Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

The sections in this article are:


1
Motor Unit Types
1.1
Muscle Fiber Types: Histochemical Profiles and Ultrastructural Correlations

1.2
Motor Unit Types: Physiological Profiles in Experimental Animals

1.3
Motor Units in Human Muscle

1.4
Stability of Motor Unit Types

1.5
Developmental Considerations

1.6
Skeletofusimotor Units




2
Anatomical Considerations
2.1
Anatomy of Muscle Units

2.2
Anatomy of Motor Nuclei

2.3
Motoneuron Anatomy in Relation to Unit Type

2.4
Electrophysiological Properties Intrinsic to Motoneurons

2.5
Organization of Synaptic Input

2.6
Control of Motoneuron Excitability: Interactive Factors




3
Control of Muscular Action: Recruitment and Rate Modulation
3.1
Motor Unit Recruitment

3.2
Precision and Stereotypy in Recruitment Process

3.3
Output Modulation by Rate and Pattern of Motoneuron Firing

3.4
Recruitment or Rate and Pattern Modulation?




4
Summary and Concluding Comments

Motor Units: Anatomy, Physiology, and Functional Organization

The sections in this article are:


1
Methodological Considerations
1.1
Selective Stimulation of Primary Afferents

1.2
Stimulation of Central Motor Systems

1.3
Methods for Investigation of Convergence at Interneuronal Level




2
Spinal Neuronal Circuits Used in Common by Segmental Afferents and Supraspinal Motor Centers
2.1
Recurrent Inhibition

2.2
Pathways From Ia-Afferents and Their Control by γ-Motoneurons

2.3
Reflex Pathways From Group Ib Tendon Organ Afferents

2.4
Reflex Pathways From Cutaneous and Joint Afferents and From Groups II and III Muscle Afferents

2.5
Propriospinal Neurons

2.6
Presynaptic Inhibition of Transmission From Primary Afferents




3
Reticulospinal Inhibition of Segmental Reflex Transmission
3.1
Dorsal Reticulospinal System

3.2
Ventral Reticulospinal Pathways

3.3
Monoaminergic Reticulospinal Pathways

3.4
Decerebrate Preparation




4
Direct Projections of Descending Pathways to α-Motoneurons

5
Ascending Pathways that Monitor Segmental Interneuronal Activity
5.1
Evidence That Ascending FRA Pathways Monitor Activity in Interneurons of Reflex Pathways

5.2
Information Via Ascending Collaterals of Interneurons

5.3
Ventral Flexor Reflex Tracts

5.4
Ventral Spinocerebellar Tract




6
General Summary and Epilogue
6.1
General Summary

6.2
Epilogue

Integration in Spinal Neuronal Systems

Inhibition of flexor burst generation by loading ankle extensor muscles in walking cats

Four selected leg muscles (gastrocnemius, soleus, vastus lateralis and intermedius) from thirty-two humans were autopsied within 25 hr of death and examined histochemically. The results of histochemical myofibrillar adenosine triphosphatase activity demonstrated that the soleus and vastus intermedius muscles have a higher proportion of slow twitch fibres (70%, 47%) than their synergists, gastrocnemius and vastus lateralis, respectively. The gastrocnemius contains about 50% slow twitch fibres and the vastus lateralis about 32%. Similar proportions of slow and fast twitch fibres have been reported for these hindlimb muscles in other mammals. Human muscles, however, differ from other mammalian muscles in that the proportion of slow and fast twitch fibres were similar in the superficial and deep regions of the muscles examined. Fast twitch oxidative glycolytic fibres in sedentary humans were observed less frequently, and they are less prominent in terms ofoxidative enzymatic activity when compared to similar fibres of several laboratory mammals studied previously.

Muscle fibre type populations of human leg muscles.

1.

The mechanical arrangement and transducing properties of Golgi tendon organs in soleus and anterior tibial muscles of anesthetized cats have been studied by noting responses of their Ib afferents to muscle stretch (passive force) and contraction (active force) of small portions of the muscle including functionally isolated motor units.




2.

Tendon organs were shown to be arranged both in-series and in-parallel with adjacent muscle fibers. There were gradations in these relations, the tightest arrangements involving the response to contraction of a single motor unit, brisk discharge from an in-series receptor and pause in the stretch-activated firing of an in-parallel receptor. Other arrangements included those in which groups of muscle fibers neither directly in-series nor in-parallel with a receptor were still found capable of influencing its firing pattern. If in-series muscle fibers maintained their contraction while in-parallel fibers were also contracting, the receptor usually responded slightly less actively than it did to the in-series force alone.




3.

Tendon organs were found to have a very low threshold to in-series force developed by muscle contraction. Responses were observed to as little as 0.5 gm of twitch tension. Minimum active force thresholds were similar for the two muscles studied, but thresholds to dynamic stretch were lower for anterior tibial receptors. Division of the dynamic stretch threshold by the minimum active force threshold gave a measure of the extent to which each tendon organ was more sensitive to active than passive force. These values (generally less than 50) did not negate the physiological significance of responses to passive stretch.




4.

The present data, together with those of Houk and his co-workers (1967, 1971) emphasize that tendon organs can participate in the moment to moment reflex control of normal muscle activity.

Mechanical arrangement and transducing properties of Golgi tendon organs.

1.

This report extends the work of Lundberg and Winsbury (1960a and 1960b) in emphasizing the usefulness of brief muscle stretch for selective activation of the primary endings of muscle spindles.




2.

In 36 experiments on anesthetized cats a distribution of rhomboid stretch (20 msec duration) thresholds was obtained for activation of 393 stretch receptors from de-efferented soleus muscles maintained at comparable degrees of initial tension by setting length to give a peak of active tension during an isometric twitch. These receptors included 153 Ia afferents with conduction velocities greater than 75 m/sec, 127 Ib afferents and 97 group II afferents with conduction velocities below 65 m/sec. A pull of 60 μ or less activated 95% of the Ia fibers in contrast to 41% of the group II fibers and only 3% of the Ib fibers. At threshold the mean latency (normalized) between the onset of stretch and impulse arrival at the spinal cord (recording electrodes) was 5.4 msec for the Ia fibers (S.D.±1.7 msec), 10.6 (± 3.9) msec for the group II fibers, and 9.1 (± 3.3) msec for the Ib afferents. In all cases only one impulse was initiated by the test stretch and no significant differences were encountered in the responses of receptors located in proximal, middle and distal portions of the muscle.




3.

In 7 experiments soleus nerve volleys were recorded at nerve-muscle and spinal cord-dorsal root levels during similar stretching procedures. A stretch of 5 μ amplitude was sufficient to generate a group I volley. As stretch amplitude was raised to 60 μ this volley grew progressively in size and decreased in duration as well as in latency from stretch onset to volley formation. A second volley, 1.5 to 2.0 msec after the first, developed with a 30 μ stretch. At 60 μ stretch its size was approximately 30–50% that of the first volley. Conduction velocity for the first volley was 90–110 m/sec and was faster than the second volley by 5–10 m/sec. Our data on isolated afferents suggest that the first volley is of Ia origin while the second volley represents predominantly a second discharge from some Ia afferents plus a minor group II input. Significant Ib contribution is excluded since only 4 of the 127 Ib fibers responded within this stretch range and then at latencies beyond formation of either volley.

Selective activation of Ia afferents by transient muscle stretch

Soleus and anterior tibial motor units of the cat

The physiological properties of motor units in cat medial gastrocnemius have been studied with particular reference to fatigue. A broad range of fatigability was encountered, particularly within the fast twitch group. These findings were consistent with the vascular and histochemical profile of this muscle.

R. L. Gerlach

Papers

Mechanical arrangement and transducing properties of Golgi tendon organs.

Selective activation of Ia afferents by transient muscle stretch

Soleus and anterior tibial motor units of the cat

Fatigability of Medial Gastrocnemius Motor Units in the Cat

Muscle receptor responses to transient stretches.