Try these articles and reference lists

Infant motor development and equipment use in the home.
Abbott, A.L.; Bartlett, D. J.
Child: Care, Health & Development. 27(3):295-306, May 2001.

Conclusion substract

A study on the effect of baby walker on mean age acquisition of motor skills in infants: PP-370Volume 97 Supplement 459, June 2008, p 219–220
Publication Type: [Abstracts from the 3rd Europaediatrics Congress 14–17 June 2008, Istanbul, Turkey: POSTER Presentation
The findings from this present study do suggest that infants with high equipmentuse tend to score lower on infant motor development.These results could also suggest that infants with low equipment use tend to score higher on infant motor development

A review of the effects of sleep position, play position, and equipment use on motor development in infants.
Pin, Tamis MSc *; Eldridge, Beverley PhD; Galea, Mary P PhD
Developmental Medicine & Child Neurology. 49(11):858-867, November 2007.

175 babies (58.33%) were male. The mean age of acquisition of all motor skills including rolling, crawling, moving on hands and feet, sitting without and with help, standing and walking dependently and independently was delayed in infants using baby walkers. This difference was statistically significant (P < 0.001).

I think the intitial poster of this question must have meant 7 months walking. This is the earliest possible time an infant has the ability to pull themselves to a standing position without assistance. At our practice we do not reccomend any forms of walkers, jolly jumpers or standing activity centres for the pure fact that these devices do not teach the child to walk/jump etc. The teach poor movement patterns such as toe walking, high extensor tone and poor pelvic position and control. I note that the instructions of these jumping devices say to let the child's toes touch the floor. Jumping feet leavng the floor is a 2-2 1/2 year old skill. A child younger than this does not have the body control (bones, muscles and co-ordination) to manage a jump action. Jumping in your arms is different and you would support the child far more than the device would. We still do not reccomend alot of weight baring before 6 months. The evidence speaks for itself!
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