The Bilateral Deficit Phenomenon in Elbow Flexion: Explanations for Its Inconsistent Occurrence and Detection

The underlying mechanism(s) of the Bilateral Deficit (BLD) phenomenon is without consensus. Methodological inconsistencies across prior works may be an important source of equivocal results and interpretations. Based on repeatability problems with the BLD measure and maximal force definition, the presence or absence of the BLD phenomenon is altered, shifting conclusions of its mechanistic cause. Our purpose in this study was to examine methodological inconsistencies in applying the BLD measure to establish optimal methods for evaluating the underlying mechanism. Eleven healthy participants engaged in one familiarity and five test sessions, completing bilateral and unilateral elbow maximal voluntary isometric contractions. We defined maximal force by averaged and absolute peak and plateau values. BLD was evident if the bilateral index (BI), the ratio of the bilateral over summed unilateral forces, was statistically different from zero. We addressed interclass correlations (ICC), Chronbach’s α, standard error of the mean, and minimal detectable change between and within sessions for all force measures and BI. We evaluated all combinations of sessions (i.e., 1–2, 3–5, 5–6) and maximal forces to establish the optimal number of sessions to achieve reliability. BLD was present for test sessions, but not for familiarization. All measures of maximal force were highly reliable between and within sessions (ICC(2,1) ≥ .895). BI was only considered significantly reliable in sessions 3–5 ( p < .027), defined by absolute and average plateau forces, but reliability was still quantifiably poor (absolute: ICC(2,1) = .392; average: ICC(2,1) = .375). These results demonstrate that high force reliability within and between sessions does not translate to stable and reliable BI, potentially exposing the lack of any defined BLD mechanism.

Finger stability in precision grips

Stable precision grips using the fingertips are a cornerstone of human hand dexterity. Occasionally, however, our fingers become unstable and snap into a hyper-extended posture. This is because multi-link mechanisms, like our fingers, can buckle under tip forces. Suppressing this instability is crucial for hand dexterity, but how the neuromuscular system does so is unknown. Here we show that finger stability is due to the stiffness from muscle contraction and likely not feedback control. We recorded maximal force application with the index finger and found that most buckling events lasted less than 50ms, too fast for sensorimotor feedback to act. However, a biomechanical model of the finger predicted that muscle-induced stiffness is also insufficient for stability at maximal force unless we add springs to stiffen the joints. We tested this prediction in 39 volunteers. Upon adding stiffness, maximal force increased by 34±3%, and muscle electromyography readings were 21±3% higher for the finger flexors (mean±standard error). Hence, people refrain from applying truly maximal force unless an external stabilizing stiffness allows their muscles to apply higher force without losing stability. Muscle recordings and mathematical modeling show that the splint offloads the demand for muscle co-contraction and this reduced co-contraction with the splint underlies the increase in force. But more stiffness is not always better. Stiff fingers would interfere the ability to passively adapt to complex object geometries and precisely regulate force. Thus, our results show how hand function arises from neurally tuned muscle stiffness that balances finger stability with compliance.

Lack of increased rate of force development after strength training is explained by specific neural, not muscular, motor unit adaptations

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data acquired from surface electromyography during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ~4 spikes/s) when reaching the plateau phase (~150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Analysis of Grip Amplitude on Velocity in Paralympic Powerlifting

(1) Background: Paralympic Powerlifting (PP) is a Paralympic modality that is predominantly about developing maximal force, as there are athletes who lift three times their body weight. Our objective was to evaluate the averages of the velocity for 30% and 50% of 1 Maximum Repetition (1 RM) on different amplitudes of the footprint in PP athletes; (2) Methods: The intervention happened over two weeks, with the first being devoted to the familiarization and testing of 1 RM, while in the second week, through the use of a linear Encoder, tests of velocity average (VA), velocity average propulsive (VAP), and velocity peak (VP) were carried out with loads of 30% and 50% of a maximum repetition 1 RM for 1× of the biacromial distance (BAD) 1.3 × BAD, 1.5 × BAD; (3) Results: There was a significant difference in the average velocity of 1 × BAD (1.16 ± 0.14 m/s, 1.07–1.26 IC; η2p 0.20) when compared to 1.3 × BAD (1.00 ± 0.17 m/s, 0.90–1.09 IC; η2p 0.20) over 30% of 1 RM. For the other velocity variables for 30% and 50% of 1 RM with different grip amplitudes, there were no significant differences; (4) Conclusions: In PP, the 1 × BAD footprint contributes significantly to VA at 30% of 1 RM when compared to the 1.3 × BAD and 1.5 × BAD footprints. For loading at 50% of 1 RM the VA, VAP and VP decreased when compared to 30% of 1 RM, to the extent that the VAP and VP generated with the 1.3 × BAD and 1.5 × BAD footprints were higher than those with 1 × BAD, other than for VA 50% of 1 RM, where the 1 × BAD footprint was superior to the others.

From Maximal Force to the Field Equations of General Relativity

We point out that field equations of general relativity are implied by a maximal force given by c4/4G, analogous to the way that special relativity is implied by a maximal speed given by c. We present some of the arguments for this equivalence. The maxi-mal force naturally plays the role of an elastic constant for space-time. Implications of the maximal force for gravitational wave measurements, cosmology and black holes are highlighted. Quantum aspects of the maximal force are discussed.

Association Between Physical Activity, Quadriceps Muscle Performance, and Biological Characteristics of Very Old Men and Women

The aim of the study was to evaluate the association between physical activity, knee extensors (KE) performance (ie, isometric strength and fatigability), and biological parameters (ie, muscle structural, microvascular, and metabolic properties) in healthy very old men and women. Thirty very old adults (82 ± 1 years, 15 women) performed an isometric Quadriceps Intermittent Fatigue (QIF) test for the assessment of KE maximal force, total work (index of absolute performance), and fatigability. Muscle biopsies from the vastus lateralis muscle were collected to assess muscle fibers type and morphology, microvasculature, and enzymes activity. Correlation analyses were used to investigate the relationships between physical activity (steps/day, actimetry), KE performance, and biological data for each sex separately. Men, compared to women, showed greater total work at the QIF test (44 497 ± 8 629 Ns vs 26 946 ± 4 707 Ns; p &lt; .001). Steps per day were correlated with total work only for women (r = 0.73, p = .011). In men, steps per day were correlated with the percentage (r = 0.57, p = .033), shape factor (r = 0.75, p = .002), and capillary tortuosity of type IIX fibers (r = 0.59, p = .035). No other relevant correlations were observed for men or women between steps per day and biological parameters. Physical activity level was positively associated with the capacity of very old women to perform a fatiguing test, but not maximal force production capacity of the KE. Physical activity of very old men was not correlated with muscle performance. We suggest that very old women could be at higher risk of autonomy loss and increasing the steps per day count could provide a sufficient stimulus for adaptations in less active women.

The beneficial effect of chronic muscular exercise on muscle fragility is increased by Prox1 gene transfer in dystrophic mdx muscle

Purpose. Greater muscle fragility is thought to cause the exhaustion of the muscle stem cells during successive degeneration/repair cycles, leading to muscle wasting and weakness in Duchenne muscular dystrophy. Chronic voluntary exercise can partially reduce the susceptibility to contraction induced-muscle injury, i.e., muscle fragility, as shown by a reduced immediate maximal force drop following lengthening contractions, in the dystrophic mdx mice. Here, we studied the effect of Prospero-related homeobox factor 1 gene ( Prox1) transfer (overexpression) on fragility in chronically exercised mdx mice, because Prox1 promotes slower type fibres in healthy mice and slower fibres are less fragile in mdx muscle. Methods . mdx mice received or not Prox1 transfer into the tibialis anterior muscle and performed voluntary running into a wheel during 1 month. We also performed Prox1 transfer in sedentary mdx mice. In situ maximal force production of the muscle in response to nerve stimulation was assessed before, during and after 10 lengthening contractions. Molecular and cellular parameters were also evaluated. Results . Interestingly, Prox1 transfer reduced the force drop following lengthening contractions in exercised mdx mice (p < 0.05 to 0.01), but not in sedentary mdx mice. It also increased the muscle expression of Myh7 (p < 0.001), MHC-2x (p < 0.01) and Trpc1 (p < 0.01), whereas it reduced that one of Myh4 (p < 0.001) and MHC-2b (p < 0.01) in exercised mdx mice. Moreover, Prox1 transfer decreased the maximal force (p < 0.01) before lengthening contraction in exercised mdx mice (p < 0.01), and reduced muscle weight (p < 0.0001) despite increased Mstn expression (p < 0.001). Conclusion . Our results indicate that the beneficial effect of Prox1 transfer on muscle fragility is only observed in chronically exercised mdx mice. Thus, Prox1 transfer combined to chronic exercise have the potential to substantially slow the progression of the dystrophic disease in the long term.

Reliability of the Single-Leg, Medial Countermovement Jump in Youth Ice Hockey Players

Appropriate performance tests are critical for documenting training, fatigue and injury-related changes. Functional performance testing can provide quantitative information on specialized sport movements. The single-leg, medial countermovement jump is an objective measure of frontal plane force, velocity and power, and is particularly applicable for ice hockey players given that ice skating involves applying lateral forces. This study assessed the short-term reliability (10 days) of the single-leg, medial countermovement jump performed by ten competitive male youth ice hockey players. Each participant performed three right and three left maximal single-leg, medial countermovement jumps from force plates. Measured variables included lateral and vertical takeoff velocity, lateral and vertical maximal force, maximal force above bodyweight, lateral and vertical peak concentric power, average concentric power, and average concentric power during the last 100 ms of push-off. Relative reliability was quantified by intraclass correlations. Absolute reliability and the smallest real difference were also calculated. The single-leg, medial countermovement jump had moderate-to-excellent test–retest reliability (ICC: 0.50–0.98) for all twelve variables of interest. These results suggest that the single-leg, medial countermovement jump is a reliable test for assessing frontal plane force, velocity and power in ice hockey players, and is a valid functional performance test for this population given the similarity to ice skating.

The Effects of Prioritizing Lead or Boulder Climbing Among Intermediate Climbers

This study compared the effects of prioritizing lead climbing or boulder climbing on climbing-specific strength and endurance, as well as climbing performance. Fourteen active climbers were randomized to a boulder climbing training group (BCT: age = 27.2 ± 4.4 years, body mass = 65.8 ± 5.5 kg, height = 173.3 ± 3.8 cm) or a lead-climbing training group (LCT: age = 27.7 ± 6.1 years, body mass = 70.2 ± 4.4 kg, height = 177.7 ± 4.4 cm). The groups participated in a 5-week training period consisting of 15 sessions, performing either two weekly bouldering sessions and one maintenance-session of lead-climbing (BCT) or two weekly lead-climbing sessions and one maintenance-session of bouldering (LCT). Pre- and post-training, maximal force and rate of force development (RFD) were measured during isometric pull-ups performed on a jug hold and a shallow rung, and during an isolated finger-strength test. Lead-climbing and bouldering performance were also measured, along with an intermittent forearm endurance test. The pre-to-post changes were not significantly different between the groups for any of the parameters (P = 0.062–0.710). However, both the BCT (ES = 0.30, P = 0.049) and LCT (ES = 0.41, P = 0.046) groups improved strength in the isometric pull-up performed using the jug, whereas neither group improved force in the rung condition (P = 0.054 and P = 0.084) or RFD (P = 0.060 and P = 0.070). Furthermore, climbing and bouldering performance remained unchanged in both groups (P = 0.210–0.895). The LCT group improved forearm endurance (ES = 0.55, P = 0.007), while the BCT group improved isolated finger strength (ES = 0.35, P = 0.015). In addition to isometric pull-up strength, bouldering can increase isolated finger strength while lead-climbing may improve forearm endurance. A 5-week period prioritizing one discipline can be safely implemented for advanced to intermediate climbers without risking declined performance in the non-prioritized discipline.