The sciatic functional index, or SFI, is a widely used metric by researchers who study the pathology and potential treatment of nerve injury.  The SFI is determined by comparing the geometric representation of the affected hind paw from an injured mouse or rat, and comparing it to the contralateral paw.  Several equations have been presented to the research community, each providing nearly the same values of ~ -10 for nearly normal function to ~-100 for dramatic injury.
The calculations have been most often based on the lengths and widths of the impressions of the paws left on a piece of paper after the animals, whose paws were dipped in ink or paint, walked across the paper, usually at indeterminate speeds.  Not widely appreciated by those who are interested in the SFI is the fact that the index is based on 3 measurements only: the paw lengths, and the paw widths, between the 1st and 5th digits (“toe-spread”, and the 2nd to 4th digits.  The most important driver in more negative values in the SFI is the paw length, followed by the 1-5 toe spread, followed by the 2-4 toe spread.
What appears even less standardized is the number or impressions used to determine the mean SFI.  While it is increasingly recognized that walking speed has a large impact on gait kinematics [stride length, cadence, propulsion duration], it is not likely that walking speed per se greatly impacts the postural maladjustments imposed by the sciatic nerve injury.  The animals’ motivation to walk, however, is probably lessened in an injured animal, making it likely that control animals or animals recovering probably take more strides.  It is likely that the standard errors for the SFI for healthier animals are lower than for sedentary and more injured animals.  It is important, therefore, to increase accuracy and sensitivity of the read out by a) ensuring that all animals walk the same speed, b) use the treadmill to increase the number of strides from which to base the calculations to keep standard error low; c) use a sufficient number of animals to for adequate statistical comparisons.
DigiGait not only reports the SFI for numerous strides with all animals walking at the same speed, it also provides several other metrics that may provide useful pre-clinical evidence of resolution of injury.  Video 1 depicts a rat with a crush to the right sciatic nerve.  Notice the clenched posture of the right hind paw.  The SFI for this animal, walking 20 cm/s, is -65, based on 12 steps at a known speed.  Figure 2 illustrates the gait signals for this animal.  The top panel depicts the superimposition of the fore limb gait signals, and the bottom the hind limb gait signals.  Note the loss of symmetry of the forelimbs, due to compensatory changes to the forelimb of the uninjured side.  In contrast, there is good symmetry between limbs of both girdles in the healthy rat.  Comprehensive gait analysis provides abundant data about numerous aspects of posture and locomotion in animals with nerve injury, providing new avenues for testing the efficacy of potential therapies.