Tying the knee joint

Posted on Sat, 14 September 2013

I was recently asked about the safety of tying on the knee joint. After all, it is quite possible for a suspension rope to slip into the apex of the bent knee instead of remaining on the thigh. I must admit, although it is something I have been taught to avoid, I have never been certain what problems it might create. So, I decided to ask my tame medical consultant. This is what he said:[private]

"The large tendons which form the medial and lateral walls of the popliteal fossa (the space in the back of the knee) are derived from the ends of the biceps femoris and semitendinous & semimembranosus muscles above and from gastrocnemius below (the largest element of the calf muscles) - in the extended position of the joint these will be under tension and so provide pretty good protection to the nerves and blood vessels running deep through the fossa. In suspension though, the knee's going to be flexed, so these tendons will be relaxed, leaving the wall of the fossa "lower" and less resistant to the weight of the limb pushing down on the rope. Pressure on the common peroneal nerve at the level of knee sufficient to cause nerve impairment can be seen in people who have managed to fall asleep with their lower legs dangling over the edge of a railing, or in prolonged squatting, a similar mechanism to a rope under here. The temporary symptoms you'd expect from this would include sensory, leg/foot gone numb, and motor, with a so called "foot drop" where the affected person can't raise their foot upwards at the ankle, so that it drags or trips them in walking.

Obviously the less fat you've got behind the knee, the more vulnerable the nerve is going to be, so as usual, thin people may be more vulnerable, although they'll tend to weigh less overall. The other thing to bear in mind is that the reason we're advised not to cross legs on long haul flights is because the pressure this causes behind the knee reduces the flow rate in the veins of the lower leg, leading to venous stasis and an increased risk of DVT. Theoretically this mechanism would apply to a behind the knee suspension, but I really don't think this would be any significant risk unless the suspension (and therefore reduced flow) was for a greater length of time than is normal.

As with all these things, it does come down to time of exposure to a degree, so a briefer suspension behind the knee will carry less risk than longer, but there are anatomical reasons why this position carries risk, so I'd suggest minimising the time for which this position is held, and keeping a keen eye out for signs of numbness of pain developing in the foot."

So there you have it. A bent knee if liable to provide little protection so watch out if your rope slips.

As an aside, on New Year's Eve at Torture Garden, I suffered a tenuosly bondage related injury. A girl, who appeared to have been over indulging in one way or another, was dancing with a considerable degree of imbalance and then started fooling about on the suspension ring doing gymnastics, while I was sitting with legs splayed out tidying up my ropes. She lost her grip. When she plummeted to the floor some part of her came into heavy contact just inside of the top of the bone of my lower leg behind the knee, on the outer side. It appears that she landed the medial femural cutantaneous nerve or thereabouts. Anyway, the end result was mild symptoms of nerve injury on the upper side of my foot, i.e. slight numbness and a tingly feeling when I ran my nails across it, and some deep bruising. Two weeks later, it still aches but it has always been a bit sensitive since I twisted it badly landing a paraglider.I later heard that another girl has actually knocked herself out doing a similar stunt. So, suspension can be dangerous, especially if those under the influence play with the kit.[/private]