The company is liable because the injury occurred on its premises and it is also liable because it hired the trainer.  An employer is liable for the negligence of an employee (trainer) which occurred during the course and scope of employment.  The gym where the trainer was also employed is liable for negligence because again an employer is liable for the negligence of an employee that occurred during the course and scope of employment.  The trainer should also be named as a defendant, but the trainer's employer(s) is/are primarily liable.An alternative to pursuing the above claims is for the injured employee to file a workers' compensation claim through his/her employer because the injury occurred on the job.