics research identified are scholarly, empirical, and dual mode. Questions considered are: Is ethics research research? What are the forms of ethics research? What is the role of empiricism in ethics research? And what is the relationship of ethics research to the moral concerns of nursing? Ethics research explores the basic moral norms undergirding nursing research, practice, and education.