Human skeletal muscle fiber type percentage and area after reduced muscle use: A systematic review and meta-analysis.

The main objective of this systematic review was to examine the effect of reduced muscle activity on the relative number of type 1 muscle fibers (%) in the human vastus lateralis muscle. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross-sectional area. We conducted systematic literature searches in eight databases and included studies assessing type 1 fiber percentage visualized by ATPase or immunohistochemical staining before and after a period (≥14 days) of reduced muscle activity. The reduced muscle activity models were detraining, leg unloading, and bed rest. Forty-two studies comprising 451 participants were included. Effect sizes were calculated as the mean difference between baseline and follow-up and Generic Inverse Variance tests with random-effects models were used for the weighted summary effect size. Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (-1.94%-points, 95% CI [-3.37, -0.51], P = .008), with no significant differences between intervention models (P = .86). Meta-regression showed no effect of study duration on type 1 fiber percentage (P = .98). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity (P < .001). The CSA of the muscle fiber types decreased after the study period (all P-values < 0.001) with greater reductions in type 2 than type 1 fibers (P < .001). The result of this meta-analysis display that the type 1 muscle fiber percentage decrease as a result of reduced muscle activity, although the effect size is relatively small.

Muscle activity and head kinematics in unconstrained movements in subjects with chronic neck pain; cervical motor dysfunction or low exertion motor output?

BACKGROUND: Chronic neck pain after whiplash associated disorders (WAD) may lead to reduced displacement and peak velocity of neck movements. Dynamic neck movements in people with chronic WAD are also reported to display altered movement patterns such as increased irregularity, which is suggested to signify impaired motor control. As movement irregularity is strongly related to the velocity and displacement of movement, we wanted to examine whether the increased irregularity in chronic WAD could be accounted for by these factors. METHODS: Head movements were completed in four directions in the sagittal plane at three speeds; slow (S), preferred (P) and maximum (M) in 15 men and women with chronic WAD and 15 healthy, sex and age-matched control participants. Head kinematics and measures of movement smoothness and symmetry were calculated from position data. Surface electromyography (EMG) was recorded bilaterally from the sternocleidomastoid and splenius muscles and the root mean square (rms) EMG amplitude for the accelerative and decelerative phases of movement were analyzed. RESULTS: The groups differed significantly with regard to movement velocity, acceleration, displacement, smoothness and rmsEMG amplitude in agonist and antagonist muscles for a series of comparisons across the test conditions (range 17-121%, all p-values < 0.05). The group differences in peak movement velocity and acceleration persisted after controlling for movement displacement. Controlling for differences between the groups in displacement and velocity abolished the difference in measures of movement smoothness and rmsEMG amplitude. CONCLUSIONS: Simple, unconstrained head movements in participants with chronic WAD are accomplished with reduced velocity and displacement, but with normal muscle activation levels and movement patterns for a given velocity and displacement. We suggest that while reductions in movement velocity and displacement are robust changes and may be of clinical importance in chronic WAD, movement smoothness of unconstrained head movements is not.

Intermuscular relationship of human muscle fiber type proportions: slow leg muscles predict slow neck muscles.

INTRODUCTION: Our aim in this study was to examine whether the muscle fiber type proportions in different muscles from the same individual are interrelated. METHODS: Samples were excised from five skeletal muscles in each of 12 human autopsy cases, and the fiber type proportions were determined by immunohistochemistry. We further examined the intermuscular relationship in fiber type proportion by reanalyzing three previously published data sets involving other muscles. RESULTS: Subjects demonstrated a predominantly high or low proportion of type 1 fibers in all examined muscles, and the overall difference between individuals was statistically significant (P < 0.001). Accordingly, the type 1 fiber proportions in most muscles were positively correlated (median r = 0.42, range -0.03-0.80). Similar results were also obtained from the three reanalyzed data sets. CONCLUSIONS: We suggest the existence of an across-muscle phenotype with respect to fiber type proportions; some individuals display generally faster muscles and some individuals slower muscles when compared with others.

Working suspended in a harness rig: A comparative study of musculoskeletal health complaints in rope access technicians and controls.

BACKGROUND: Rope access technique is an alternative method for gaining access to challenging work locations. There is limited knowledge about possible adverse effects of this technique on the workers' health. OBJECTIVE: To compare the frequency of bodily regions with pain in rope access technicians with craft workers and the working population in general. METHODS: The one-month prevalence of pain in the head, neck, distal upper extremities, lower back and lower extremities was recorded in rope access technicians (n = 95), "craft workers" (n = 289) and "all occupations" (n = 1563). RESULTS: An increased prevalence of pain in the neck, distal upper extremities and lower extremities was found for the rope access technicians compared with all occupations (p-values <0.01). Compared with the craft workers, relatively more rope access technicians reported pain in the lower extremity region (p <0.01) while the groups were similar for the other body regions. CONCLUSIONS: The prevalence of pain in the lower extremities was higher in rope access technicians compared with craft workers, while no differences were found for other body regions. The increased prevalence of pain in the neck and distal upper extremities in the technicians compared with all occupations may therefore be related to the work tasks and not the access technique.

Muscular performance after concentric and eccentric exercise in trained men.

PURPOSE: We studied previously resistance-trained men and compared the effects of concentric and eccentric training on performance and structural muscle parameters. METHODS: Seventeen trained individuals (age 26.9 +/- 3.4 yr) participated in 12 wk of either maximum concentric (N = 8) or eccentric (N = 9) resistance training of the elbow flexors. The functional performance was measured as the maximum concentric and eccentric strength and angular velocity at standard loads. Muscle cross-sectional area and cross-sectional area of single cells were used as measures of muscular hypertrophy. Fiber-type proportions were assessed by staining cells for myofibrillar ATPase. RESULTS: Both eccentric and concentric training increased concentric strength to a similar extent (14 vs 18%), whereas eccentric training led to greater increases in eccentric strength than concentric training did (26 vs 9%). The maximum angular velocity at all loads was enhanced equally in both training groups. The cross-sectional area of both the elbow flexors (+11%) and of the type I and type IIA fibers increased only after the eccentric training. In addition, the relative cross-sectional area occupied by the type II fibers increased from 64 to 73% after the eccentric training. There were only minor changes in the fiber-type proportions. CONCLUSION: The present data suggest that for resistance-trained men, increases in concentric strength and velocity performance after eccentric training are largely mediated by changes in fiber and muscle cross-sectional area. However, hypertrophy alone could not explain the increase in eccentric strength. Because the increases in strength and velocity performance after concentric training could not be ascribed to muscular adaptations alone, we suggest that they may be attributable to additional neural factors.

Acute hemodynamic and cardiovascular responses following resistance exercise to voluntary exhaustion. Effects of different loadings and exercise durations.

BACKGROUND: The aim of this study was to compare the acute hemodynamic and cardiovascular responses of high load/low repetition resistance training (RT) to low load/high repetition RT. METHODS: Thirteen healthy men performed four sets of 4 repetition maximum (RM) and 20RM leg-extensions without breath-holding. The RT was conducted in a randomized order and with 48 hours between bouts. Non-invasive beat-to-beat systolic and diastolic blood-pressure (SBP/DBP) was measured on the finger, while non-invasive cardiac output (CO) was assessed beat-to-beat by impedance-cardiography. RESULTS: Mean±SD resting SBP/DBP and CO were 126±14/73±9 mmHg and 5.6±9 L min-1, respectively. Exercise SBP/DBP values increased to 154±22/99 ±18 and 203±33/126±19 mmHg following 4RM and 20RM RT, respectively (compared to rest, all; P<0.001), and 20RM SBP/DBP values were higher than 4RM values (both, P<0.001). The SBP increased from the first to the fourth set of exercise following the 20RM load (P<0.01), but not so for the 4RM load. Exercise SBP/DBP values following the 4th rep of 20RM exercise (154±18/91±14), was similar to the 4RM values, but different to the 20th rep of the 20 RM loading (both; P<0.001). CO increased to 10.8±2.6 and 13.9±2.2 L min-1, following 4RM and 20RM RT, respectively (compared to rest, both; P<0.001) and 20RM CO was higher than 4RM CO (P<0.01). CONCLUSIONS: 20RM RT resulted in higher blood-pressure than 4RM RT when performed to voluntary exhaustion. Differences in hemodynamic responses seems to be related to training duration and not to difference in loading.

The smoothness of unconstrained head movements is velocity-dependent.

Non-smooth, irregular movements reported in persons with neck pain have been suggested to signify motor impairment. However, irregular movements are additionally observed during slow movements in healthy participants. We therefore examined whether the smoothness of head movements is related to the movement speed in 26 healthy participants. Six unconstrained small and large amplitude head movements were completed in the sagittal plane at three different self-chosen speeds. Kinematic variables were calculated from position data and overall smoothness of the movement was assessed by the normalized jerk cost (NJC). Relationship between NJC and average movement angular velocity was analyzed using a mixed factor model. Movement duration, angular velocity, NJC and number of submovements differed significantly between speed conditions for all movement directions and amplitudes (all p<.05). We found a strong relationship between the average angular velocity and NJC across all movement directions and amplitudes (all p<.0001). Large amplitude movements showed higher NJC for a given movement velocity than small amplitude movements (p<.001). We have shown that the smoothness of head movements is strongly related to the movement velocity, thus fast movements are smooth while slow movements are jerky. In addition, movements of larger amplitude are less smooth than movements of smaller amplitude.

Effect of concentric or eccentric weight training on the expression of heat shock proteins in m. biceps brachii of very well trained males.

Increased HSP expression in response to acute exercise is well documented in animal studies, and there is growing evidence that similar responses occur in man. In general, many human exercise studies have investigated the HSP response to low force continuous activity, while the knowledge about the HSP response to high force intermittent type of activity, like weight training, is so far sparse. In addition, most studies have used untrained subjects, and a common observation is that acute low force continuous activity in untrained individuals increases the HSP expression in these individuals. The main scope of this study was to investigate the HSP response in very well trained males subjected to longitudinal high intensity exercise, and if this response was dependent on exercise modality [i.e. eccentric (ECC) or concentric (CON) contractions]. Very well trained males performed progressive strength training consisting of either high force ECC or high force CON elbow flexions 2-3 times a week for 12 weeks. Compared with pre-exercise levels, HSP72 expression decreased by 46.1% (P<0.05) after CON contractions. GRP75 expression was unchanged after ECC or CON contractions, while ubiquitin expression decreased by 19.9% (P<0.02) after ECC contractions. These findings imply that chronic, intensive exercise may attenuate the HSP response in well-trained males.