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It is known that the behavior of fast and slow skeletal muscles differs, but a comparison between fast and slow muscles with different stimulation patterns at various muscle lengths is lacking. The twitch, tetanus, and double pulse responses with a number of interval times have been investigated for a fast (EDL) and a slow (soleus) muscle of the rat at three muscle lengths (in vivo, at 37 degrees C, pentobarbital sodium anesthesia). The twitches of EDL and soleus change with muscle length, but the relative form of the ascending phase is independent of muscle length. The tetanus amplitude is independent of the muscle length over a considerable length range below the optimum twitch length. The form of the ascending phases of EDL-tetanus and double pulse responses (with certain interval times between the stimuli) varies with muscle length in contrast with the comparable soleus force patterns. The maximum slope in the descending phase of all contraction patterns decreases with increasing muscle length above the optimum twitch length.

191 SUMMARYA new ep!muscular method for stimulating single motor units (m.u.'s) in free prepared muscle is described. Unit isolation is stable and can be continued for long periods. The method is compared with an intramuscular stimulation technique and histological evidence is presented to confirm the validity of both techniques.Conventional methods for stimulation of individual motor-units {m.u.'s), i.e. stimulation of a.motoneuron or isolated ventral roots, are time-consuming and technically demanding. Mczeover such niethods are not suitable for human muscles. Buchthal et al.[1] stimulated in human muscle 'bundles of muscle fibres' with intramuscular electrodes. Several investigators [ 7,10--12 ] showed that such intramuscular stimulation may excite fine motor nerve twigs, which may lead to contraction of single m.u.'s.In this paper we will: (i) describe the difficulties involved in intramuscular stimulation of small musclm (EDL and soleus of rat); (fi) introduce 'epimuscul~ stimulation' to overcome these difficulties; (ifl) provide histological confirmation of the validity of both iutra-and epimuscular methods for single unit stimulation; (iv) ~ their limitations. The criterion we used ~~t all-or-nothing force and EMG adjured Close to the threshold value(1). ~ce our ex~" n~ dmmmd long term stable simulation of the same m:a.a second critmion~wmin~oduced, i.e. unvarying force and EMG reswhen ~ ~ ~al ~hbld stimulus amplitude (II), which

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~ ~i i !i i ĩi !ĩ!(i !i i ĩĩ ~ pic volur)om eering Division, PO muscle ffi lcroscop source,, ' of singh 7branes l rose cal homc ted fibre ivity to var Pt Within a, ;11 as the ] "onduction 7e radial d, est foJ th~ to an activ tential, Skeletal 1 Introduction IN HOMOGENEOUS volume conductor models of skeletal muscle tissue (used for example by ROSENFALCK, 1969; NANDEDKAR and ST~,LB~RG, 1983) the tissue has no structure. It is considered as a bulk material with anisotropic conductivities tr= (in the longitudinal fibre direction) and a, (radial). Usually, az and a, are taken real-valued. ALBERS et al. (1986) introduced a microscopic, frequency-dependent volume conductor model to study the relationship between SFAP and tissue structure. This network model accounted explicitly for the fibrous structure of skeletal muscle tissue. It was used for the calculation of the transfer function between a moving current source and the resulting extracellular potential (ALBERS et al., 1988). It was shown that, close to the source, the transfer function is significantly different from the transfer function calculated with a homogeneous, frequency-independent volume conductor model. This is a direct effect of the fibrous structure of the tissue in the inhomogeneous, capacitive model. In the