The strength of associations between ultrasound measures of upper limb muscle morphology and isometric muscle strength: An exploratory study

INTRODUCTION/AIMS

Assessing upper limb muscle strength is important for understanding health outcomes, such as daily function and mortality. Ultrasound (US) is increasingly used to evaluate muscle health, but the relationship between its measures of morphology and isometric strength has not been thoroughly explored in upper limb muscles. The aim of this study was to evaluate the associations between US morphological measures and isometric strength in functionally relevant upper limb muscles in healthy adults.

METHODS

Twenty-four healthy volunteers (30.0 ± 10.8 years) underwent B-mode, axial US scans of the first dorsal interosseus (FDI), flexor pollicis longus (FPL), biceps brachii (BB), brachialis (BR), and triceps brachii lateral head (TB). Participants performed corresponding maximal voluntary contractions (MVC), including first digit distal phalanx flexion, second digit abduction, and elbow flexion and extension. US images were segmented to obtain maximal muscle thickness (MT) and cross-sectional area (CSA).

RESULTS

Strong positive correlations were found between muscle strength and BB MT (r = .83; p < .001), BR CSA (r = .84; p < .001), and TB MT (r = .70; p < .001). Moderate positive correlations were found for strength and FDI CSA (r = .67; p < .001), FDI MT (r = .47; p < .05), FPL CSA (r = .54; p < .01), and FPL MT (r = .42; p < .05). No significant correlation was found between strength and BR MT (r = .16; p > .05).

DISCUSSION

Our data showed moderate-to-strong associations between US muscle morphology and strength, suggesting that US is likely a good biomarker for strength. However, its use is not "one size fits all." Future investigations should continue to assess this relationship in different muscles and expand the generalizability to clinical populations.

Post-activation potentiation after isometric contractions is strongly related to contraction intensity despite the similar torque-time integral

Post-activation potentiation (PAP) is defined as an enhanced contractile response of a muscle following its own contractile activity and is influenced by the intensity and duration of the conditioning contraction. The aim of this study was to determine if the combination of intensity and duration, that is, torque-time integral (TTI) is a determinant of PAP amplitude. We compared PAP amplitude following low-to-maximal voluntary conditioning contraction intensities with and without similar TTI in the knee extensors. Twelve healthy males completed two experimental sessions. Femoral nerve stimulation was applied to evoke single twitches on the relaxed quadriceps before and after isometric conditioning contractions of knee extensors. In one session, participants performed conditioning contractions without similar TTI (6 s at 100, 80, 60, 40 and 20% maximal voluntary contraction (MVC)), while they performed conditioning contractions with similar TTI in the other session (6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC). In both sessions, PAP amplitude was related to conditioning contraction intensity. The higher the conditioning contraction intensity with or without similar TTI, the higher PAP. Significant correlations were found (i) between PAP and conditioning contraction intensity with (r2 = 0.70; P < 0.001) or without similar TTI (r2 = 0.64; P < 0.001), and (ii) between PAP with and without similar TTI (r2 = 0.82; P < 0.001). The results provide evidence that TTI has a minor influence on PAP in the knee extensors. This suggests that to optimize the effect of PAP, it is more relevant to control the intensity of the contraction rather than the TTI.

Influence of temperature on twitch potentiation following submaximal voluntary contractions in human plantar flexor muscles

This study aimed to clarify the influence of temperature on post-activation twitch potentiation, a possible indicator of fast-twitch fiber activation during a preceding voluntary contraction. Ten healthy males immersed their left lower leg in water of different temperatures (cold: 0°C, neutral: 32-35°C, hot: ~43°C) for 20 min each. In each temperature condition, they performed submaximal (10%-50% of maximal voluntary contraction torque measured before water immersion) and maximal plantar flexions. Immediately after each voluntary contraction, twitch contractions were evoked with supramaximal stimulation of the posterior tibial nerve. The magnitude of twitch potentiation, defined as a percent increase in twitch torque following a voluntary contraction, increased with the intensity of the preceding voluntary contraction. The magnitude of twitch potentiation after the maximal voluntary contraction was smaller in Cold than in the other temperature conditions. However, temperature had no influence on the relative magnitude of twitch potentiation following the submaximal contractions. In addition, there was no difference in electromyographic activity between the gastrocnemius and soleus muscles in any temperature conditions. Collectively, the temperature dependence was not observed when using twitch potentiation or electromyographic amplitude as an indicator of fast-twitch fiber activation during brief submaximal voluntary contractions.

Age and sex differences in force steadiness and intermuscular coherence of lower leg muscles during isometric plantar flexion

Age- and sex-related alterations in the control of multiple muscles during contractions are not well understood. The purpose of the present study was to examine the age and sex differences in force steadiness and intermuscular coherence (IMC), and thereby to clarify the functional role of IMC during plantar flexion. Twenty-six young (YNG, 23-34 years), thirty middle-aged (MID, 35-64 years) and twenty-four older adults (OLD, 65-82 years) performed submaximal isometric contractions of plantar flexion, while electromyography was recorded from the soleus (SOL), gastrocnemius lateralis/medialis (GL/GM) and tibialis anterior (TA) muscles. Coefficient of variation (CV) of torque and IMC in the alpha, beta and gamma bands was calculated. We found that OLD demonstrated significantly higher torque CV than YNG and MID, and males demonstrated significantly higher torque CV than females (both p < 0.05). The IMC in the gamma band (five out of the six pairs) was significantly higher in YNG than MID and/or OLD (p < 0.05), while the gamma band IMC between GL and SOL was significantly higher in females. However, age or sex differences were not detected in the alpha or beta band. Moreover, the gamma band IMC between SOL and TA had a weak (r =  - 0.229) but significant (p < 0.05) negative correlation with torque CV. These results suggest that force steadiness differs with age and sex, and that the higher gamma band IMC may contribute to more stable force control during plantar flexion.

Human thrust in aquatic environment: The effect of post-activation potentiation on flutter kick

Herein, we analyse by experimental techniques the human kicking thrust and measure the effect of a warm-up routine that includes post-activation potentiation (PAP) sets on front-crawl flutter kick thrust, kinematics, and performance. Sixteen male competitive swimmers with 22.13 ± 3.84 years of age were randomly assigned in a crossover manner to undergo a standard warm-up (non-PAP; control condition) and a warm-up that included PAP sets (PAP; experimental condition) consisting in 2 × 5 repetitions of unloaded countermovement jump. Participants performed a 25 m all-out trial in front-crawl with only flutter kicks eight min after each warm-up. Kinetics (i.e., peak thrust, mean thrust, and thrust-time integral) and kinematics (i.e., speed, speed fluctuation and kicking frequency) were experimentally collected by an in-house customized system composed of differential pressure sensors, speedo-meter, and underwater camera. Peak thrust (P = 0.02, d = 0.66) and mean thrust (P = 0.10, d = 0.40) were increased by 15% in PAP compared to non-PAP. Large and significant differences were noted in speed (P = 0.01, d = 0.54) and speed fluctuation (P = 0.02, d = 0.58), which improved by 10% in PAP compared with non-PAP. In conclusion, a warm-up that includes PAP sets improves kicking thrust, kinematics and performance.

Multiscale Hill-type modeling of the mechanical muscle behavior driven by the neural drive in isometric conditions

In this study, we present a new model describing the mechanical behavior of the skeletal muscle during isometric contraction. This model is based on a former Hill-inspired model detailing the electromechanical behavior of the muscle based on the Huxley formulation. However, in this new multiscale model the muscle is represented at the Motor Unit (MU) scale. The proposed model is driven by a physiological input describing the firing moments of the activated MUs. Definition of both voluntary and evoked MU recruitment schemes are described, enabling the study of both contractions in isometric conditions. During this type of contraction, there is no movement of the joints and the tendon-muscle complex remains at the same length. Moreover, some well-established macroscopic relationships such as force-length or force-velocity properties are considered. A comparison with a twitch model using the same input definition is provided with both recruitment schemes exhibiting limitations of twitch type models. Finally, the proposed model is validated with a comparison between simulated and recorded force profiles following eight electrical stimulations pulses in isometric conditions. The simulated muscle force was generated to mimic the one recorded from the quadriceps of a patient implanted with a functional electrical stimulation neuroprosthesis. This validation demonstrates the ability of the proposed model to reproduce realistically the skeletal muscle contractions and to take into account subject-specific parameters.

What are the best isometric exercises of muscle potentiation?

PURPOSE

The aim of this study was to follow post-activation potentiation (PAP), low-frequency fatigue (LFF), metabolic-induced fatigue and post-contractile depression (PCD) in response to different isometric muscle contraction modalities.

METHODS

Young healthy men (N = 120) were randomly assigned to one of ten exercise modality groups which differed in contraction duration (5-60 s), activation pattern (intermittent or continuous contractions), activation mode (voluntary or stimulated), and intensity [maximal or submaximal (50%)]. Isometric maximal voluntary contraction (MVC), and electrically induced knee extension torque were measured at baseline and at regular intervals for 60 min after exercise.

RESULTS

Muscle contraction modalities involving 5 s MVC were the most effective for PAP, whereas the lowest PAP effectiveness was found after the 12 × 5-MVC modality. After all of the 5-15 s MVC and 6 × 5-MVC protocols, the potentiation of the twitch rate was significantly higher than that recorded after continuous 30-60 s protocols (P < 0.001). Tetanic maximal torque (100 Hz) potentiation occurred 5 min after 15-30 s repetitive MVC modalities and after modality involving 15 electrical stimuli (P < 0.05).

CONCLUSIONS

The findings demonstrate that post-activation potentiation was most effective after brief duration continuous and repetitive MVC protocols. To understand the resultant warm-up of motor performance, it is necessary to recognize the coexistence of muscle PAP, tetanic maximal force potentiation, rapid recovery of metabolic muscle, and central muscle activation processes, as well as prolonged LFF and prolonged PCD.

Utility of electromyographic fatigue threshold during treadmill running

INTRODUCTION

We investigated 2 different methods for determining muscle fatigue threshold by electromyography (EMG).

METHODS

Thirteen subjects completed an incremental treadmill running protocol for EMG fatigue threshold (EMGFT ) determination based on the critical power concept (EMGFT 1) and the breakpoint in the linear relationship between EMG amplitude and exercise intensity (EMGFT 2). Then, both the EMGFT 1 and EMGFT 2 were tested in a continuous treadmill running protocol. EMG was recorded from the rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), and lateral gastrocnemius (LG) muscles.

RESULTS

For BF, EMGFT 2 was higher than EMGFT 1, and EMGFT 1 for BF was lower than EMGFT 1 for LG. EMG of RF was higher at EMGFT 2 than at EMGFT 1, and LG EMG was lower at EMGFT 2.

CONCLUSIONS

EMGFT can be determined during a single treadmill running test, and EMGFT 1 may be the most appropriate method to estimate the muscle fatigue threshold during running.

Effect of conditioning contraction intensity on postactivation potentiation is muscle dependent

We aimed to examine whether the influence of conditioning contraction intensity on the extent of postactivation potentiation (PAP) is muscle dependent. Eleven healthy males performed both thumb adduction and plantar flexion as a conditioning contraction. The conditioning contraction intensities were set at 20%, 40%, 60%, 80%, or 100% of the maximal voluntary isometric contraction (MVC). Before and after the conditioning contraction, twitch torque was measured for the respective joint to calculate the extent of PAP. In plantar flexion, the extent of PAP became significantly larger as the conditioning contraction intensity increased up to 80% MVC (p<0.05). In contrast, the extent of PAP in thumb adduction increased significantly only up to 60% MVC (p<0.05), but not at higher intensities. These results indicate that the influence of the conditioning contraction intensity on the extent of PAP is muscle dependent. Our results suggest that a conditioning contraction with submaximal intensity can sufficiently evoke sizable PAP in the muscle where most of muscle fibers are recruited at submaximal intensities, thereby attenuating muscle fatigue induced by the conditioning contraction.

Passive muscle length changes affect twitch potentiation in power athletes

INTRODUCTION

A conditioning maximal voluntary muscle action (MVC) has been shown to induce postactivation potentiation, that is, improved contractile muscle properties, when muscles are contracted isometrically. It is still uncertain how the contractile properties are affected during ongoing muscle length changes. The purpose of this study was to investigate the effects of a 6-s conditioning MVC on twitch properties of the plantarflexors during ongoing muscle length changes.

METHODS

Peak twitch, rate of torque development, and rate of torque relaxation, rising time, and half relaxation time were measured from supramaximal twitches evoked in the plantarflexors of 11 highly trained athletes. Twitches were evoked before a 6-s MVC and subsequently on eight different occasions during a 10-min recovery for five different modes: fast lengthening, slow lengthening, isometric, fast shortening, and slow shortening of the plantarflexors.

RESULTS

The magnitude and the duration of effects from the conditioning MVC were significantly different between modes. Peak twitch, rate of torque development, and rate of torque relaxation significantly increased for all modes but more so for twitches evoked during fast and slow shortening as compared with lengthening. Rising time was reduced in the lengthening modes but slightly prolonged in the shortening modes. Half relaxation time was significantly reduced for all modes, except fast lengthening.

CONCLUSIONS

The findings show that the effects of a conditioning MVC on twitch contractile properties are dependent on direction and velocity of ongoing muscle length changes. This may imply that functional enhancements from a conditioning MVC might be expected to be greatest for concentric muscle actions but are still present in isometric and eccentric parts of a movement.