W ith the latissim us dorsi aiTanged as an in vivo preparation, there are tw o m utually exclusive options for organising a fixed and a mobile end to the muscle during contraction. One possibility is to clam p the humeral tendon firmly (usually achieved by clam ping the fore limb), and dissect away the distal attachm ents of the m uscle to the low er ribs, lumbai' fascia, sen atis anterior and thoracic portion of the trapezius muscle, attach this firm ly to an extensive clam p of some description and allow this to shorten against a force-length transducer. In addition, the proxim al portion of the muscle m ust also be exposed in order to place stimulating electrodes neai' the thoracodorsal nei*ve.
The single advantage of using this method is that it allows fiiTn and easy clam ping o f the hum eral tendon so that com pliance at this point is reduced to a m inim um , and does not interfere with the derived force records. The disadvantages aie that the en the muscle has to be exposed, and for a long testing procedure the muscle preparation has to be kept m oist and warm. In addition, significant distal muscle perforators have to be ligated and divided w hich could cause distal muscle ischaem ia with a consequent reduction in the m uscle performance.
The alternative is to attem pt to im m obilise the distal attachm ents of the m uscle, w hile ex p o sin g and d iv id in g the hum eral tendon, and attaching this to the fo rce-len g th tiansducer. The advantages of this m ethod aie that the m ajority of the m uscle rem ains unexposed, thereby m aintaining w arm th, and its blood supply is left totally intact; the
humeral tendon with its attached periosteum can m ore easily be clam ped than the w ide distal m uscle edge, and proxim al dissection of the m uscle is also ap p ro p ria te for stim ulating electrode placem ent. The disadvantages are that a reliable system o f distal m uscle clam ping m ust be designed and built, and, for the transform ed m uscles, m uscle testing requires re-operation into the site of the previously implanted stim ulator leads. The second approach described was adopted because it was felt to be im perative to m aintain the m uscle in peak condition, paiticularly during the intended long fatigue test. A fter initial work on cadavers and then in a pilot study it was found to be p ossible to reoperate on the proxim al portion of the m uscle follow ing previous lo ng-term lead im plantation and chronic electiical stimulation: It was also felt that the m uscle testing procedures w ould occur in a m ore physiologically appropriate direction since m uscle shortening w ould occur in a sim ilar m anner and direction to the norm al w orking o f the muscle, though this is not such an important consideration.
In order to com pletely im m obilise the distal portion of the m uscle a heavy "secon daiy" baseboard was constructed capable of holding firm ly the thoracic spine, ribs and lum bar fascia o f the animal which form the muscle's distal attachments. This consisted of tw o 200 mm steel 'runners' bolted to the baseboard 20 mm apart and drilled at 10 m m centres. The animal was laid supine along these runners and the thoracic spine fastened to it via two loops of 7 metric stainless steel wire (Ethicon Ltd) previously passed through the bodies of two suitable vertebrae using a 55 mm heavy trocar point needle and seized dow n to the runners with cardiothoracic wire needle holders. This technique is in com m on use in clinical caidiac surgei"y to appose the sternum post-operatively. The rest of the animal was passed into a stiff polypropylene 200 mm diam eter tube itself bolted firm ly to the base. The near end of the tube was cut to form a series of grooves running around the rim 1 cm from the edge and the whole tube was w rapped in m ultiple layers o f close-knit tubular bandage (tubigrip) with the inside cavity of the tube m aintained by the use o f 4 plastic sleeves bolted to the base through the tube wall.
Using 4 metric m onofilam ent polyam ide sutures (Ethicon Ltd) and a 90 mm 105^ curved needle, the ribs and lum bar fascia of the animal w ere sutured to the tube in a parachute fashion. This provided very firm longitudinal fixation of the m uscle w ith m inim al co m p liance but allow ed som e lateral rib m ovem ent and abdom inal ex cu rsio n for respiration.
This base board was itself bolted to the prim ary test bed which form ed the base for the force-length transducer and other ancillary equipment. In the event of the need to test other m uscles or species, the secondary base board can easily be rem oved and replaced with another suitably airanged to im mobilise the muscle under the test. Larger animal species could be accom m odated by bolting additional holding equipm ent to the fai' end of the rig platform , and in this regaid a sheep or goat pen was specifically considered. In addition, we w ere m indful of the lim ited space available in the bioscience d ep artm en ts and laboratories, and the entire rig was therefore built in a vertical stacking arrangem ent and placed on castors for ease of m ovement and storage (Fig. 3a. 1).
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a) Main te s t b ed b) C am bridge 31 OB force/length tra n sd u c e r d) 8 ch a n n e l m u scle stim ulator (one c h a n n el lead is show n)
f) S p a re s d raw er g) H arvard heating blanket control i) P e rc u ta n e o u s O2/C O2 m onitor j) P aediatric ventilator an d tubing
I) STA 1800 screw term inal a sse m b ly m) D rip-stand for adm inistration of IV fluids
0) EGG le a d s p) P e rc u ta n e o u s C O2/O2 monitor - a tta c h e d to left e a r
c) S e c o n d a ry te s t b e d for anim al preparation e) C am b rid g e 31 OB control electro n ics h) ECO m onitor
k) 4 8 6 DX c o m p u ter an d printer
n) Monitor a n d m o u se for ex p erim en t control