We evaluated abdominal muscle contractility and fatigue by measuring twitch gastric pressure (Pga t ) after percutaneous supramaximal electrical stimulation of the abdominal wall before and after sit-ups to task failure. Mouth pressures during maximal voluntary expulsive maneuvers (P E max) at TLC and FRC with superimposed twitches, and maximum voluntary ventilation (MVV) were also assessed. Mean fresh Pga t was 36.1 6 3.0 cm H 2 O with a coefficient of variation that ranged between 3.0 to 4.8%. Pga t decreased by 25% (p , 0.001) and 37% (p , 0.001) at 1 and 30 min after sit-ups. During maximal voluntary contraction twitch occlusion never occurred. P E max at TLC and FRC decreased by 15% (p , 0.001) and 11% (p , 0.017) at 1 min, and 8% (p , 0.036) and 9% (p , 0.030) at 30 min after sit-ups, respectively. Despite the abdominal muscle fatigue, MVV values at 1 and 30 min after situps were not significantly different from the value obtained before the sit-ups. We conclude that ( 1 ) Pga t is a useful objective indicator of abdominal muscle contractility and fatigue; ( 2 ) during maximal voluntary expulsive maneuvers the abdominal muscles are never fully activated; ( 3 ) sit-ups lead to substantial low-frequency fatigue but little high-frequency fatigue of the abdominal muscles, which has little effect on maximal breathing capacity. Suzuki J, Tanaka R, Yan S, Chen R, Macklem PT, Kayser B. Assessment of abdominal muscle contractility, strength, and fatigue. AM J RESPIR CRIT CARE MED 1999;159:1052‐1060.

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Assessment of Abdominal Muscle Contractility, Strength, and Fatigue

The proportions of fibers of different phenotypes in the rabbit masseter muscle differ strikingly in adult males and females. Muscles from females contain similar proportions of small fibers that expr

Sex differences in rabbit masseter muscle function.

Skeletal muscle has been used for biomechanical assist in experimental and clinical studies. Central to the success of these procedures is the generation of sufficient muscle force for the lifetime of the subject. Burst (tetanic) stimulation results in summation of individual twitches and generates higher power output. However, the superiority of paraneural versus intramuscular as well as proximal versus middle and distal intramuscular stimulations remains unclear. Electrophysiological mapping and mechanical performance of seven canine latissimus dorsi muscles were analyzed. The mechanism of higher tension generation produced by: (1) increased temporal summation; (2) greater motor units activated; or (3) result of both were determined. The parameters primarily dependent on the number of activated motor units are significantly greater following paraneural and proximal intramuscular stimulations. The parameters mainly related to temporal summation are not different between various electrode configurations. For intramuscular stimulation, it is the location of interelectrode field rather than the location of the cathode per se that determines the mechanical performance of the skeletal muscle. Furthermore, tension development of skeletal muscle is primary nerve activation rather than direct muscle stimulation. The higher tension generation that resulted from different electrode configurations is produced by activating a higher number of muscle fibers through the neuromuscular junctions.

James J. Elson

Papers

Correlation of electrophysiological activation patterns to tension generation in stimulated latissimus dorsi muscle.