Voluntary muscle strength must be assessed to determine optimal human work capacity. Overexertion is considered a major factor causing musculoskeletal injuries. High force for a working requirement has been identified as a risk factor for hand-wrist cumulative trauma disorders [5]. The present study experimentally investigated the risk of forearm musculoskeletal disorders while driving screws. During the screw driving task, the muscles involved concurrently exerted torque about the pronation–supination axis of the forearm combined with ulnar deviation of the hand under elbow flexion. Inserting a screw involves applying three force types simultaneously: the subject must hold the handle of the screwdriver and apply a combination of forearm torsion with a pushing force to insert the screw. The three force types, GF, DT and PF, are the major forces acting during a driving task. Changes in the MIF and EMG signal amplitude before and after inserting a screw were used to evaluate forearm muscle strength and fatigue in subjects. Our experimental results showed that after the subjects inserted eight screws, the MGF, MDT and MPF decreased by an average of 32%, 24% and 27%, respectively, with GF showing the most substantial FL.
We designed an eight screw driving task to investigate intensity of muscle activity (FL). Figure 5(A) and 5(B) show that the percentages of MIF (pre-fatigue) relative to DT and PF during screwing decreased from 13.6% (0.72 N m) to 6.7% (0.35 N m) and from 14.8% (20.11 N) to 7.0% (9.54 N), respectively. Both forces applied on the eighth screw were reduced by approximately half of that applied to the first screw. According to the DT and PF measured for the first, fourth and eighth screws, an approximate linear decrease in force was observed as the number of screws inserted increased.
The screwdriver used in this study had a cylindrically shaped handle and a circular cross-section that is comfortable to hold in the hand. Some studies have reported that discomfort in handle shape, handle size and handle material may reduce the GF and DT and cause hand-arm musculoskeletal disorders [42–51]. The handle of the screwdriver was made of polypropylene with hardness similar to that of the human hand, providing a comfortable holding sensation. In some situations, gloves are used to protect the hands from human/tool hazards. Wearing gloves reduces the friction between the hand and the hand–tool surface as well as prevents localised hand discomfort [52]; thus, industrial gloves are designed with greater thickness to protect the hands from injuries. Swain et al. [53] reported that wearing industrial gloves reduces torque exertion when operating knobs. Mital et al. [42] showed that gloved supination/pronation during maximum volitional torque exertion (MVTE) is greater than barehanded MVTE when operating a screwdriver.
Muscle fatigue is complex with important implications for ergonomics. It can be defined as a decrease in maximal force or power production in response to contractile activity of a muscle. When fatigue occurs, maintaining the same force level becomes difficult; thus, increasing the likelihood of sustaining an injury. To measure the degree of muscle fatigue is a challenge as it cannot be easily quantified, particularly as it increases significantly during repetitive motion. Several studies have measured the time to failure to assess muscle fatigue while subjects sustained an isometric contraction [54, 55].
Soo et al. [56] proposed a recovery model to establish the relationship between muscle fatigue and rest time. They used a maximal isometric grip force of 50% for 10, 30 and 50 s to explore the effect with/without recovery on fatigue, and recovery during the operation was crucial to relieve muscle fatigue. Some studies have investigated fatigue characteristics using isometric muscle contractions; however, muscles are not persistently in the state of isometric contraction during dynamic work because muscle recovery occurs simultaneously. Our study evaluated muscle fatigue during a screw insertion operation in a dynamic environment. The muscle fatigue characteristics were not analysed using long-term, accumulated data because a short rest period was taken between screws to recover from muscle fatigue; rather, they were analysed using MVC and EMG variations before and after driving the screws.
The degree of muscle fatigue can be estimated based on the physiological and biomechanical signal changes in the muscles during fatiguing contractions. Surface EMG (sEMG) has been widely used as a non-invasive technique to quantify the level of total activity of working muscles and to identify muscle fatigue. Although this study only measured the skin sEMG signals of muscles during which cross-talk interference of the underlying adjacent muscles still occurred, EMG has become a popular tool for assessing muscle responses owing to its application advantages in situ, as well as non-invasive and real-time monitoring. This study only explored the immediate risks of BB, BR and ECU injuries related to the force applied while using a screwdriver. Figure 6(A) shows that the loss of EMG amplitude (ΔEMG) was higher in the BR and ECU than that in the BB when the subjects performed the MGF test after the screw insertion operation. This finding indicates that the percentage of muscle FL in the BR and ECU was greater than that in the BB owing to the grip force applied on the screwdriver handle during screwing. Figure 6(B) also shows that the percentages of muscle FL in the BR and ECU were larger than that in the BB, indicating that the BR and ECU experienced a greater muscle loss percentage during DT than did the BB. Therefore, the BR and ECU were more likely to be fatigued than the BB in terms of GF and DT during insertion of the screws. Additionally, Fig. 6(C) shows that the muscle FL percentage of the BB increased to 36.5% because a downward vertical force had to be applied during screw insertion. At this point, the participants had to lift their upper limbs and engage in elbow flexion (this force is mainly derived from contraction of the BB) to provide a downward force. Therefore, the PF shown in Fig. 6(C) on the BB muscle (ΔEMG) is greater than that of the GF [17.4% in Fig. 6(A)] and DT [16.7% in Fig. 6(B)]. This study suggests that the PF caused more force loss and fatigue to the BB than the GF and DT during the screw driving task.
When performing the driving task, the applied force varied with time, unlike typical fatigue tasks in which subjects are asked to perform at a given intensity until failure. The participants adopted a self-selected technique, operating time, operating process and posture based on personal physiological conditions, and they may have spontaneously decreased the forces exerted and extended the operating time to avoid excessive local muscle fatigue. This extended time recovered the motor deficit; thus, potentially increasing the time to exhaustion from the muscle force. Although exertion force is not maintained at a constant level during sub-cycling, Eq. (5) provided the average forces for inserting a screw to estimate muscle fatigue of the dynamic (cyclic) tasks. In our study, the average DT and PF values for the eighth screw were reduced by approximately half of that of the first screw (from 13.6–6.7% and from 14.8–7.0%, respectively) (Fig. 4). Significantly different IR values were found beginning with the fourth screw, implying that forearm muscle fatigue may have developed in the subjects when inserting four screws.
Ciriello et al. [57, 58] reported that the average maximum acceptable torque (MAT) for males was between 1.15 and 1.88 N m, with an approximate ratio of 15–35% relative to maximal isometric torque. For females, the MAT was between 0.33 and 0.65 N m with ratios of 14–24%. However, they recommended values that were 75% (0.62–1.02 N m, with ratios of 14% and 11%) of their present findings for practical implications. In our study, the participants included 10 male and 2 female adults, and the average percentages of DT relative to the MDT for the first, fourth, and eighth screws were 13.6%, 12.1% and 6.7% and the corresponding applied driving torque values were 0.72, 0.58 and 0.33 N m, respectively. The measured DT of the participants during screw insertion did not exceed the maximum acceptable torque for males except the first screw. Although our experimental results did not show that the subjects had a high risk of forearm muscle injury after performing this type of screw insertion task, the current study performed the screw driving tasks under the cyclic (dynamic) action condition. As mentioned above, subjects may have spontaneously decreased the forces exerted and extended the operating time to avoid excessive local muscle fatigue. The lower force or torque and extended operating time could have recovered muscle activation and decreased fatigue during the sub-cycles. Furthermore, the two females involved in our experiment contributed smaller measured applied force values during the experiments compared with the male subjects. Therefore, this decreased total driving torque. The possible risks faced by female subjects that perform these types of screw driving tasks should be further studied.
The experiments in this study were relatively limited. Although many factors can affect insertion torque, such as the geometry of the driver handle, screw type, insertion depth, screw length, thread pitch, density, major/minor (outer/inner) screw diameters and pilot-hole size [59–63], the findings of this study were based on the experimental results of eight screws (diameter = 3.6 mm; length = 30.5 mm and pitch = 1.0 mm). This screw type is frequently used in industry, wood working, modular furniture and construction. Other screw types, the operating technique and physical work exposure were not quantified in this study.