Muscle strength was assessed from the maximum force that could be exerted voluntarily by triceps brachii muscles of 72 people with chronic cervical spinal cord injury (SCI) at or above C7, and 18 able-bodied (A-B) subjects. The magnitude of co-activation was estimated from the ratio of biceps brachii surface EMG to triceps plus biceps brachii surface EMG (biceps EMG/ triceps + biceps EMG). Maximum voluntary forces exerted by triceps brachii muscles of SCI subjects were significantly lower than those of controls (p < 0.01). Strength differences between muscles of SCI men and women were not evident. Significant positive relationships were found (linear or curvilinear) between triceps surface EMG and force for all control muscles (n = 19) and for 54% of the muscles of SCI subjects (n = 73). The remaining muscle of SCI subjects (n = 63) were either so weak that only one EMG and force value could be measured or EMG occurred without detectable force. For control muscles (n = 19), the mean triceps-biceps EMG ratio was 0.15+/-0.05 for all voluntary contraction force levels. For muscles of SCI subjects, 41 had EMG ratios similar to those of controls, co-activity largely attributed to EMG cross talk; 19 muscles had constant EMG ratios, but these were three standard deviations above the control means; 13 muscles had EMG ratios that decreased or increased as force increased. Muscles of SCI subjects with greater than control levels of co-activity during maximum voluntary contractions (high EMG ratios) were as strong as muscles with EMG ratios similar to controls. These results provide quantitative descriptions of voluntary muscle weakness after SCI and a database from which to evaluate improvements in muscle strength. These data also show that, for many SCI subjects, any triceps-biceps co-activation is similar to that of controls and does not necessarily distort muscle control unduly.