Firefighters are paramount to public safety with over 360,000 career firefighters currently employed in the United States, protecting nearly 70% of the population (7). Occupational injuries are common within the fire service with over 60,000 firefighters being injured on duty in 2021 (5). These injuries result in a high economic burden, with subsequent expenses between $2.8 billion and $7.8 billion per year (25). One potential contributing factor is that these individuals are required to perform a number of critical and essential tasks in hazardous conditions while wearing personal protective equipment weighing around 22 kg (22). Specifically, the International Association of Fire Chiefs and International Association of Firefighters have identified eight critical firefighter tasks in the Candidate Physical Abilities Test (32). The stair climb (SC) task is often considered one of the most critical and challenging tasks. For example, previous studies have demonstrated a significant cardiometabolic response during the SC including near maximal heart rates and increased oxygen consumption and blood lactate levels (12,28,35). Further, simulated SC assessments have been shown to be reliable (28) and rated as a highly relevant occupational task among firefighters with a mean response of 3.6 on a 0 – 4 scale (0 = not relevant at all, 4 = very relevant) (31).

Inter-limb leg lean tissue mass (LTM) asymmetries occur when there are inter-limb differences in skeletal muscle mass. These asymmetries have been examined in various populations ranging from athletes to older adults (2,11,19,21). Previous studies have found leg inter-limb asymmetries adversely impact countermovement jump height (2), contribute to greater leg inter-limb strength differences (11), and are associated with reduced performance in common functional tasks (i.e., chair sit-to-stands, walking, timed-up-and-go) (18). Previous authors have suggested inter-limb leg LTM asymmetries are influenced by performing various tasks on a favored side over time (14,17,20,33). For example, Australian football players tend to have larger psoas muscle size in their kicking (favored) limb, when compared to their stance leg (14,33). It is possible firefighters are exposed to occupational tasks that are performed on a favored side such as specific victim drag techniques and/or asymmetric load carriage (1,14,33). Due to the unilateral limb demands during a SC task (29), inter-limb leg LTM asymmetries may negatively impact SC performance. For instance, previous authors (29) have indicated that inter-limb leg asymmetry adversely affects SC speed more than gait speed in older adults due to the higher demand on unilateral leg function. These previous studies indicate a need to assess and examine the relationship between inter-limb leg asymmetries and occupational performance in career firefighters. Therefore, the purpose of this study is to examine the relationship between inter-limb leg LTM differences and SC performance in career firefighters. We hypothesize that firefighters with greater LTM asymmetries will have reduced performance (longer time to completion) during a timed SC task.

All mean and standard deviation values for limb LTM, asymmetry, and SC variables are presented in Table 1. Our participants age and %BF were similar to and representative of career firefighters reported in previous studies (7,30). One participant was identified as an outlier (> 4 SD above the mean) for the SC time and was removed prior to analyses. The point biserial correlation indicated LTM asymmetry was associated with longer SC task time (poorer performance) (r = 0.432, P = 0.005). When adjusting for age and %BF, a similar positive relationship was observed between LTM asymmetry and SC time (r = 0.502, P = 0.001). Lastly, there was no significant relationship between participant stature and SC time (r = −0.095, P = 0.562).

Firefighters are required to perform a number of essential occupational activities (32), however, the SC is often considered one of the most critical and challenging tasks (34). The current study examined the impact of lower body LTM asymmetry on simulated SC performance in career firefighters. The primary findings of the current study indicated that inter-limb leg LTM asymmetry is significantly associated with impaired SC task performance (longer completion times) in career firefighters before and after accounting for age and %BF.

The finding that greater inter-limb leg LTM asymmetry is associated with reduced performance is in agreement with previous research in both athletic and older adult populations (2,11,18,21). For example, investigators have reported that greater inter-limb leg LTM asymmetries are associated with poorer countermovement jump height (2), greater inter-limb strength differences (unilateral isometric back squat on force plates) (11), poorer performance during common functional tasks (chair sit-to-stands, walking, timed-up-and-go) (18), and reduced lower-extremity function (400 m walk test, 30 sec chair sit-to-stands) (21). These authors (18,21) also noted that inter-limb leg LTM asymmetries likely contribute to limb strength/power capabilities that may impact subsequent performance. For example, Bell et al. (2) reported LTM asymmetry accounted for 20% of the variance in leg inter-limb force differences in collegiate athletes. Thus, it is possible that the inter-limb leg LTM asymmetry reported in the current study may have contributed to strength/power asymmetries whereby the weaker limb is unable to maintain the unilateral demands required during the weighted SC climb (24,29). This may be supported by Portegijis et al. (29) who reported leg inter-limb asymmetry adversely affects SC speed more than gait speed in older adults, given that the SC is a more challenging mobility task requiring a higher demand on unilateral leg function.

Previous studies have suggested inter-limb leg LTM differences occur as a result of the mechanical stress stimulating increases in muscle mass on the favored limb through chronic exposure to asymmetric tasks, and is commonly seen in athletic populations (14,17,20,33). For example, Australian football athletes tend to have greater psoas major muscle size in their dominant (kicking) leg compared to their contralateral (stance) limb, and field hockey athletes exhibit lower extremity asymmetries between their lead side and contralateral limb (14,17,33). We are unaware of any previous studies that have examined the impact of inter-limb leg LTM asymmetries on occupational performance in firefighters. Based on previous studies examining LTM in athletic populations (2,11), the muscle atrophy that occurs following injury (19), and recent work demonstrating the impact of firefighter demographics on performance during other simulated occupational tasks (22,31), it is possible that these asymmetries may exist due to chronically performing various firefighter tasks, injury history, age, and/or %BF.

Previous injury (i.e., ACL tear) can cause prolonged underloading resulting in muscle atrophy of the injured limb and the development of inter-limb leg LTM asymmetry (18,19). However, inter-limb leg LTM asymmetry reported in the current study may occur from repetitively performing occupational tasks targeting the favored side given that we excluded participants with any major musculoskeletal surgeries in the lower leg and back, any recent injuries, and found no impact of age and %BF when controlled for our in our analyses. For example, firefighters perform many occupational duties including specific victim drag techniques and asymmetric load carriage that may impact inter-limb leg LTM asymmetries. However, future work (e.g., occupational analysis) is needed to identify specific issues that cause LTM asymmetries in firefighters. Additionally, it is important to note that inter-limb leg LTM asymmetry accounted for 18.7 – 25.2% of the variance in SC time, which aligns with previous studies (2). Along with LTM asymmetries, it is likely additional physiological parameters such as lower extremity strength and power (31), aerobic capacity (12,15,26), and/or steadiness (31) may also be important contributors to SC performance. Therefore, future studies are needed to identify additional variables that further explain the remaining variance in firefighter SC performance. Lastly, it is important to note that previous investigations have utilized different percent asymmetry calculations (2,11). However, when using these calculations (2,11) to determine the ±3% threshold, there were no differences in the outcome (r = 0.432, P = 0.005, 7 asymmetry cases identified) between these methods and the asymmetry calculation used in the current paper.