Assessment of the rate of force development scaling factor for the hip muscles

Upper-Limb Muscle Fatigability in Para-Athletes Quantified as the Rate of Force Development in Rapid Contractions of Submaximal Amplitude

This study aimed to compare neuromuscular fatigability of the elbow flexors and extensors between athletes with amputation (AMP) and athletes with spinal cord injury (SCI) for maximum voluntary force (MVF) and rate of force development (RFD). We recruited 20 para-athletes among those participating at two training camps (2022) for Italian Paralympic veterans. Ten athletes with SCI (two with tetraplegia and eight with paraplegia) were compared to 10 athletes with amputation (above the knee, N = 3; below the knee, N = 6; forearm, N = 1). We quantified MVF, RFD at 50, 100, and 150 ms, and maximal RFD (RFDpeak) of elbow flexors and extensors before and after an incremental arm cranking to voluntary fatigue. We also measured the RFD scaling factor (RFD-SF), which is the linear relationship between peak force and peak RFD quantified in a series of ballistic contractions of submaximal amplitude. SCI showed lower levels of MVF and RFD in both muscle groups (all p values ≤ 0.045). Despite this, the decrease in MVF (Cohen's d = 0.425, p < 0.001) and RFDpeak (d = 0.424, p = 0.003) after the incremental test did not show any difference between pathological conditions. Overall, RFD at 50 ms showed the greatest decrease (d = 0.741, p < 0.001), RFD at 100 ms showed a small decrease (d = 0.382, p = 0.020), and RFD at 150 ms did not decrease (p = 0.272). The RFD-SF decreased more in SCI than AMP (p < 0.0001). Muscle fatigability impacted not only maximal force expressions but also the quickness of ballistic contractions of submaximal amplitude, particularly in SCI. This may affect various sports and daily living activities of wheelchair users. Early RFD (i.e., ≤50 ms) was notably affected by muscle fatigability.

Characterization of neuromuscular performances in adults with late-onset Pompe disease: A control case cross-sectional study

Adults with late-onset Pompe disease (aLOPD) are characterized by muscular contractile tissue deterioration. However, their neuromuscular performances are poorly known. We aimed to compare maximal muscle strength, activation, explosive strength and neuromuscular fatigue between aLOPD and controls. We studied 20 aLOPD and 20 matched controls. Isometric maximum voluntary contraction (MVC) torque was obtained for the hip, knee and ankle muscles. The voluntary activation level (VAL) during knee extensor MVC was assessed using interpolated twitch technique. Explosive strength was evaluated for knee and ankle muscles through the rate of torque development (RTD) during fast contractions. Neuromuscular fatigue was measured during a 30-second contraction of knee flexors and extensors. All muscle MVC torques were significantly lower in aLOPD than controls (p <0.05). The weakest muscles were the hip extensors followed by hip abductors and abductors. Raw value of RTD was lower in aLOPD for the majority of muscles (p <0.05). No intergroup differences were reported for normalized RTD, VAL and neuromuscular fatigue (p-values> 0.05). Our study shows that maximal strength was the only neuromuscular characteristic affected in aLOPD with a proximal-distal intensity gradient. This suggests that the surviving muscle tissue of aLOPD is as functionally efficient as that of control individuals.

Reliability of Maximal Strength and Peak Rate of Force Development in a Portable Nordic Hamstrings Exercise Device

The Nordic hamstring exercise (NHE) is a very popular exercise used to improve eccentric strength and prevent injuries. The aim of this investigation was to assess the reliability of a portable dynamometer that measures maximal strength (MS) and rate of force development (RFD) during the NHE. Seventeen physically active participants (34.8 ± 4.1 years; n = 2 women and n = 15 men) participated. Measurements occurred on two different days separated by 48-72 h. Test-retest reliability was calculated for bilateral MS and RFD. No significant test-retest differences were observed in NHE (test-retest [95% CI, confidence interval]) for MS [-19.2 N (-67.8; 29.4); p = 0.42] and RFD [-70.4 N·s^-1 (-178.4; 37.8); p = 0.19]. MS showed high reliability (intraclass correlation coefficient [ICC] [95% CI], =0.93 [0.80-0.97] and large within-subject correlation between test and retest [r = 0.88 (0.68; 0.95)]. RFD displayed good reliability [ICC = 0.76 (0.35; 0.91)] and moderate within-subject correlation between test and retest [r = 0.63 (0.22; 0.85)]. Bilateral MS and RFD displayed a coefficient of variation of 3.4% and 4.6%, respectively, between tests. The standard error of measurement and the minimal detectable change for MS was 44.6 arbitrary units (a.u.) and 123.6 a.u., and 104.6 a.u. and 290.0 a.u. for peak RFD. This study shows that MS and RFD can be measured for NHE using a portable dynamometer. However, not all exercises are suitable to apply to determine RFD, so caution must be taken when analyzing RFD during NHE.

A New Portable Device to Reliably Measure Maximal Strength and Rate of Force Development of Hip Adduction and Abduction

ABSTRACT

Gonçalves, BM, Mesquita, RNO, Tavares, F, Brito, J, Correia, P, Santos, P, and Mil-Homens, P. A new portable device to reliably measure maximal strength and rate of force development of hip adduction and abduction. J Strength Cond Res 36(9): 2465-2471, 2022-Groin injuries are a major issue in sports involving kicking or quick changes of direction. Decreased hip adduction and abduction strength have been indicated as one of the main risk factors for groin injury. The methods currently available to measure hip adduction and abduction strength are reliable but highly dependent on the evaluator skills. Furthermore, several studies have reported the reliability of maximal strength (MVIC), but very few studies investigated the reliability of explosive strength (RFD), a parameter that has been previously shown to have a higher functional value. The aim of the current investigation was to assess the reliability of a user-independent portable dynamometer that concurrently measures MVIC and RFD. Twenty-five healthy young subjects performed maximal isometric hip adduction and abduction in both sitting and supine positions. Measurements occurred in 2 different days separated by 48-72 hours. Test-retest reliability was calculated for both MVIC and RFD. Both MVIC and RFD showed good relative reliability (intraclass correlation coefficient = 0.77-0.98) with no differences between positions or muscle actions. Measurement error was similar between positions for MVIC in both hip adduction and abduction. Measurements of RFD showed higher reliability using a time window of at least 0-100 milliseconds, and lower measurement error was observed in sitting for adduction and in supine for abduction. This study shows that portable dynamometry can be used to concurrently measure hip adduction and abduction maximal and explosive strength, with levels of reliability that are similar to previously described methods.

Relationship between hip abductor strength, rate of torque development scaling factor and medio-lateral stability in older adults

BACKGROUND

Recently, the rate of torque development scaling factor (RTD-SF) has been proposed as a useful tool that could contribute to a more comprehensive insight into muscular capacity. While lower RTD-SF is associated with ageing and certain neuromuscular diseases, it remains unknown whether this novel measure is associated with the postural control in the older adults.

RESEARCH QUESTION

Are hip abductor muscle strength, RTD and RTD-SF associated with responses to external postural perturbations in medio-lateral direction in older adults?

METHODS

Twenty healthy older adults (14 females, 6 males) were assessed for hip abductor muscle strength, RTD and RTD-SF, using a custom-built dynamometer. Perturbations were applied at waist level (4 perturbation intensities, 15 repetitions each) using a wire-pull paradigm, with centre-of-pressure (CoP) being recorded with force plates. For each condition (i.e. perturbation intensity), medio-lateral displacement and velocity of the CoP were computed. For both parameters, within-individual variation (representing consistency of the responses), expressed by the standard deviation (SD) of CoP parameters was also considered. Pearson correlation coefficients were computed between parameters of hip muscle capacity and CoP responses and SD values of CoP responses.

RESULTS

RTD-SF was moderately positively related to the consistency of the responses of both CoP displacement and velocity (r = 0.53-0.56; p = 0.011-0.016) at the lowest level of the perturbation magnitude (15 N). No other statistically significant relationships were found (all r < 0.35).

SIGNIFICANCE

RTD-SF could play a role in preserving postural balance in older adults when low-intensity perturbations are applied. RTD-SF is a novel outcome measure that could represent an important alternative clinical tool to traditional strength assessments. It could represent a supplementary tool to assess the risk of falls, however, several limitations and ambiguities need to be resolved by future research before it can be utilized in practice.

Quantification of Inter-Limb Symmetries With Rate of Force Development and Relaxation Scaling Factor

The inter-limb (a)symmetries have been most often assessed with the tests that quantify the maximal muscle capacity. However, the rapid force production and relaxation during submaximal tasks is equally important for successful sports performance. This can be evaluated with an established rate of force development and relaxation scaling factor (RFD-SF/RFR-SF). The aims of our study were (1) to assess the intra-session reliability of shortened RFD-SF/RFR-SF protocol and its absolute and symmetry outcome measures, (2) to compare the main absolute RFD-SF/RFR-SF outcome measures (slopes of RFD-SF and RFR-SF: kRTD-SF and kRFR-SF, theoretical peak RFD/RFR: TPRFD and TPRFR) across gender and sports groups, and (3) to compare inter-limb symmetries across gender and sports groups for main outcome measures (kRFD-SF, kRFR-SF, TPRFD, and TPRFR). A cross-sectional study was conducted on a group of young health participants (basketball and tennis players, and students): 30 in the reliability study and 248 in the comparison study. Our results showed good to excellent relative and excellent absolute reliability for the selected absolute and symmetry outcome measures (kRFD-SF, kRFR-SF, TPRFD, and TPRFR). We found significantly higher absolute values for kRFD-SF and TPRFD in males compared to females for the preferred (kRFD-SF: 9.1 ± 0.9 vs. 8.6 ± 0.9/s) and the non-preferred leg (kRFD-SF: 9.1 ± 0.9 vs. 8.5 ± 0.8/s), while there was no effect of sport. Significantly lower symmetry values for kRFR-SF (88.4 ± 8.6 vs. 90.4 ± 8.0%) and TPRFR (90.9 ± 6.8 vs. 92.5 ± 6.0%) were found in males compared to females. Moreover, tennis players had significantly higher symmetry values for kRFR-SF (91.1 ± 7.7%) and TPRFR (93.1 ± 6.0%) compared to basketball players (kRFR-SF: 88.4 ± 8.7% and TPRFR: 90.9 ± 6.7%) and students (kRFR-SF: 87.6 ± 8.7% and TPRFR: 90.5 ± 6.7%). Our results suggest that the reduced RFD-SF/RFR-SF protocol is a valuable and useful tool for inter-limb (a)symmetry evaluation. Differences in symmetry values in kRFR-SF and TPRFR (relaxation phase) were found between different sports groups. These may be explained by different mechanisms underlying the muscle contraction and relaxation. We suggest that muscle contraction and relaxation should be assessed for in-depth inter-limb symmetry investigation.

Advancements in the Protocol for Rate of Force Development/Relaxation Scaling Factor Evaluation

Brief submaximal actions are important for wide range of functional movements. Until now, rate of force development and relaxation scaling factor (RFD-SF and RFR-SF) have been used for neuromuscular assessment using 100-120 isometric pulses which requires a high level of attention from the participant and may be influenced by physiological and/or psychological fatigue. All previous studies have been conducted on a smaller number of participants which calls into question the eligibility of some of the outcome measures reported to date. Our aims were: (1) to find the smallest number of rapid isometric force pulses at different force amplitudes is still valid and reliable for RFD-SF slope (k _{R F D -SF}) and RFR-SF slope (k _RFR-SF ) calculation, (2) to introduce a new outcome measure - theoretical peak of rate of force development/relaxation (TP _RFD and TP _RFR ) and (3) to investigate differences and associations between k _RFD-SF and k _RFR-SF . A cross-sectional study was conducted on a group of young healthy participants; 40 in the reliability study and 336 in the comparison/association study. We investigated the smallest number of rapid isometric pulses for knee extensors that still provides excellent reliability of the calculated k _RFD-SF and k _RFR-SF (ICC₂,₁ ≥ 0.95, CV < 5%). Our results showed excellent reliability of the reduced protocol when 36 pulses (nine for each of the four intensity ranges) were used for the calculations of k _RFD-SF and k _RFR-SF . We confirmed the negligibility of the y-intercepts and confirmed the reliability of the newly introduced TP _RFD and TP _RFR . Large negative associations were found between k _RFD-SF and k _RFR-SF (r = 0.502, p < 0.001), while comparison of the absolute values showed a significantly higher k _RFD-SF (8.86 ± 1.0/s) compared to k _RFR-SF (8.03 ± 1.3/s) (p < 0.001). The advantage of the reduced protocol (4 intensities × 9 pulses = 36 pulses) is the shorter assessment time and the reduction of possible influence of fatigue. In addition, the introduction of TP _RFD and TP _RFR as an outcome measure provides valuable information about the participant's maximal theoretical RFD/RFR capacity. This can be useful for the assessment of maximal capacity in people with various impairments or pain problems.

Adapted protocol of rate of force development and relaxation scaling factor for neuromuscular assessment in patients with knee osteoarthritis

BACKGROUND

The linear relationship between muscle torque and rate of torque rise/relaxation during rapid muscle contractions has been recently introduced as a novel measure of muscle quickness, termed rate of torque development/relaxation scaling factor (RTD-SF/RTR-SF). Because the standard assessment protocol includes potentially painful muscle contractions, the first purpose of this study was to validate an adapted RTD-SF/RTR-SF protocol for knee extensor muscles that utilizes lower submaximal intensities and can be used in knee osteoarthritis patients.

METHODS

A cross-sectional study was performed on a group of healthy controls (n = 24) who underwent the standard RTD-SF/RTR-SF protocol (20-80% of maximum) and the knee osteoarthritis group (n = 24) who underwent the adapted protocol (20-60% of maximum). We calculated the RTD-SF, RTR-SF and the linearity (r²) for both relationships, based on both protocols in controls.

RESULTS

The validity of the adapted protocol was acceptable (intraclass correlation coefficient = 0.77-0.93), with low within-participant variation (coefficient of variability <10%) for both outcome measures. Compared with the control group, the knee osteoarthritis group had similar RTD-SF, but lower linearity of RTD-SF (0.90 vs. 0.82). The RTR-SF (8.0/s vs. 6.7/s) and its linearity (0.87 vs. 0.73) were significantly reduced. Comparing the affected and the unaffected leg in the knee osteoarthritis group, the unaffected leg had greater maximal torque (96.2 vs. 84.1 Nm) and higher linearity for RTD-SF (0.86 vs. 0.80) and RTR-SF (0.82 vs. 0.73).

CONCLUSIONS

We confirmed the validity of the adapted RTD/RTR-SF protocol and its sensitivity to impairments associated with knee osteoarthritis.

Interlimb Asymmetries and Ipsilateral Associations of Plantar Flexors and Knee Extensors Rate-of-Force Development Scaling Factor

Rate of force/torque development scaling factor (RFD-SF/RTD-SF) was recently introduced as a tool to quantify the neuromuscular quickness, and it could have potential for interlimb asymmetry identification. Moreover, positive relationships in RFD-SF ability among different muscle groups were shown, but not in the lower extremity. The first aim of our study was to use RTD-SF for interlimb asymmetry identification. The second aim was to determine associations between plantar flexors (PF) and knee extensors (KE). Forty young healthy athletes (14.8 ± 1.2 years) performed explosive isometric contractions to a span of torque levels for PF and KE. From rapid isometric contractions, the RTD-SF and linearity (r2) of the regression line were calculated. Using RTD-SF we identified 10% (PF) and 15% (KE) of subjects with contralateral asymmetries (>15% criterion). The results revealed significant positive moderate correlation in RTD-SF between PF and KE (r = 0.401, p < 0.05). We supported that RTD-SF can be a useful tool for interlimb asymmetry identification. Future research about observed asymmetry in rapid submaximal contractions deserves more attention, as most of the functional sport specific activities put high demands on rapid force production. Our study as first confirmed positive associations in RTD-SF ability between muscle groups in lower limbs.

Introduction of dynamic rate-of-force development scaling factor in progressive drop jumps

Rapid force generation across submaximal levels has been evaluated with the rate of force development scaling factor (RFD-SF) in different isometric tasks, while such measurement was still not verified in dynamic tasks. Our study was designed to evaluate the feasibility of the RFD-SF in dynamic drop jump (DJ) task (RFD-SF_DJ). A total of 55 young athletes performed isometric plantarflexion at different submaximal intensities and 60 DJs (6 different drop heights). For each participant we calculated linearity (r²) and slope in isometric task (RFD-SF_PF), eccentric part of DJ (RFD-SF_DJ-ECC) and concentric part of DJ (RFD-SF_DJ-CON), as well as average jump height (DJ_H) from each drop height. Our results revealed strong linear force-RFD relationship for isometric plantarflexion (r² = 0.90 ± 0.06), eccentric (r² = 0.87 ± 0.09) and concentric phase of DJ (r² = 0.80 ± 0.18). Significant moderate positive correlations were calculated between RFD-SF_PF and RFD-SF_DJ-ECC (r = 0.311, p < 0.05) and small negative correlations between RFD-SF_DJ-CON and RFD-SF (r = -0.276, p < 0.05)_. Significant positive moderate correlations were seen only between RFD-SF_DJ-ECC and DJ_H from 10 cm (r = 0.459, p < 0.001) and 15 cm (r = 0.423, p < 0.01). This is the first study to introduce and confirm that RFD-SF_DJ can be obtained from the multi-joint tasks (60 jumps) and still provide acceptable reliability and linear relationship. Furthermore, RFD-SF_DJ may have greater practical application than RFD-SF assessed under the isometric conditions. This verification of RFD-SF_DJ opens opportunities for further research regarding its practical application.

Age- and Sex-Related Differences in the Maximum Muscle Performance and Rate of Force Development Scaling Factor of Precision Grip Muscles

The aim of this study was to explore the effects of age and sex on the rate of force development scaling factor (RFD-SF) and maximum performance (i.e., maximum grip force [GFMax] and maximum rate of grip force development [RGFDMax]) of precision handgrip muscles. Sixty-four subjects, allocated in four groups according to their age and sex, were asked to hold an instrumented handle with the tip of the digits and perform two tests: maximum voluntary contraction and RFD-SF tests. In the maximum voluntary contraction test, GFMax and RGFDMax were assessed. In the RFD-SF test, the subjects generated quick isometric force pulses to target amplitudes varying between 20% and 100% of their GFMax. The RFD-SF and R2 values were obtained from the linear relationship between the peak values of the force pulses and the corresponding peak values of the rate of force development. Younger adults and males produced higher GFMax and RGFDMax and presented higher R2 and RFD-SF than older adults and females, respectively. No correlations between GFMax and RFD-SF and between RGFDMax and RFD-SF were observed.

Relevance of evaluating the rate of torque development in ballistic contractions of submaximal amplitude

OBJECTIVE

The neuromuscular quickness capacity can be assessed by calculating the rate of torque development (RTD) during ballistic contractions of maximal (RTD_maximal) or submaximal (RTD_submaximal) amplitudes. In a series of ballistic contractions of submaximal amplitudes, the RTD scaling factor (RTD-SF) represents the slope of the linear regression between achieved peak torques and the corresponding RTD. First we investigated whether the RTD-SF contributes to the prediction, together with maximal voluntary torques (MVT), of the RTD_maximal. Then, we evaluated the agreement between the z-scores of RTD_maximal and RTD_submaximal.

APPROACH

The MVT was obtained for the quadriceps and hamstrings muscles of 22 elite young soccer players. RTD-SF was quantified in a series of ballistic contractions of submaximal and maximal amplitudes. RTD_submaximal was estimated from the regression relationship between the peak torques and the corresponding RTD.

MAIN RESULTS

MVT, RTD-SF and y -intercept in total accounted for 76.9 and 61.2% of the variance in RTD_maximal in quadriceps and hamstrings, respectively. Specifically, RTD-SF accounted for 13.7% and 18.7% of the variance in RTD_maximal, respectively. Generally, the agreement between the z-scores of RTD_maximal and RTD_submaximal was poor both in quadriceps and hamstrings.

SIGNIFICANCE

These results suggest that RTD-SF may have a functional relevance in the relationship between MVT and RTD_maximal and influence the amount of torque that can be achieved in a quick muscle contraction. Moreover, evaluating the RTD_submaximal does not provide results that are interchangeable with RTD_maximal. Thus, evaluating the RTD across the whole range of torque could provide additional meaningful information about neuromuscular quickness.

Interlimb Asymmetries Identified Using the Rate of Torque Development in Ballistic Contraction Targeting Submaximal Torques

Evaluating the rate of torque development (RTD) in isometric ballistic contraction targeting submaximal torques is usually overlooked in the literature. In a series of isometric ballistic contractions targeting a range of submaximal torque values, there is a linear relationship between the peak torque and the peak RTD obtained in each contraction. RTD scaling factor (RTD-SF) represents the slope of this relationship. In this study, we investigated the prevalence of interlimb asymmetry in the RTD-SF and in the RTD calculated across submaximal torques. Furthermore, we compared these asymmetry indices with those calculated adopting more classical approaches, such as ballistic contraction targeting maximal torque and isokinetic concentric conditions. Quadriceps and hamstrings strength was evaluated in both limbs of elite under 17 and under 19 soccer players (20 males, 17 ± 1 years). Participants performed three concentric isokinetic contractions at 240°/s and a series of isometric ballistic contractions targeting from 20 to 100% of maximal isometric torque. The interlimb difference was calculated for each parameter and players presenting an interlimb difference >15% were identified. A total of 40% (for quadriceps) and 60% (for hamstring) of players showed an interlimb asymmetry in isometric RTD for at least 50% of submaximal torque range. The RTD-SF was able to identify more players with asymmetry than the classical isokinetic tests. However, isokinetic and isometric indices of asymmetry were in general poorly or not correlated with each other. Most players presented an interlimb asymmetry in RTD for a wide part of the torque range and the adopted protocol was able to highlight important interindividual differences. Furthermore, players showed a large prevalence of RTD-SF asymmetry in both quadriceps and hamstrings. It is still to be determined if these asymmetries are functionally relevant. Nevertheless, the adopted protocol provided meaningful information for identifying interlimb asymmetries that could not be gathered when adopting the classical method of ballistic contractions targeting only maximal torques.

Neuromuscular Fatigue Does Not Impair the Rate of Force Development in Ballistic Contractions of Submaximal Amplitudes

The effect of muscle fatigue on rate of force development (RFD) is usually assessed during tasks that require participants to reach as quickly as possible maximal or near-maximal force. However, endurance sports require athletes to quickly produce force of submaximal, rather than maximal, amplitudes. Thus, this study investigated the effect of muscle fatigue induced by long-distance running on the capacity to quickly produce submaximal levels of force. Twenty-one male amateur runners were evaluated before and shortly after a half-marathon race. Knee extensors force was recorded under maximal voluntary and electrically evoked contractions. Moreover, a series of ballistic contractions at different submaximal amplitudes (from 20 to 100% of maximal voluntary force) was obtained, by asking the participants to reach submaximal forces as fast as possible. The RFD was calculated for each contraction. After the race, maximal voluntary activation, resting doublet twitch, maximal force, and RFD during maximal contraction decreased (-12, -12, -21, and -19%, respectively, all P-values < 0.0001). Nevertheless, the RFD values measured during ballistic contractions up to 60% of maximal force were unaffected (all P-values > 0.4). Long-distance running impaired the capacity to quickly produce force in ballistic contractions of maximal, but not of submaximal, amplitudes. Overall, these findings suggest that central and peripheral fatigue do not affect the quickness to which muscle contracts across a wide range of submaximal forces. This is a relevant finding for running and other daily life activities that rely on the production of rapid submaximal contractions rather than maximal force levels.

High-Speed Cycling Intervention Improves Rate-Dependent Mobility in Older Adults

PURPOSE

The aim was to determine the feasibility of a 6-wk speed-based exercise program that could be used to initiate new exercise behaviors and to improve rapid movement in older adults approaching frailty.

METHODS

The intervention group included 14 older adults (3 males and 11 females; mean ± SD, age = 70 ± 7.6 yr, height = 1.6 ± 0.11 m, mass = 76.8 ± 12.0 kg, body mass index = 27.7 ± 4.7 kg·m). The control group included 12 older adults (6 males and 6 females; mean ± SD, age = 69.2 ± 6.9 yr, height = 1.7 ± 0.09 m, mass = 78.2 ± 10.9 kg, body mass index = 25.3 ± 2.7 kg·m). Subjects included active older adults, including regular exercisers, but none were engaged in sports or exercises with an emphasis on speed (e.g., cycling spin classes or tennis). Stationary recumbent cycling was selected to minimize fall risk, and low pedaling resistance reduced musculoskeletal and cardiovascular load. Two weekly 30-min exercise sessions consisted of interval training in which subjects pedaled at preferred cadence and performed ten 20-s fast cadence intervals separated by 40 s of active recovery at preferred cadence.

RESULTS

Significant group-time interactions (P < 0.05) supported a 2-s improvement in the timed up and go test and a 34% improvement in rapid isometric knee extension contractions in the exercise group but not in controls. Central neural adaptations are suggested because this lower extremity exercise program also elicited significant improvements in the untrained upper extremities of the exercise group (elbow extension rate of force development scaling factor and Nine-Hole Peg Test, P < 0.05).

CONCLUSION

These results demonstrate that a relatively low dose of speed-based exercise can improve neuromuscular function and tests of mobility in older adults. Such a program serves as a sensible precursor to subsequent, more vigorous training or as an adjunct to a program where a velocity emphasis is lacking.

Simultaneous assessment of hand function and neuromuscular quickness through a static object manipulation task in healthy adults

Both hand function [as seen through the coordination between grip force (GF) and load force (LF)] and the ability to produce a submaximal force quickly (i.e., neuromuscular quickness) are two important qualities of motor function that could be seriously affected by the presence of neurological diseases. Therefore, their quantitative assessment is very important in clinical settings. Within this study, we aimed to develop, standardize, and measure the within-session reliability of a clinically meaningful test that assesses both hand function and neuromuscular quickness simultaneously. Thirteen healthy young adults produced around 90 rapid isometric LF pulses to varying submaximal magnitudes by either pulling down or pulling up on an externally fixed GF- and LF-measuring device. Results revealed high indices of force coordination (i.e., GF scaling as assessed by GF/LF and GF coupling as assessed by maximum cross-correlation between GF and LF) in both force directions, while GF coupling was higher in downward than in upward direction (p < 0.001). Regarding the indices of neuromuscular quickness (i.e., the regression parameters obtained from the relationship between peak force and it's rate of development and half-relaxation time), results, in general, revealed a higher slope (named as rate of force development scaling factor; p < 0.01), similar R ² (p > 0.05), and shorter half-relaxation time (p < 0.05) for LF than for GF. Furthermore, all of the selected variables showed moderate to excellent within-session reliability with only 45 pulses. Findings suggest that brief force production tasks should be further evaluated as clinical tests of hand function and neuromuscular quickness in various populations.

Rate of force development: physiological and methodological considerations

The evaluation of rate of force development during rapid contractions has recently become quite popular for characterising explosive strength of athletes, elderly individuals and patients. The main aims of this narrative review are to describe the neuromuscular determinants of rate of force development and to discuss various methodological considerations inherent to its evaluation for research and clinical purposes. Rate of force development (1) seems to be mainly determined by the capacity to produce maximal voluntary activation in the early phase of an explosive contraction (first 50–75 ms), particularly as a result of increased motor unit discharge rate; (2) can be improved by both explosive-type and heavy-resistance strength training in different subject populations, mainly through an improvement in rapid muscle activation; (3) is quite difficult to evaluate in a valid and reliable way. Therefore, we provide evidence-based practical recommendations for rational quantification of rate of force development in both laboratory and clinical settings.