Sex comparisons of the bilateral deficit in proximal and distal upper body limb muscles

Bilateral Force Deficit in Proximal Effectors Versus Distal Effectors in Lower Extremities

Purpose: Bilateral force deficit occurs when the maximal generated force during simultaneous bilateral muscle contractions is lower than the sum of forces generated unilaterally. Neural inhibition is stated as the main source for bilateral force deficit. Based on differences in bilateral neural organization, there might be a pronounced neural inhibition for proximal compared to distal effectors. The aim of the present experiment was to evaluate potential differences in bilateral force deficit in proximal compared to distal effectors in lower extremities. Methods: Fifteen young adults performed single-joint maximal voluntary contractions in isometric dorsiflexion of ankle (distal) and knee (proximal) extension unilaterally and bilaterally. Results: Results showed a significant absolute bilateral force deficit for both proximal (123.46 ± 59.51 N) and distal effectors (33.00 ± 35.60 N). Interestingly, the relative bilateral force deficit for knee extension was significantly larger compared to dorsiflexion of ankle, 19.98 ± 10.04% and 10.27 ± 9.57%, respectively. Our results indicate a significantly higher bilateral force deficit for proximal effectors compared to distal effectors. Conclusion: Plausible explanations are related to neuroanatomical and neurophysiological differences between proximal effectors and distal effectors where proximal muscles have a higher potential for bilateral communication compared to distal muscles. In addition, higher forces produced with proximal effectors could cause a higher perceived exertion and cause a more pronounced bilateral force deficit to proximal effectors.

Variability of unilateral and bilateral isometric muscle strength of lower extremities extensors in young females and males

BACKGROUND

The muscle strength of the lower extremity extensors can be evaluated in the closed kinetic chain (CKC) during unilateral or bilateral conditions. Factors such as the mass and length of the muscle, joint angle, type of contraction, and gender influence the magnitude of the muscle strength. The aim of this study was to compare the isometric strength of lower extremity extensors between the different knee extension angles (KEs) as well as between bilateral and unilateral conditions.

METHODS

Nineteen female students (age: 20.2 ± 0.6 years) and nineteen male students (age: 20.3 ± 0.7 years) participated in the study. The muscle strength was evaluated in CKC using the strain gauge dynamometer. The analysis included values of the maximum muscle strength normalized to body mass (MS/BM) for the six KEs of 80°, 70°, 60°, 50°, 40° and 30°.

RESULTS

A significant main effect in the MS/BM values for the angle factor (p < 0.001) and condition factor (p < 0.001) was found. Moreover, there was a non-significant interaction effect between the angle factor and gender factor (p = 0.476) as well as between the condition factor and gender factor (p = 0.770). Comparisons showed significant differences in the MS/BM values between the six KEs (p < 0.001). Furthermore, significantly lower MS/BM values for bilateral conditions than unilateral conditions at the 30° KE were observed (p < 0.001).

CONCLUSION

The decrease in KE by 10° significantly increased the muscle strength of the lower extremity extensors. Gender did not affect the change in MS/BM values with the change in KE and conditions. Findings also revealed significant bilateral deficit, i.e., significantly a lower summed muscle strength during bilateral conditions than unilateral conditions. The study emphasized the importance of selecting the 30° KE as the optimal angle to assess the maximum strength developed in CKC.

Effect of bilateral contraction on the ability and accuracy of rapid force production at submaximal force level

The present study aims to clarify the effects of bilateral contraction on the ability and accuracy of rapid force production at the submaximal force level. Eleven right-handed participants performed rapid gripping as fast and precisely as they could in unilateral (UL) and bilateral (BL) contractions in a standing position. Participants were required to impinge a grip force of 30% and 50% of their maximal voluntary contraction (MVC). Ability and accuracy of rapid force production were evaluated using the rate of force development (RFD) and force error, respectively. The data analysis did not observe a significant difference in the RFD between UL and BL contractions in both 30% (420±86 vs. 413±106%MVC/s, p = 0.34) and 50% of MVC (622±84 vs. 619±103%MVC/s, p = 0.77). Although the RFD to peak force ratio (RFD/PF) in BL contraction was lower than in UL in 30% of MVC (12.8±2.8 vs. 13.4±2.7, p = 0.003), it indicated a small effect size (d = 0.22) of the difference between UL and BL in RFD/PF. The absolute force error of BL contraction was higher than of UL contraction in 30% (4.67±2.64 vs. 3.64±1.13%MVC, p = 0.005) and 50% of MVC (5.53±2.94 vs. 3.53±0.71%MVC, p = 0.009). In addition, medium and large effect sizes were observed in absolute force error from 30% (d = 0.51) and 50% of MVC (d = 0.94), respectively. In conclusion, results indicated that the bilateral contraction reduced in the ability and accuracy of rapid force production at the submaximal force level. Nevertheless, the present results suggest that the noticeable effect of bilateral contraction is more prominent on the accuracy than in the ability of rapid force production at the submaximal force level.

Bilateral deficit in strength but not rapid force during maximal handgrip contractions

The purpose of this experiment was to evaluate the bilateral index in force and electromyographic (EMG) responses for the dominant and non-dominant hands during maximal handgrip contractions in males and females. Thirty-two right-handed participants (16 females) performed maximal unilateral and bilateral handgrip contractions on two separate visits. Bilateral indices were computed for maximal force, rate of force development (RFD₁₀₀), EMG amplitude, and the rate of EMG rise (RER). There was a bilateral deficit for maximal force in the dominant (-4.98 ± 7.39%, p < 0.001; d = 0.674) but not the non-dominant hand (-1.57 ± 9.10%, p = 0.334; d = 0.173). No deficits were observed for rapid force. The non-dominant flexor carpi radialis showed a bilateral facilitation in EMG amplitude (+12.32 ± 19.29%, p < 0.001; d = 0.638), yet a bilateral deficit for RER (-22.10 ± 27.80%, p < 0.001; d = 0.795). No sex differences were observed for any of the bilateral indices. These data suggest that maximal but not rapid force is susceptible to a bilateral deficit during contractions of the hands. The EMG responses did not parallel the force data. We show sex does not influence the magnitude or direction of the bilateral index in this muscle group.

Changes in corticospinal excitability during bilateral and unilateral lower-limb force control tasks

Ankle dorsiflexion force control is essential for performing daily living activities. However, the involvement of the corticospinal pathway during different ankle dorsiflexion tasks is not well understood. The objective of this study was to compare the corticospinal excitability during: (1) unilateral and bilateral; and (2) ballistic and tonic ankle dorsiflexion force control. Fifteen healthy young adults (age: 25.2 ± 2.8 years) participated in this study. Participants performed unilateral and bilateral isometric ankle dorsiflexion force-control tasks, which required matching a visual target (10% of maximal effort) as quickly and precisely as possible during ballistic and tonic contractions. Transcranial magnetic stimulation (TMS) was applied over the primary motor cortex to elicit motor-evoked potentials (MEPs) from the right tibialis anterior during: (i) pre-contraction phase; (ii) ascending contraction phase; (iii) plateau phase (tonic tasks only); and (iv) resting phase (control). Peak-to-peak MEP amplitude was computed to compare the corticospinal excitability during each experimental condition. MEP amplitudes significantly increased during unilateral contraction compared to bilateral contraction in the pre-contraction phase. There were no significant differences in the MEP amplitudes between the ballistic tasks and tonic tasks in any parts of the contraction phase. Although different strategies are required during ballistic and tonic contractions, the extent of corticospinal involvement appears to be similar. This could be because both tasks enhance the preparation for precise force control. Furthermore, our results suggest that unilateral muscle contractions may largely facilitate the central nervous system during movement preparation for unilateral force control compared to bilateral muscle contractions.