Mechanics of normal muscular activity

[On tremor during movement (author's transl)]

An analysis of the movement of stretching the right forearm showed that the frequency of tremor slightly decreases, whereas the average (absolute) acceleration of tremor and the average electromyoactivity (EMA) of the agonist increase as expected.

Human strength: terminology, measurement, and interpretation of data

Application of strength data to human engineering problems is often hampered by ambiguities of both terminology and data. This paper attempts to point out some of the problems. After defining “strength” and clarifying related terms, mechanical, physiological, and statistical implications of strength testing are discussed. It becomes obvious that strength data are fully relevant to human engineering problems only if the operator must exert maximal static muscle force; if submaximal forces are required, the applicability of strength data is very limited. Research is needed to establish relations between human static force capacity and the abilities to perform maximal or submaximal dynamic work. At present there is little evidence that static force data accurately predict dynamic performance.

Effect of alterations in the thyroid state on the intrinsic contractile properties of isolated rat skeletal muscle

Contractile properties of soleus muscles isolated from 31 euthyroid (EU), 20 hyperthyroid (HT), and 18 myxedematous (MY) rats were studied in a myograph. At 100 stimuli/sec maximum isometric tension was essentially identical in EU (17.2 +/-0.5 g/mm(2)) and HT (17.7 +/-0.5 g/mm(2)) muscles, but was significantly depressed in MY muscles (11.5 +/-0.7 g/mm(2)). The rate of tension development was increased in HT (103 +/-4.5 g/sec per mm(2)) as compared to both EU (86.2 +/-4.6 g/sec per mm(2)) and MY (38.4 +/-2.2 g/sec per mm(2)) muscles, while the duration of the active state was shortened in HT (77.1 +/-2.3 msec) as compared to EU (105.1 +/-1.1 msec) muscles and was prolonged in MY muscles (153.3 +/-6.0 msec). The mean rate of isometric relaxation was 26.5 +/-4.9 g/mm(2) per sec in EU muscles, more rapid in HT muscles (33.1 +/-1.3 g/sec per mm(2)), and slower in MY muscles (16.0 +/- g/mm(2) per sec). The fusion frequency was greater in HT muscles, averaging 68.5 +/-3.6 stimuli/sec compared to EU muscles (38.1 +/-1.2 stimuli/sec) and to MY muscles (33.3 +/-4.0 stimuli/sec). At 40 stimuli/sec tension averaged 16.4 +/-0.8 g/mm(2) in EU muscles while at the same frequency tension was reduced in HT muscle, averaging 14.2 +/-0.5 g/mm(2). All differences were significant (P < 0.01). In conclusion, HT and MY result in profound alterations in the intrinsic contractile properties of skeletal muscle. While tension in HT muscles is maintained in vitro at a stimulus frequency of 100 stimuli/sec, the reduction in duration of active state may lower tension in vivo by preventing complete fusion of contractile events. In MY tension is reduced as a consequence of the lowered intensity of the active state. These changes explain, at least in part, the weakness of muscle activity in both HT and MY.

ENERGY OF CONTRACTION IN MUSCLE

A large body of the present day theory of production and absorption of work and heat by the contracting muscle is inferred from the experimental study of excised amphibian muscle at 0°. The question arises as to how well the results apply to in situ human muscles. In a study of the question, the thermodynamics of these muscles is discussed and an attempt made to evaluate directly their contraction energy characteristics. The study presumes that the muscle is analogous in some of its properties to both a mechanical spring and an electromagnet. The analogy is utilized to derive an analytical expression of the “biomechanical energy” of a body system in activity. The law of conservation of energy allows a comparison of this expression with the experimental measurement—assumed obtainable—of the “regional metabolism” of the system associated with the activity. The comparison would furnish the magnitudes of two heat coefficients which characterize the contraction energy of the in situ muscle.