The effects of concentric and eccentric training in murine models of dysferlin-associated muscular dystrophy

INTRODUCTION

Dysferlin-deficient murine muscle sustains severe damage after repeated eccentric contractions.

METHODS

With a robotic dynamometer, we studied the response of dysferlin-sufficient and dysferlin-deficient mice to 12 weeks of concentrically or eccentrically biased contractions. We also studied whether concentric contractions before or after eccentric contractions reduced muscle damage in dysferlin-deficient mice.

RESULTS

After 12 weeks of concentric training, there was no net gain in contractile force in dysferlin-sufficient or dysferlin-deficient mice, whereas eccentric training produced a net gain in force in both mouse strains. However, eccentric training induced more muscle damage in dysferlin-deficient vs dysferlin-sufficient mice. Although concentric training produced minimal muscle damage in dysferlin-deficient mice, it still led to a prominent increase in centrally nucleated fibers. Previous exposure to concentric contractions conferred slight protection on dysferlin-deficient muscle against damage from subsequent injurious eccentric contractions.

DISCUSSION

Concentric contractions may help dysferlin-deficient muscle derive the benefits of exercise without inducing damage.

Reduced corticospinal responses in older compared with younger adults during submaximal isometric, shortening, and lengthening contractions

The aim of this study was to assess differences in motor performance, as well as corticospinal and spinal responses to transcranial magnetic and percutaneous nerve stimulation, respectively, during submaximal isometric, shortening, and lengthening contractions between younger and older adults. Fifteen younger [26 yr (SD 4); 7 women, 8 men] and 14 older [64 yr (SD 3); 5 women, 9 men] adults performed isometric and shortening and lengthening dorsiflexion on an isokinetic dynamometer (5°/s) at 25% and 50% of contraction type-specific maximums. Motor evoked potentials (MEPs) and H reflexes were recorded at anatomical zero. Maximal dorsiflexor torque was greater during lengthening compared with shortening and isometric contractions ( P < 0.001) but was not age dependent ( P = 0.158). However, torque variability was greater in older compared with young adults ( P < 0.001). Background electromyographic (EMG) activity was greater in older compared with younger adults ( P < 0.005) and was contraction type dependent ( P < 0.001). As evoked responses are influenced by both the maximal level of excitation and background EMG activity, the responses were additionally normalized {[MEP/maximum M wave (M_max)]/root-mean-square EMG activity (RMS) and [H reflex (H)/M_max]/RMS}. (MEP/M_max)/RMS and (H/M_max)/RMS were similar across contraction types but were greater in young compared with older adults ( P < 0.001). Peripheral motor conduction times were prolonged in older adults ( P = 0.003), whereas peripheral sensory conduction times and central motor conduction times were not age dependent ( P ≥ 0.356). These data suggest that age-related changes throughout the central nervous system serve to accommodate contraction type-specific motor control. Moreover, a reduction in corticospinal responses and increased torque variability seem to occur without a significant reduction in maximal torque-producing capacity during older age. NEW & NOTEWORTHY This is the first study to have explored corticospinal and spinal responses with aging during submaximal contractions of different types (isometric, shortening, and lengthening) in lower limb musculature. It is demonstrated that despite preserved maximal torque production capacity corticospinal responses are reduced in older compared with younger adults across contraction types along with increased torque variability during dynamic contractions. This suggests that the age-related corticospinal changes serve to accommodate contraction type-specific motor control.

Heat stress enhances mTOR signaling after resistance exercise in human skeletal muscle

This study investigated the effect of heat stress (HS) on mammalian target of rapamycin (mTOR) signaling involved in translation initiation after resistance exercise in human skeletal muscle. Eight young male subjects performed four sets of six maximal repetitions of knee extension exercises, with or without HS, in a randomized crossover design. HS was applied to the belly of the vastus lateralis by using a microwave therapy unit prior to and during exercise. Muscle biopsies were taken from the vastus lateralis before, immediately after, and 1 h after exercise. HS significantly increased the phosphorylation of Akt/PKB, mTOR, and ribosomal protein S6 at 1 h after exercise (P < 0.05), and the 4E-BP1 phosphorylation level, which had initially decreased with exercise, had recovered by 1 h after exercise with HS. In addition, the phosphorylation of ribosomal S6 kinase 1 was significantly increased immediately after exercise with HS (P < 0.05). These results indicate that HS enhances mTOR signaling after resistance exercise in human skeletal muscle.