THE OCCUPATIONAL Safety and Health Administration (OSHA) says its proposed rule on ergonomic injuries will extend government protection to a huge new category of workers severely harmed by repetitive lifting, pulling or straining motions required in their jobs. Industry and employers' groups opposed to the regulation say it will open them to almost infinite liability for a set of ailments nobody can define and whose cause nobody can pin down. Their fears are exaggerated but not completely unfounded. Ergonomic injuries are real, and workers should have protection against them. But the rule is vague enough to cause problems.

The OSHA rule targets so-called "musculoskeletal disorders," which include back pain, tendinitis, carpal-tunnel and repetitive-stress syndromes and others. Numerous studies have shown that certain workplace conditions increase the likelihood workers will get these conditions: Reaching repeatedly above one's head, lifting too-heavy loads and working in cold temperatures all play a role. But no one can yet say exactly why one worker gets hurt and another doesn't, or whether non-workplace factors are more important.

Setting the definition of what constitutes a work-related danger, then, is a key element in any rule. When does an employer become liable, and what is required for compliance? The two aspects of the proposed rule that have drawn the most fire deal with this line-drawing. The OSHA rule is triggered if any workplace reports a single "recordable" ergonomic injury--one that the job itself can reasonably be said to have caused or contributed to.

At that point the employer must analyze all the jobs in that category and create a program to deal with them. Employers say one injury isn't enough--maybe the employee was bowling after hours--and that "contribute to" is too general a standard. A related complaint is that the proposed medical benefits for an ergonomic injury are too generous (six months leave at 90 percent salary) given the difficulty of ascertaining when an employee is hurt and when he or she is fully healed.

Neither of these weaknesses in the regulation is beyond fixing in the public comment period, which lasts till February. OSHA has already given ground on both since an earlier draft. A California version of the rule has a two-injury trigger, which some groups opposing the OSHA rule say they would support.

A more general problem, ironically, is the emphasis OSHA places on flexibility and common-sense language: Instructional materials keep repeating that most existing programs already meet the standard and that employers can do anything that works, which leaves discretion--and liability--almost wholly in the hands of individual inspectors. If OSHA can pull free of election-year politics, particularly a keen desire by the administration to please the unions, a good rule can still emerge.