Work hardening mainly provides training to increase muscle strength, including the traditional resistance training and work-related functional resistance training. The combination of these two training modalities offers a great boost to one’s physical capacity . In this study, the participants had substantial improvement in the moderate or more loads of bilateral carrying and bilateral floor-to-knuckle lifting subtests after the training compared to the pre-training results. More importantly, a higher load in the bilateral carrying subtest before the training was associated with higher odds of RTW.
The two strength subtests of bilateral carrying and floor to knuckle lifting showed that all participants were improved to be at least capable of a moderate load after work hardening training, indicating that the participants had increased carrying and lifting ability. According to the biomechanical principle of the lever, the shorter the resistance arm, the lesser force needed (Fig. S1) , and according to the length-tension relationship in the exercise physiology, the magnitude of a force depends on the length, speed and tension of the muscle (Fig. S2) . Therefore, the weight to be carried or lifted is closely associated with the length of the muscles involved. Compared to bilateral knuckle-to-shoulder lifting and bilateral shoulder-to-overhead lifting, bilateral carrying and bilateral floor-to-knuckle lifting are less demanding and are easier action modalities, and they have also been shown to have the greatest improvement after the work hardening training [25,26,27].
There are several possible reasons why the pre-training bilateral carrying ability predicted a higher rate of RTW. A better capacity before training indicates better recovery from the injury, hence a better chance to RTW. From the biomechanical perspective, only bilateral carrying requires walking, and when one foot swings forward, the other must support the entire weight of the body, generating a moment of single-limb support (SLS) [28,29,30]. In order to hold the body steadily with one foot, one has to call for the muscles on his/her legs and torso, such as the gluteus medius, tensor fasciae latae and quadriceps femoris, and according to some studies, the more strength provided by the leg and torso muscles, the better chance to RTW [31, 32]. Compared to the other strength subtests, bilateral carrying requires not only the upper limbs but also the lower limbs to be able to bear weight. A better result in the pre-training bilateral carrying subtest represents better strength in bilateral training of the subject, which can be translated into a less serious injury to the subject, hence a better chance for the subject’s RTW. Our results showed no associations between post-training results on the strength subtests and successful RTW. Multiple factors could account for this finding, including the degree of subjects’ recovery, presence of chronic pain, and non-injury related factors impacting RTW . The subjects in this study were from a pool of workers receiving government-subsidized rehabilitation. The rehabilitation lasted for 2 months, including 48 hours of work-hardening training. Once a subject finished the 48-hour training, a post-training evaluation was performed immediately. The subjects had improved physical capacity after the training primarily due to neural adaptations, while there was little material changes in the physical health of muscles [33,34,35], and a substantial proportion of the subjects had not reached the physical capacity required for their pre-injury work or had their pains resolved by the end of the training [12, 36]. According to Gibson et al. , the evaluation of whether an injured athlete can return to play depends primarily on his/her pre-injury level of activity and full capacity. Therefore, the strength subtest explored in this study was “one-time” capacity of the subjects, similar to the one-repetition maximum (1RM) in the resistance training. However, RTW requires approximately 8-hour work every day, and muscle endurance must be considered.
In this study, RTW was defined as going back to the pre-accident position. This study found that the higher weight successfully handled in the pre-training bilateral carrying subtest, the easier to RTW. Similarly, Gouttebarge et al. also found that bilateral carrying predicted the probability of RTW and the future work disability . Meanwhile, other studies showed that the bilateral lifting subtests were associated with RTW [9, 14]. For example, Gross and Battié found that the knuckle-to-overhead lifting predicted RTW, and that the better bilateral floor-to-knuckle lifting, the better chance to return to the original position . However, the injury sites of participants and RTW were defined differently in their study compared to the present study, and most previous studies defined RTW simply as working again [8, 9, 14, 16]. It has also been found that the longer one stays out of work, the less possible for him/her to go back to work . In summary, there is evidence for both bilateral carrying and bilateral lifting to predict RTW, but a consistent conclusion is yet to reach due to different injury types, RTW definitions and the duration of non-working period.
This study was limited by the small number of participants, which is why we grouped the five load grades of the six strength subtests into two categories. In addition, although evidence supports the importance of work-hardening [25,26,27], we did not find associations between any post-training strength subtest result and successful RTW. Other factors necessary for successful RTW that require additional investigation include resolution of chronic pain and recovery of muscle endurance. Practical FCE and reporting are lengthy and time-consuming for healthcare professionals. As this study revealed, the heavier the load handled in pre-training bilateral carrying, the more likely it was for participants to RTW. It is therefore recommended that clinical healthcare professionals with limited time use strength subtests, especially bilateral carrying, to quickly determine the physical capacity of a patient and the possibility of RTW, and provide appropriate RTW recommendations and training. If evaluation shows a good load in the pre-training bilateral carrying but the patient fails to RTW for a long time, further investigation into psychological and social factors may be necessary.