If anyone would like a copy please contact me.The letter highlights my concerns with the authors conclusions in light of the methodology employed and follows on to a degree from the letter I wrote in response to Steiger et al. last year in European Spine Journal.
The main issues refer to the poor control of resistance training variables, in particular the intensity of effort, load and also frequency.
In their paper Willemink et al suggest that adaptations on the multifidus musculature are not related in any way to clinical improvement i.e. pain and disability. However another issue with their methodology relates to the lack of control group. Their training protocol was likely insufficient to induce muscular adaptation anyway, but it is also not possible to rule out that the clinical improvements may have been due to placebo type effects.
I think it is premature to suggest that changes in muscular morphology or performance are unrelated to clinical improvements at present. Especially when our own research utilising very specific isolated lumbar extension exercise has just reported that improvements in isolated lumbar extension strength are in fact significantly related to clinical improvements.