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A key determinant of geriatric frailty is sarcopenia, the age-associated loss of skeletal muscle mass and strength (1,2). Although the etiology of sarcopenia is unknown, the correlation during aging between the loss of activity of satellite cells, which are endogenous muscle stem cells, and impaired muscle regenerative capacity has led to the hypothesis that the loss of satellite cell activity is also a cause of sarcopenia (3,4). We tested this hypothesis in male sedentary mice by experimentally depleting satellite cells in young adult animals to a degree sufficient to impair regeneration throughout the rest of their lives. A detailed analysis of multiple muscles harvested at various time points during aging in different cohorts of these mice showed that the muscles were of normal size, despite low regenerative capacity, but did have increased fibrosis. These results suggest that lifelong reduction of satellite cells neither accelerated nor exacerbated sarcopenia and that satellite cells did not contribute to the maintenance of muscle size or fiber type composition during aging, but that their loss may contribute to age-related muscle fibrosis., Recent estimates indicate that up to one-third of elderly people suffer from frailty, characterized by a common set of symptoms including loss of muscle strength, increased fatigability, modest levels of [...]

McBride, JM, Skinner, JW, Schafer, PC, Haines, TL, and Kirby, TJ. Comparison of kinetic variables and muscle activity during a squat vs. a box squat. J Strength Cond Res 24(12): 3195-3199, 2010--The purpose of this investigation was to determine if there was a difference in kinetic variables and muscle activity when comparing a squat to a box squat. A box squat removes the stretch-shortening cycle component from the squat, and thus, the possible influence of the box squat on concentric phase performance is of interest. Eight resistance trained men (Height: 179.61 [+ or -] 13.43 cm; Body Mass: 107.65 [+ or -] 29.79 kg; Age: 24.77 [+ or -] 3.22 years; 1 repetition maximum [1RM]: 200.11 [+ or -] 58.91 kg) performed 1 repetition of squats and box squats using 60, 70, and 80% of their 1RM in a randomized fashion. Subjects completed the movement while standing on a force plate and with 2 linear position transducers attached to the bar. Force anal velocity were used to calculate power. Peak force and peak power were determined from the force--time and power--time curves during the concentric phase of the lift. Muscle activity (electromyography) was recorded from the vastus lateralis, vastus medialis, biceps femoris, anal longissimus. Results indicate that peak force and peak power are similar between the squat and box squat. However, during the 70% of 1 RM trials, the squat resulted in a significantly lower peak force in comparison to the box squat (squat = 3,269 [+ or -] 573 N, box squat = 3,364 [+ or -] 575 N). In addition, during the 80% of 1RM trials, the squat resulted in significantly lower peak power in comparison to the box squat (squat = 2,050 [+ or -] 486 W, box squat = 2,197 [+ or -] 544 W). Muscle activity was generally higher during the squat in comparison to the box squat. In conclusion, minimal differences were observed in kinetic variables and muscle activity between the squat and box squat. Removing the stretch-shortening cycle during the squat (using a box) appears to have limited negative consequences on performance. KEY WORDS force, power, stretch-shortening cycle, strength

The purpose of this investigation was to examine the relationship between maximal squat strength and sprinting times. Seventeen Division I-AA male football athletes (height = 1.78 [+ or -] 0.04 m, body mass [BM] = 85.9 [+ or -] 8.8 kg, body mass index [BMI] = 27.0 [+ or -] 2.6 kg/[m.sup.2], 1 repetition maximum [1RM] = 166.5 [+ or -] 34.1 kg, 1RM/BM = 1.94 [+ or -] 0.33) participated in this investigation. Height, weight, and squat strength (1RM) were assessed on day 1. Within 1 week, 5, 10, and 40 yard sprint times were assessed. Squats were performed to a 70[degrees] knee angle and values expressed relative to each subject's BM. Sprints were performed on a standard outdoor track surface with timing gates placed at the previously mentioned distances. Statistically significant (p [less than or equal to] 0.05) correlations were found between squat 1 RM/BM and 40 yard sprint times (r = -0.605, p = 0.010, power = 0.747) and 10 yard sprint times (r = -0.544, p = 0.024, power = 0.626). The correlation approached significance between 5 yard sprint times and 1RM/BM (r = -0.4502, p = 0.0698, power = 0.4421). Subjects were then divided into those above 1RM/BM of 2.10 and below 1RM/BM of 1.90. Subjects with a 1RM/BM above 2.10 had statistically significantly lower sprint times at 10 and 40 yards in comparison with those subjects with a 1RM/BM ratio below 1.90. This investigation provides additional evidence of the possible importance of maximal squat strength relative to BM concerning sprinting capabilities in competitive athletes. KEY WORDS power, running, football, athletes

Abstract Background The use of the Cre/loxP system for gene targeting has been proven to be a powerful tool for understanding gene function. The purpose of this study was to create and characterize an inducible, skeletal muscle-specific Cre transgenic mouse strain. Methods To achieve skeletal muscle-specific expression, the human α-skeletal actin promoter was used to drive expression of a chimeric Cre recombinase containing two mutated estrogen receptor ligand-binding domains. Results Western blot analysis, PCR and β-galactosidase staining confirmed that Cre-mediated recombination was restricted to limb and craniofacial skeletal muscles only after tamoxifen administration. Conclusions A transgenic mouse was created that allows inducible, gene targeting of floxed genes in adult skeletal muscle of different developmental origins. This new mouse will be of great utility to the skeletal muscle community.

The purpose of this investigation was to determine the relationship between relative net vertical impulse and jump height in a countermovement jump and static jump performed to varying squat depths. Ten college-aged males with 2 years of jumping experience participated in this investigation (age: 23.3 ± 1.5 years; height: 176.7 ± 4.5 cm; body mass: 84.4 ± 10.1 kg). Subjects performed a series of static jumps and countermovement jumps in a randomized fashion to a depth of 0.15, 0.30, 0.45, 0.60, and 0.75 m and a self-selected depth (static jump depth = 0.38 ± 0.08 m, countermovement jump depth = 0.49 ± 0.06 m). During the concentric phase of each jump, peak force, peak velocity, peak power, jump height, and net vertical impulse were recorded and analyzed. Net vertical impulse was divided by body mass to produce relative net vertical impulse. Increasing squat depth corresponded to a decrease in peak force and an increase in jump height and relative net vertical impulse for both static jump and countermovement jump. Across all depths, relative net vertical impulse was statistically significantly correlated to jump height in the static jump (r = .9337, p < .0001, power = 1.000) and countermovement jump (r = .925, p < .0001, power = 1.000). Across all depths, peak force was negatively correlated to jump height in the static jump (r = -0.3947, p = .0018, power = 0.8831) and countermovement jump (r = -0.4080, p = .0012, power = 0.9050). These results indicate that relative net vertical impulse can be used to assess vertical jump performance, regardless of initial squat depth, and that peak force may not be the best measure to assess vertical jump performance. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this investigation was to determine the effect of stable and unstable conditions on one repetition maximum strength and muscle activity during dynamic squatting using absolute and relative loading. Methods: Ten recreationally weight-trained males participated in this study (age = 24.1 ± 2.0 y, height = 178.0 ± 5.6 cm, body mass = 83.7 ± 13.4 kg, 1RM/body mass = 1.53 ± 0.31), which involved two laboratory sessions separated by 1 wk. Linear position transducers were used to track bar displacement while subjects stood on a force plate for all trials. Vastus lateralis (VL), biceps femoris (BF) and erector spinae (L1) muscle activity (average integrated EMG [IEMG]) was also recorded during all trials. During the first session subjects complete a one repetition maximum test in a stable dynamic squat (S1RM = 128.0 ± 31.4 kg) and an unstable dynamic squat (U1RM = 83.8 ± 17.3 kg) in a randomized order with a 30-min rest period between conditions. The second session consisted of the performance of three trials each for 12 different conditions (unstable and stable squats using three different absolute loads [six conditions] and unstable and stable squats using three different relative loads [six conditions]). Results: Results revealed a statistically significant difference between S1RM and U1RM values (P ≤ .05). The stable trials resulted in the same or a significantly higher value for VL, BF and L1 muscle activity in comparison with the unstable trials for all twelve conditions. Conclusions: Unstable squatting is of equal or less (depending on the loading condition) benefit to improving or maximizing muscle activity during resistance exercise. [ABSTRACT FROM AUTHOR]