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The purpose of this study was to examine the influence of repeated bouts of shiftwork on lower extremity maximal and rapid strength and reaction time in career firefighters. Thirty-five firefighters (3 females; 34.3 ± 9.1 years) performed a psychomotor vigilance test (PVT) and reactive maximal isometric strength assessment prior to and following a full shift rotation (three 24-hr on-off shifts). Reaction time (RT), maximal, absolute and normalised rapid strength (50, 100, 150, 200 ms), and PVT measures were assessed on-site. Separate linear regression models were used to evaluate the POST-PRE change in variables adjusted for BMI, age, sleep, and call duration. Early (50 ms) absolute rapid strength was the only variable significantly reduced (-25.9%; p  = 0.031) following the full shift rotation. Our findings indicate that early rapid strength may be a sensitive measure in detecting work-related fatigue, despite minimal changes in sleep between work and non-work nights and a low call duration. Practitioner summary: We examined the impact of repeated shiftwork on changes in reaction time and neuromuscular function. Early rapid strength was a sensitive, portable lab assessment that feasibly measured work-related fatigue in career firefighters. Interventions that mitigate work-related fatigue may be impactful at preventing falls and/or risk of musculoskeletal injury.

Purpose: Although firefighters are required to perform various high-intensity critically essential tasks, the influence of neuromuscular function on firefighter occupational performance is unclear. The primary aim of the current study was to identify the key neuromuscular determinants of stair climb (SC) performance in firefighters., Methods: Leg extension isometric peak torque (PT), peak power (PP), torque steadiness at 10% (Steadiness 10% ) and 50% (Steadiness 50% ) of PT, fatigability following 30 repeated isotonic concentric contractions at 40% of PT, percent body fat (%BF), and a weighted and timed SC task were examined in 41 (age: 32.3 ± 8.2 yrs; %BF: 24.1 ± 7.9%) male career firefighters., Results: Faster SC times (74.7 ± 13.4 s) were associated with greater PT and PP, less fatigability, younger age, and lower %BF (r = -0.530-0.629; P ≤ 0.014), but not Steadiness 10% or Steadiness 50% (P ≥ 0.193). Stepwise regression analyses indicated that PP and Steadiness 50% were the strongest predictors of SC time (R 2  = 0.442, P < 0.001). However, when age and %BF were included in the model, these variables became the only significant predictors of SC time (R 2  = 0.521, P < 0.001) due to age and %BF being collectively associated with all the neuromuscular variables (excluding Steadiness 10% )., Conclusions: Lower extremity neuromuscular function, specifically PP and steadiness, and %BF are important modifiable predictors of firefighter SC performance, which becomes increasingly important in aging firefighters., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Resistance exercise is an important strategy to reduce injuries and improve performance in the fire service. However, given the large age range of firefighters, it is important to determine if age influences their recovery following an acute bout of resistance exercise., Purpose: To examine the changes in indirect markers of muscle damage in young and older firefighters following a feasible worksite resistance exercise routine., Methods: Nineteen young (25.5 ± 3.4 years) and 19 older male career firefighters (50.3 ± 3.5 years) completed pre-testing, an acute bout of resistance exercise, and post-testing at 24, 48, and 72 h post-exercise at their fire station. Prior to all testing, firefighters completed a work-related fatigue (WRF) questionnaire to account for potential unanticipated differences in previous shift workloads. Testing included perceived muscle soreness, ultrasonography to quantify muscle size and echo intensity (EI) of the vastus lateralis (VL) and biceps brachii (BB), countermovement jump (CMJ) height and velocity, upper body (UB) peak force (PF), lower body (LB) PF and rapid force, and electromyographic (EMG) amplitude of the VL. The resistance training session included 3 sets of 8-10 repetitions of a deadlift, shoulder press, lunge, and upright row exercise at 80% 1-RM. All recovery variables were analyzed using a linear mixed model, controlling for WRF., Results: There was no interaction (age × time) for any of the variables and a similar training volume were completed between groups (P ≥ 0.171). Muscle soreness, CMJ height and velocity, UB PF, LB rapid force and EMG amplitude at later time intervals (100-200 ms), and VL and BB muscle size were altered from baseline (P ≤ 0.044) for 48, 24, 48, 72, and 72 h post-exercise, respectively. Young firefighters exhibited greater CMJ height and velocity, LB PF, LB rapid force (200 ms), and lower VL EI values than the older firefighters (P ≤ 0.047)., Conclusions: Age did not influence the recovery from an acute bout of worksite resistance exercise in firefighters. However, UB muscle strength, CMJ performance, and LB rapid force production were reduced 24-72 h post-exercise. Appropriately scheduled and chronic on-duty resistance training may mitigate these decrements., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Objective . Muscle quality has received considerable attention within the literature due to its influence on health and performance. However, it has been assessed by various measures (e.g. ultrasound imaging, normalized strength), contributing to a lack of a consensus definition. The purpose of this study was to investigate the association between common muscle quality measures: vastus lateralis echo intensity (EI) and peak torque (PT) and power (PP) normalized to thigh lean mass (TLM) and thigh fat mass (TFM). Approach . Participants' ( n  = 39; age = 32.7 ± 8.2 years; %BF = 24.1% ±8.1%) whole body and leg composition was assessed via dual-energy x-ray absorptiometry and custom thigh analyses. Subcutaneous fat-corrected muscle EI was assessed via ultrasonography. Maximal PT and PP of the dominant leg extensors were examined on a calibrated dynamometer and were normalized to both TLM and TFM, respectively. Pearson product-moment correlations were used to examine the associations between EI and PT/TLM, PT/TFM, PP/TLM, and PP/TFM. Main results . Greater muscle EI was significantly related with lower PT/TFM and PP/TFM ( r  = -0.74 to -0.70, P  < 0.001), but unrelated to PT/TLM or PP/TLM ( P  ≥ 0.205). Significance . These findings suggest that ultrasound imaging (muscle EI) and normalized strength and power may not be used interchangeably to define muscle quality, and muscle EI may be more sensitive to the adiposity of the limb than TLM. Future research should consider using separate and consistent definitions when referring to imaging-derived or normalized strength and power values of muscle quality., (© 2021 Institute of Physics and Engineering in Medicine.)

The purpose of the present study was to examine the acute effects of an herbal supplement containing a 70:30 blend of Rhaponticum carthamoides extract and Rhodiola rosea extract, respectively, on performance fatigability and the affective responses prior to and following exercise. Thirty men (age = 22.3 ± 4.1 years) volunteered for this randomized, placebo-controlled, double-blind, cross-over study. Following familiarization, participants visited the laboratory on three separate occasions where they consumed one 350 mg dose, a 175 mg dose of the supplement plus 175 mg of maltodextrin, or a placebo. Participants completed feeling, arousal, and mood surveys at supplement ingestion and following testing. Ninety minutes following ingestion, participants performed five sets of 30 maximal, concentric, leg extension muscle actions at 180°/sec, with one minute of rest between sets. Peak torque, average torque, and total work were calculated for each set, and the rating of perceived exertion (RPE) was assessed after each set. Data were analyzed with a repeated measures analysis of variance and intent-to-treat approach. For the performance variables, there was no interaction ( p  ≥ 0.317) or treatment effect ( p  ≥ 0.660), however, all torque variables decreased across sets and plateaued at sets 4-5, whereas RPE increased across sets ( p  < 0.001). For the affective responses, there was no interaction ( p  ≥ 0.781) or treatment effect ( p  ≥ 0.314); however, participants felt better, had an improved mood, and felt more aroused following testing ( p  < 0.047). This combination of herbal extracts likely does not acutely influence performance fatigability during high-intensity, repeated muscle actions.

The purpose of this study was to determine if muscular dimensional changes with increases in torque production are influenced by age- and obesity-related increases in intramuscular fat, and its relationship to percent body fat (%BF), echo intensity (EI), strength, and maximum walking speed. Sixty-six healthy men were categorized into 3 groups based on age and body mass index status (young normal weight [YNW], older normal weight [ONW], and older obese [OB]). Participants underwent %BF assessments, resting ultrasonography to determine muscle size (cross-sectional area [CSA]) and EI of the superficial quadriceps, and a 10-m maximum walking speed assessment. Maximal and submaximal (rest-100% MVC in 10% increments) isometric leg extension strength was assessed while changes in rectus femoris (RF) CSA, width, and depth were obtained with ultrasonography. Echo intensity and %BF were different among all groups (p ≤ .007), with the YNW and OB groups exhibiting the lowest and highest %BF and EI values, respectively. The RF increased in depth and decreased in width with increases in torque intensity for all groups. The ONW group demonstrated no change (-0.08%) in RF CSA across torque intensities, whereas the YNW group (-11.5%) showed the greatest decrease in CSA, and the OB group showed a more subtle decrease (-4.6%). Among older men, a greater change in RF CSA was related to poorer EI (r = -0.355) and higher %BF (r = -0.346), while a greater decrease in RF width was associated with faster walking speeds (r = -0.431). Examining muscular dimensional changes during contraction is a unique model to investigate the influence of muscle composition on functional performance., (© The Author(s) 2020. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Objective: Little is known regarding how individual characteristics and changes in muscle strength across a shift cycle are associated with the Occupational Fatigue Exhaustion Recovery scale in career firefighters., Methods: Thirty-two career firefighters completed lower body maximal strength testing prior to and following three 24-hour on-off shifts. Demographic data were collected prior to the first shift and the OFER15 questionnaire was completed following the last shift., Results: Increased age was associated with greater acute and chronic fatigue and poorer recovery. A positive percent change in strength following a shift cycle was related to better recovery. Age was the only significant predictor of acute and chronic fatigue, while age and the change in strength were significant predictors of inter-shift recovery., Conclusions: Increased age influences all dimensions of work-related fatigue, with changes in muscle strength influencing inter-shift recovery in career firefighters.

Purpose: The purpose of the present investigation was to identify 1) the prevalence of overweight and obesity and 2) factors associated with previous work-related musculoskeletal injury, and physical activity (PA) levels in North Carolina probation officers., Methods: North Carolina probation officers (N = 1866) were sent a questionnaire on demographics, work history, injury history, and PA. A multivariable logistic regression model estimated the odds of reporting work-related musculoskeletal injury history within the past year, and multivariable ordinal logistic regression estimated the odds of reporting lower PA levels in the previous month. Odds ratios (OR) with 95% confidence intervals (CI) excluding 1.00 were deemed significant., Results: Complete data were available for 1323 probation officers (70.9% completion rate; 46.5% female; mean ± SD age, 39.9 ± 10.0 yr). Officers that were classified as overweight and obese were 80.8% (body mass index [BMI] ≥ 25 kg·m), with 49.9% and 9.5% categorized as obese (BMI ≥ 30 kg·m) and severely obese (BMI ≥ 40 kg·m), respectively. Being older (1-yr increase; OR, 1.06; 95% CI, 1.04-1.09) and severely obese (compared to normally weighted; OR, 2.56; 95% CI, 1.19-5.51) was associated with a greater odds of sustaining a work-related musculoskeletal injury in the past year. A higher number of years of employment (1-yr increase; OR, 1.03; 95% CI, 1.01-1.04), being overweight or obese (compared to normal weight; OR, 1.57-3.22) and being female (compared with male: OR, 0.38; 95% CI, 0.31-0.47) was associated with the greater odds of lower PA levels in the previous month., Conclusions: The prevalence of obesity is alarmingly high in probation officers, and is associated with a greater likelihood of sustaining previous work-related musculoskeletal injury and engaging in lower amounts of PA. Public safety administrators may consider worksite exercise/diet interventions and annual physical employment standards to combat obesity.

Background: Limited research exists examining the influence of age on the passive mechanical properties of the muscle-tendon unit. The purpose of the present study was to 1) investigate the age-related differences in maximum range of motion (MRoM), stretch tolerance (maximal tolerable torque), and the dissipation coefficient (DC), and 2) examine the influence of plantarflexor muscle size and quality on the age-related differences in the DC., Methods: Twenty-one young and 14 older men completed a MRoM assessment to determine peak torque (PT pass ), energy stored (area under the loading curve), energy restituted (area under the unloading curve), and the DC (normalized hysteresis) from the angle-torque curve. Ultrasonography was used to determine muscle cross-sectional area (CSA) and subcutaneous fat corrected echo intensity (EI) of the gastrocnemii., Findings: The PT pass , MRoM, and energy restituted were greater (P ≤ 0.046) in the young men, whereas the DC and EI were greater (P ≤ 0.024) in the older men. When accounting for CSA, differences between groups for the DC remained (P = 0.028), while there were no longer differences between groups when accounting for EI (P = 0.120). PT pass was also strongly related to MRoM (r = 0.755, P < 0.001)., Interpretation: Older men exhibited lower dorsiflexion MRoM and a greater DC, which may be explained by an altered stretch tolerance and qualitative changes (i.e., non-contractile tissue infiltration) in aged skeletal muscle leading to greater energy dissipated, respectively., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The purpose of the present study was to examine the relationships between neuromuscular function and functional balance performance in firefighters. Fifty career firefighters (35.1 ± 7.5 yr) performed isometric leg extension and flexion muscle actions to examine peak torque (PT), and absolute (aTQ) and normalized (nTQ; %PT) rapid torque variables at 50, 100, 150, and 200 ms. A performance index (PI) was determined from the functional balance assessment completion time. Partial correlations were used to examine the relationship between the PI and the maximal and rapid TQ variables for each muscle and the composite value, while controlling for demographic data related to the PI. Multiple regression analyses examined the relative contributions of the maximal and rapid aTQ variables, and demographic data on the PI. After controlling for age and %BF, the majority of the later aTQ and nTQ variables (100-200 ms) and PT were associated with the PI (r = -0.501--0.315). Age, %BF, and aTQ 100 explained 42-50% of the variance in the PI. Lower rapid strength, increased age, and poorer body composition were related to worse performance during the functional balance assessment. Strategies to improve rapid strength and %BF, especially in aging firefighters may impact dynamic balance abilities in firefighters.

Gerstner, GR, Giuliani, HK, Mota, JA, and Ryan, ED. Influence of muscle quality on the differences in strength from slow to fast velocities in career firefighters. J Strength Cond Res 32(10): 2982-2986, 2018-The purpose of this study was to examine the relationship between ultrasound-derived muscle quality and the %decrease in peak torque (PT) from slow to fast velocities in career firefighters. Thirty-nine male career firefighters (mean ± SD: age = 36.90 ± 6.87 years; stature = 180.93 ± 6.98 cm; mass = 109.30 ± 20.57 kg; and body mass index [BMI] = 33.24 ± 4.95 kg·m) performed 3 maximal concentric isokinetic contractions at a slow (1.05 rad·s) and fast (4.19 rad·s) velocity in random order. The velocity-related decrease in PT was calculated as the %decrease from 1.05 to 4.19 rad·s. Ultrasonography was used to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality before strength testing. The results indicated that there was a significant relationship between EI and %decrease in PT while adjusting for age (rpartial = 0.331, p = 0.042), and age and BMI (rpartial = 0.325, p = 0.050). These findings suggest that velocity-impaired isokinetic strength may be influenced by alterations in skeletal muscle tissue composition. Future studies are needed to determine whether the %decrease in PT is related to performance during critical firefighter tasks.

The examination of mechanisms contributing to the age-related reductions in rapid velocity development is limited. The purpose of this study was to examine the influence of muscle cross-sectional area (CSA) and echo intensity (EI) on plantarflexor rate of velocity development (RVD) in older adults. Twenty-two men (mean ± SD age = 69 ± 3 years) performed three maximal plantarflexion voluntary isokinetic muscle actions at a velocity of 2.09 rad · s -1 . Peak RVD was determined as the peak derivative of the velocity-time curve from the onset of velocity development to the onset of the load range. B-mode ultrasound was used to examine CSA and EI from the lateral and medial gastrocnemius. Plantarflexor RVD was related to EI (r = -0.491, P = 0.020), but not CSA (r = -0.003, P = 0.989). The findings of the present investigation suggest that alterations in muscle tissue composition (i.e., increase in intramuscular fat and/or fibrous tissue) may influence the ability of older adults to rapidly accelerate their limb.

Age-related changes in the relative differences in isokinetic strength and power may reflect fast twitch fiber function. We aimed to examine the influence of muscle quality on the relative differences in strength and power in younger and older men. Twenty younger (20.1±1.5yrs) and 20 older (69.5±3.1yrs) healthy, recreationally active men performed two plantarflexion maximal voluntary isometric contractions (MVCs) and three maximal concentric isokinetic contractions at a slow (0.52rad·s -1 ) and fast (2.09rad·s -1 ) velocity. Absolute and normalized (%MVC) isokinetic peak torque (PT), mean power (MP), peak power (PP), the relative differences in PT (%decrease), MP and PP (%increase) from 0.52 to 2.09rad·s -1 , and electromyographic (EMG) amplitude were examined. Ultrasonography was used to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality. The younger men exhibited greater absolute isometric PT, isokinetic PT, MP, and PP at 0.52 and 2.09rad·s -1 (P=0.001-0.003); but these differences were no longer present following normalization (P=0.079-0.954). The older men exhibited similar EMG amplitude values but higher EI values (P<0.001), a greater %decrease in PT (43.6% vs. 38.9%; P=0.006), and a lower %increase in MP (167.5% vs. 186.3%; P=0.049) and PP (125.5% vs. 144.5%; P=0.006). Echo intensity was related to the %decrease in PT (r=0.605; P<0.001), %increase in MP (r=-0.419; P=0.009), and %increase in PP (r=-0.605; P<0.001) from 0.52 to 2.09rad·s -1 . The absolute age-related reductions in isokinetic strength and power were no longer present following normalization to isometric strength. However, age-related differences in strength and power remained intact when examining the relative differences from slow to fast velocities, which appear to be influenced by the qualitative changes in skeletal muscle., (Copyright © 2017 Elsevier Inc. All rights reserved.)