Comparison of the activities listed in Table 2 indicates that violin playing and housekeeping use up to 30 kcal/hr, or
of power, more than standing. Most of this power is generated by moving the upper limbs. Empirical studies done by [Braune and Fischer, 1987] at the turn of the century show that for a particular 58.7 kg man, the lower arm plus hand masses 1.4 kg, the upper arm 1.8 kg, and the whole arm 3.2 kg. The distance through which the center of mass of the lower arm moves for a full bicep curl is 0.335 m, while raising the arm fully over the head moves the center of mass of the whole arm 0.725 m. Empirically, bicep curls can be performed at a maximum rate of 2 curls/sec and lifting the arms above the head at 1.3 lifts/sec. Thus, the maximum power generated by bicep curls is
while the maximum power generated by arm lifts is
Obviously, housekeeping and violin playing do not involve as much strenuous activity as these experiments. However, these calculations do show that there is plenty of energy to be recovered from an active user. The task at hand, then, is to recover energy without burdening the user. A much more reasonable number, even for a user in an enthusiastic gestural conversation, is attained by dividing the bicep curl power by a factor of 8. Thus, the user might make one arm gesture every two seconds. This, then, generates a total of 3 W of power. By doubling the normal load on the user's arms and mounting a pulley system on the belt, 1.5 W might be recovered (assuming 50% efficiency from loss due to friction and the small parts involved), but the system would be extremely inconvenient.
A less encumbering system might involve mounted pulley systems in the elbows of a jacket. The take-up reel of the pulley system could be spring-loaded so as to counter-balance the weight of the user's arm. Thus, the system would generate power from the change in potential energy of the arm on the downstroke and not require additional energy by the user on the upstroke. The energy generation system, the CPU, and the interface devices could be incorporated into the jacket. Thus, the user would simply don his jacket to use his computer. However, any pulley or piston generation system would involve many inconvenient moving parts and the addition of significant mass to the user.
A more innovative solution would be to use piezoelectric materials at the joints which would generate charge from the movement of the user. Thus, no moving parts per se would be involved, and the jacket would not be significantly heavier than a normal jacket. However, as will be seen in the next sections, materials with the appropriate flexibility have only 11% efficiency, making the recoverable power 0.33 W.