SUMMARY1. In human subjects sustained static contractions of the quadriceps femoris in one leg were performed with the same absolute and the same relative intensity before and after partial neuromuscular blockade with either decamethonium or tubocurarine which reduced strength to about 50 % of the control value. During the contractions performed with the same absolute force, the magnitude of the cardiovascular responses (heart rate and blood pressure) was greater during neuromuscular blockade than during control contractions. During the contractions involving the same relative force the magnitude of the cardiovascular responses was almost the same with and without neuromuscular blockade. These findings were independent of the drug used.2. The metabolic part of the exercise pressor reflex was assessed by the application of an arterial cuff I min before cessation of exercise and for the following 3 min of rest. Although heart rate and blood pressure decreased after cessation of exercise, application of the tourniquet resulted in higher post-exercise values and this effect was seen both with and without neuromuscular blockade.3. Muscle biopsies from the subjects' m. vastus lateralis were analysed for fastand slow-twitch fibre composition showing 27-66 % slow-twitch fibres. No correlation was found between cardiovascular responses to static exercise, with or without neuromuscular blockade, and fibre type predominance. 4. The results suggest that the involvement of fast-or slow-twitch muscle fibres does not play a dominant role in the cardiovascular responses to static exercise in man. Both central command and reflex neural mechanisms are of importance, and it appears that these two control mechanisms are redundant and that neural occlusion may be operative. However, when partial neuromuscular blockade induces a disproportion between an increase in central command and a constant or decreasing muscle tension and metabolism, the larger signal arising from central command determines the magnitude of the cardiovascular responses.

Two- and one-leg extension strengths were compared in 155 female and male, untrained and trained (eight bicyclists, 38 weight-lifters) subjects and in a polio patient with almost no strength in one leg. On average two-leg strength was 82 +/- 1.3% (SE) of the sum of the strength of the right and left legs (leg-strength ratio) with no significant difference between the untrained and trained subjects. In the polio patient two-leg strength was lower than the strength of the strongest leg. A similar leg-strength ratio was found when measurements were made with a knee angle of 150 degrees (n = 128) and 90 degrees (n = 25). The ratio did not change when re-determined after 1 year (n = 9). Moderate changes in the subjects' position during measurements did not affect one-leg strength (n = 13). However, familiarization with the experimental apparatus increased the leg-strength ratio from 80 +/- 2.5% to 97 +/- 2.9% (P less than 0.01) (n = 18). In contrast, two-arm extension strength was similar to the sum of the strength of the right and left arms (n = 15). The results demonstrate that two-leg strength in general is lower than the sum of the strength of the right and left legs, and that this phenomenon is found even after intensive endurance or strength training. However, familiarization with the experimental apparatus can increase the relative two-leg strength which suggests that muscle strength in man may be limited by the central nervous systems' ability to activate maximally all synergistic muscles and/or to control antagonistic muscles.

The effect of a bolus injection of tubocurarine (0.1-0.13 mg X kg-1 i.v.) was followed in six young male subjects by registration of the rectified smoothed electromyogram (rsEMG) from the m. soleus (71 +/- 5.1 (s.e. mean) % slow twitch muscle fibers) and from the m. gastrocnemius (54 +/- 3.1% slow twitch muscle fibers). Volitional muscle strength was recorded in isometric plantar flexions with the knee fully (0 degrees) extended where m. soleus and m. gastrocnemius both are active, and with the knee bent 90 degrees where m. soleus is dominating force development. During maximal action of the drug, the rsEMG from the soleus muscle was reduced to 30 +/- 4.0% of the control value, while the rsEMG from the gastrocnemius muscle was reduced to 53 +/- 5.2% (P less than 0.01). Muscle strength with the knee extended showed 53 +/- 7.5% force left, while 44 +/- 6.4% of the muscle strength remained when the knee was bent (P less than 0.01). The results suggest that tubocurarine affects human muscles in proportion to their slow twitch muscle fiber content.