Transitory force decrease following a sudden reduction in stimulation frequency in motor units of rat medial gastrocnemius

Effect of Calcitriol and Vitamin D Receptor Modulator 2 on Recovery of Injured Skeletal Muscle in Wistar Rats

Muscle injuries often result in functional limitations due to insufficient healing. This study assessed the influence of calcitriol and vitamin D Receptor Modulator 2 (VDRM2) on muscle regeneration in male Wistar rats following open blunt muscle injury. The injured left soleus muscle of the rats was treated for the first four days after trauma with local injections of either calcitriol, VDRM2, or a 10% ethanol solution (control). Although muscle strength significantly decreased post-injury, all groups showed gradual improvement but did not achieve full recovery. By the 14th day, calcitriol-treated rats significantly outperformed the control group in the incomplete tetanic force, with VDRM2-treated rats showing muscle strength values that fell between the control and calcitriol groups. Similar trends were observed in complete tetanic contractions and were confirmed histologically via muscle cell width quantification. Additionally, histological analysis showed increased cellular turnover on the fourth postoperative day in the calcitriol group, as indicated by elevated cell proliferation rates and fewer apoptotic cells. VDRM2-treated animals showed only an increased proliferative activity on day 4 after injury. No noticeable differences between the groups for CAE-positive cells or visible muscle tissue area were found. In conclusion, predominantly calcitriol positively influenced post-trauma muscle recovery, where VDRM2 had substantially lower biological activity.

Biomechanical conditioning of the motor unit transitory force decrease following a reduction in stimulation rate

BACKGROUND

The biomechanical background of the transitory force decrease following a sudden reduction in the stimulation frequency under selected experimental conditions was studied on fast resistant motor units (MUs) of rat medial gastrocnemius in order to better understand the mechanisms of changes in force transmission.

METHODS

Firstly, MUs were stimulated with three-phase trains of stimuli (low-high-low frequency pattern) to identify patterns when the strongest force decrease (3-36.5%) following the middle high frequency stimulation was observed. Then, in the second part of experiments, the MUs which presented the largest force decrease in the last low-frequency phase were alternatively tested under one of five conditions to analyse the influence of biomechanical factors of the force decrease: (1) determine the influence of muscle stretch on amplitude of the force decrease, (2) determine the numbers of interpulse intervals necessary to evoke the studied phenomenon, (3) study the influence of coactivation of other MUs on the studied force decrease, (4) test the presence of the transitory force decrease at progressive changes in stimulation frequency, (5) and perform mathematical analysis of changes in twitch-shape responses to individual stimuli within a tetanus phase with the studied force decrease.

RESULTS

Results indicated that (1) the force decrease was highest when the muscle passive stretch was optimal for the MU twitch (100 mN); (2) the middle high-frequency burst of stimuli composed of at least several pulses was able to evoke the force decrease; (3) the force decrease was eliminated by a coactivation of 10% or more MUs in the examined muscle; (4) the transitory force decrease occured also at the progressive decrease in stimulation frequency; and (5) a mathematical decomposition of contractions with the transitory force decrease into twitch-shape responses to individual stimuli revealed that the force decrease in question results from the decrease of twitch forces and a shortening in contraction time whereas further force restitution is related to the prolongation of relaxation.

CONCLUSIONS

High sensitivity to biomechanical conditioning indicates that the transitory force decrease is dependent on disturbances in the force transmission predominantly by collagen surrounding active muscle fibres.

Initial force production before sag is enhanced by prior contraction followed by a 3-minute rest period in fast motor units of the rat medial gastrocnemius

Unfused tetanic contractions evoked in fast motor units exhibit extra-efficient force production at the onset of contraction, an effect called "boost". Boost is diminished in subsequent contractions if there is a short rest period between contractions, but can be re-established with a longer period of rest. We tested the hypothesis that contractile activity and rest could enhance boost-related metrics. Two sets of 3 unfused tetani were evoked 3 min apart in fast fatigable (FF) and fast fatigue-resistant (FR) motor units of the rat medial gastrocnemius. The greatest changes occurred in the first unfused tetanic contractions. Relative to the first contraction in the first set, the first contraction in the second set exhibited higher peak force during boost in a subset of motor units (76% of FF and 48% of FR). Enhanced force during boost was influenced by interaction of slowing of twitch contraction time (up to 20% and 25%, for FF and FR motor units, respectively), half-relaxation time (up to 37% and 49% for FF and FR motor units, respectively), and potentiation of the first twitch (up to 13% and 5% for FF and FR motor units, respectively). Examination of twitches evoked between sets suggested opportunity for greater enhancement of boost with shorter intervening rest periods. The phenomenon of enhanced boost following motor unit activity may interest sports scientists.