Dave Estrada of West Roxbury stuck electrodes to his thighs and hamstrings as he prepared to exercise on an indoor rowing machine at Spaulding Rehabilitation Hospital. An avid weightlifter before he was paralyzed at 23, Estrada's muscle atrophy was so severe that a biopsy of his thigh turned up almost no remaining muscle. For more than a decade, he had no way to stop the deterioration of his lower body until he discovered, in 2006, that paraplegics in England were rebuilding their legs through the sport of rowing.