Rate of force development: physiological and methodological considerations

<p>Typical rehabilitation strategies and performance tests after knee surgery are often based on peak lower extremity strength. However, people rarely generate maximal knee force in both daily and sports activities, which are characterized by brief periods of rapid muscle activation and relaxation. Thus, the ability to rapidly develop or modulate force may be more meaningful and more relevant to function. It is unclear how knee surgery influences the neuromuscular mechanisms controlling the ability to rapidly develop leg muscle force and produce power, or the functional relevance of these characterizations of muscle performance in relation to injury, surgery, and recovery.</p><p>The primary purpose of this collection of studies was to assess rapid quadriceps muscle activation and lower extremity force production in people undergoing arthroscopic knee surgery for meniscal debridement and anterior cruciate ligament (ACL) reconstruction.</p><p>People undergoing arthroscopic partial meniscectomy (APM) presented with significant deficits in knee extensor rate of torque development (RTD), leg press power, and rapid quadriceps muscle activation both prior to and in the initial month following surgery. Subjective knee function was significantly correlated with RTD variables but not with peak strength or quadriceps volume. Limitations in the ability to rapidly activate the involved quadriceps suggests that impaired centrally mediated neural function of the involved quadriceps may limit RTD and lower extremity power post-surgery.</p><p>Next, the speed and intensity of quadriceps exercise performed in the early post-surgical period of patients post-APM and the relationships between training parameters, strength, quadriceps RTD, and subjective knee function were investigated. Subjects performed high intensity quadriceps contractions 2-3x/week in the first month following surgery. All subjects increased quadriceps strength, but people who trained with greater RTD following APM demonstrated greater improvements in RTD and had better patient-based outcomes scores than those who trained with a slower rate of torque rise.</p><p>Finally, power and rate of force development (RFD) in people ≤ 1 year following ACL reconstruction were evaluated along with movement biomechanics, typical clinical measures of readiness to return to activity, and patient-based outcomes. Significant side-to-side asymmetries in quadriceps strength, RFD, leg press strength and power, and knee joint kinetics were noted. Deficits in voluntary quadriceps strength paralleled the deficits in early phase RFD, indicating that in this population RFD was limited by the intrinsic properties and force production capacity of the quadriceps, not the ability to rapidly activate the muscle. However, strong to very strong correlations were found between quadriceps RFD, movement biomechanics and subjective knee function, which were predominantly stronger than the correlations with peak quadriceps strength. Leg press strength, power, and acceleration were very strongly correlated with movement biomechanics and subjective knee function.</p><p>In summary, this series of studies provides important insight into the neuromuscular mechanisms related to rapid lower extremity force development and muscle activation in the context of knee joint injury and recovery after arthroscopic knee surgery. Collectively, this work suggests that the inability to quickly develop or modulate quadriceps force may have significant functional consequences, and that rehabilitation efforts following arthroscopic knee surgery to incorporate both specific dosage of and earlier performance of rapid leg muscle contractions should be explored.</p>

Changes in the rate of isometric force development with fatigue were measured in vitro (25 degrees C) using mouse soleus and extensor digitorum longus (EDL) muscles. Muscles were fatigued using 30 tetanic contractions. Rate of force development was determined from the rate constant of an exponential curve fitted to the rising force phase of a tetanus. For both muscles, when the intertetanus interval was 3 s, maximum isometric force and relaxation rate were significantly reduced in the final tetanus relative to the values in the first tetanus. Rate of force development in soleus muscles transiently increased and then decreased a small amount. The final rate was 92.7 +/- 3.3% (n = 4) of the initial rate. In contrast, the rate of force development in EDL muscles increased to 133.7 +/- 3.3% (n = 4) of the initial rate. This increased rate was evident from the second tetanus of the series, was fully established after 5 tetani, and the magnitude of the increase in rate was inversely proportional to intertetanus interval and was independent of presumed energy expenditure. The enhanced rate decayed with a time constant of 14.3 +/- 2.0 s and was independent of presumed energy expenditure. Most of these observations can be explained by the effects of P(i) on cross bridge kinetics. Other possible mechanisms, involving more rapid activation, are also suggested.

Only a few studies have been conducted concerning the effects of aging on explosive force production capacity and on functional reflex activity (FRA). PURPOSE: Therefore, the purpose of this study was to investigate the impact of aging on (a) rate of force development (RFD) during maximal isometric leg extension force (MIF), (b) the first 30ms of the force-time curve (FTC) during MIF, and (c) on the ability to compensate for gait perturbations (GP). METHODS: 28 healthy young (Y) (n=14, age 27±3.1yrs, BMI=23.6±0.8kg/m2) and old men (O) (n=14, age 67.3±3.7yrs, BMI=25.5±2.1kg/m2) volunteered as subjects in this study. All subjects were tested for: (a) treadmill deceleration impulses at a walking speed of 3.5km/h, and (b) their MIF and RFD on a leg-press. RFDmax was defined as the maximal slope at deflection of the FTC. RFD30 was calculated as the mean slope of the FTC over the time interval 0-30ms. EMG activity was recorded from M. tibialis (TI) and M. vastus medialis (VM) of the right leg using surface electrodes. The analysed time interval for GP was 120ms after the first physiological response in the goniometer signal. Integrated and time normalized EMG (mean amplitude Voltage (MAV)) of TI during GP was normalized on the swing phase of a regular gait cycle. For MIF, MAV was analysed in the time interval 0-30ms and normalized on MAV of VM 100ms pre and post MIF. RESULTS: MIF, RFDmax and RFD30 were significantly reduced in O compared to Y (MIF: 1307±83 vs. 2390±103N, p<.01; RFDmax: 9.2±1.3 vs. 18.4±1.2N/ms, p<.01; RFD30: 0.28±0.06 vs. 1.66±0.21N/ms, p<.01). Normalized MAV of VM in the time interval 0-30ms was decreased in O compared to Y (84.5±30.8 vs. 95.5±56.6%, n.s.). For GP, latency of TI was enhanced in O compared to Y (77.7±3.6 vs. 75.1±2.5ms, n.s.). Normalized MAV of TI during GP was significantly reduced in O compared to Y (216±66 vs. 306±145%, p<.05). However, no correlation could be found between MAV of TI during GP and MIF, RFDmax and RFD30. DISCUSSION: Aging leads to tremendous decreases in MIF, RFD and FRA, with RFD being more affected than MIF. Heavy resistance strength training seems to be a wellsuited method to counteract age related decreases in MIF and RFD. Furthermore, there is evidence that balance training has the potential to enhance FRA even in old age (1).

In young adults, improvements in the rate of force development as a result of resistance training are accompanied by increases in neural drive in the very initial phase of muscle activation. The purpose of this experiment was to determine if older adults also exhibit similar adaptations in response to rate of force development (RFD) training. Eight young (21-35 years) and eight older (60-79 years) adults were assessed during the production of maximum rapid contractions, before and after four weeks of progressive resistance training for the elbow flexors. Young and older adults exhibited significant increases (P<0.01) in peak RFD, of 25.6% and 28.6% respectively. For both groups the increase in RFD was accompanied by an increase in the root mean square (RMS) amplitude and in the rate of rise (RER) in the electromyogram (EMG) throughout the initial 100 ms of activation. For older adults, however, this training response was only apparent in the brachialis and brachioradialis muscles. This response was not observed in surface EMG recorded from the biceps brachii muscle during either RFD testing or throughout training, nor was it observed in the pronator teres muscle. The minimal adaptations observed for older adults in the bifunctional muscles biceps brachii and pronator teres are considered to indicate a compromise of the neural adaptations older adults might experience in response to resistance training.

Adequacy of activation during rapid voluntary contractions is limited in young and old adults and can be shown by comparing the voluntary rate of force development (RFD) and electrically evoked RFD. Nonetheless, it is unknown whether the activation of rapid voluntary contractions differs between young men and women. PURPOSE: The purpose of this study was to compare the maximal RFD of young men and women during electrically evoked isometric contractions and rapid voluntary contractions with the knee extensor muscles across a range of torques. METHODS: Eight young adults (18-26 years; 4 men, 4 women) consented to sets of single and double pulse (10 ms interval) stimulations of the femoral nerve at maximal intensities followed by rapid voluntary isometric knee extensions at target torques matched to the electrically evoked torques. For the rapid voluntary isometric contractions, subjects were instructed to kick as fast as possible rather than as accurately as possible. Voluntary target torques ranged between 10-40% of maximal voluntary contraction (MVC) and were set to match the electrically evoked torques. Maximal RFD for each trial (voluntary and electrically evoked) was calculated as the peak values of the first derivative of the torque signal. RESULTS: Torques (between 10-40% MVC) were similar for the electrically evoked and voluntary contractions (F1,6=0.54, P=0.49) for both the men (25.0 ± 0.6 %MVC vs. 26.4 ± 0.7 %MVC) and women (26.0 ± 2.0 %MVC vs. 26.5 ± 2.7 %MVC). Although torques were similar, RFD from electrically evoked contractions in men (514.8 ± 30.4 % MVC.s−1) and women (475.1 ± 78.0 %MVC.s−1) was 36% greater than the voluntary contractions (men; 330.2 ± 18.8 vs. women; 305.9 ± 21.2 %MVC.s−1; F1.6 = 23.1, P = 0.003). The linear association between relative torque (%MVC) and electrically evoked RFD (R2 = 0.85, b = 0.92, P<0.001) was larger and steeper than the association with voluntary RFD (R2 = 0.72, b = 0.85, P<0.001) indicating that the difference in RFD between the electrically evoked and voluntary contractions increased at the higher forces. CONCLUSIONS: Young men and women had marked reductions in the ability to voluntarily generate isometric knee extension torque rapidly compared with electrically evoked contractions that were independent of the central nervous system. Supported by NIA R01 AG048262 to SKH

Objective: Within this methodological study, we aimed to explore the effects of seven different commonly applied routines for rate of force development (RFD) calculation on the calculation of rate of force development scaling factor (RFD-SF) and r2 and the between session reliability of RFD-SF in three different muscle groups. Approach: We tested grip force muscles, elbow extensors and knee extensors of 13 healthy subjects asking them to produce around 120 rapid isometric force pulses per muscle to varying submaximal amplitudes. Then, we applied different filtering and analysis procedures, such as different filtering cut-off frequencies (no filter, 5 Hz cut-off, 10 Hz cut-off) and time windows during which RFD was calculated (0–50 ms, 0–75 ms, 0–100 ms and time from onset of contraction to peak force (PF) value). RFD-SF and r2 were obtained from the relationship between the PF and the corresponding RFD. Results: Our results showed that the magnitude of the calculated RFD-SF and r2 values significantly differed among different RFD calculation methods (p < 0.0005), but that r2 was high in most applied methods (RFD-SF ranged between 4.6 and 17.7, while r2 ranged between 0.63 and 0.98). Regardless of the tested muscle group and applied method to calculate RFD, the reliability of the calculated RFD-SF was moderate (0.5 < ICC3,1 < 0.75) to good (ICC3,1 > 0.75). The highest r2, highest ICC, lowest SEM% and MDD% were observed in case of RFD calculated during first 100 ms and when cut-off filter at 5 Hz was applied, indicating those two methods as the most appropriate ones. Significance: Our study contributes to the establishment of methodological suggestions that will help in maximizing the reliability of RFD-SF measurements, collecting normative data, and comparing results among different populations and lab settings.

Though similar motor unit (MU) discharge properties have been observed during slow sustained contractions between chronically strength-trained (ST) and untrained (UT) individuals, it is currently unknown whether differences between these groups exist for when maximal in vivo MU discharge rate is assessed during rapid, maximal rate of force development (RFD) contractions. Therefore, we compared MU discharge characteristics and RFD during rapid contractions in chronic ST and UT individuals. The investigations were performed in two independent cohorts of chronically ST men, with trained elbow flexors (experiment 1, n = 13, 6 ± 4 yr of training experience) or knee extensors (experiment 2, n = 11, 9 ± 4 yr of experience), and compared with those of UT (n = 12 and n = 10, respectively). ST individuals had greater absolute elbow flexion and knee extension RFD throughout the first 150 ms of rapid contractions compared with UT, but this difference was absent for relative RFD. ST exhibited higher initial MU discharge rate in both biceps brachii (74 [68, 80] vs. 56 [50, 63] pulses per second (pps), P < 0.0001) and vastus lateralis (102 [90, 115] vs. 76 [63, 90] pps, P = 0.0025) and a greater average number of MU discharges per second in both trained muscles in the early phase of rapid contractions. We provide novel evidence for a higher maximal MU discharge rate in strength-trained individuals. Interestingly, despite the augmented output of the spinal cord, no differences in relative RFD were observed, which suggests either greater maximal force enhancement of ST compared with UT and/or slowing of the intrinsic contractile properties by prolonged strength training.NEW & NOTEWORTHY Chronically strength-trained and untrained individuals show similar motor unit discharge rates during slow sustained contractions, however, potential differences in motor unit discharge rates during rapid contractions remained unclear. Here, we show greater maximal motor unit discharge rates during rapid contractions of chronically strength-trained individuals. However, the augmented spinal cord output of strength-trained individuals did not lead to greater relative maximal rate of force development compared with untrained men.

Effect of resistance training on muscle strength and rate of force development in healthy older adults: A systematic review and meta-analysis

Correlation between motor unit discharge times, often referred to as motor unit synchronization, is determined by common synaptic input to motor neurons. Although it has been largely speculated that synchronization should influence the rate of force development, the association between the degree of motor unit synchronization and rapid force generation has not been determined. In this study, we examined this association with both simulations and experimental motor unit recordings. The analysis of experimental motor unit discharges from the tibialis anterior muscle of 20 healthy individuals during rapid isometric contractions revealed that the average motor unit discharge rate was associated with the rate of force development. Moreover, the extent of motor unit synchronization was entirely determined by the average motor unit discharge rate (R > 0.7, P < 0.0001). The simulation model demonstrated that the relative proportion of common synaptic input received by motor neurons, which determines motor unit synchronization, does not influence the rate of force development (R = 0.03, P > 0.05). Nonetheless, the estimates of correlation between motor unit spike trains were significantly correlated with the rate of force generation (R > 0.8, P < 0.0001). These results indicate that the average motor unit discharge rate, but not the degree of motor unit synchronization, contributes to most of the variance of human contractile speed among individuals. In addition, estimates of correlation between motor unit discharge times depend strongly on the number of identified motor units and therefore are not indicative of the strength of common input. NEW & NOTEWORTHY It is commonly assumed that motor unit synchronization has an impact on the rate of force development of a muscle. Here we present computer simulations and experimental data of human tibialis anterior motor units during rapid contractions that show that motor unit synchronization is not a determinant of the rate of force production. This conclusion clarifies the neural determinants of rapid force generation.

The maximal rate of rise in muscle force [rate of force development (RFD)] has important functional consequences as it determines the force that can be generated in the early phase of muscle contraction (0-200 ms). The present study examined the effect of resistance training on contractile RFD and efferent motor outflow ("neural drive") during maximal muscle contraction. Contractile RFD (slope of force-time curve), impulse (time-integrated force), electromyography (EMG) signal amplitude (mean average voltage), and rate of EMG rise (slope of EMG-time curve) were determined (1-kHz sampling rate) during maximal isometric muscle contraction (quadriceps femoris) in 15 male subjects before and after 14 wk of heavy-resistance strength training (38 sessions). Maximal isometric muscle strength [maximal voluntary contraction (MVC)] increased from 291.1 +/- 9.8 to 339.0 +/- 10.2 N. m after training. Contractile RFD determined within time intervals of 30, 50, 100, and 200 ms relative to onset of contraction increased from 1,601 +/- 117 to 2,020 +/- 119 (P < 0.05), 1,802 +/- 121 to 2,201 +/- 106 (P < 0.01), 1,543 +/- 83 to 1,806 +/- 69 (P < 0.01), and 1,141 +/- 45 to 1,363 +/- 44 N. m. s(-1) (P < 0.01), respectively. Corresponding increases were observed in contractile impulse (P < 0.01-0.05). When normalized relative to MVC, contractile RFD increased 15% after training (at zero to one-sixth MVC; P < 0.05). Furthermore, muscle EMG increased (P < 0.01-0.05) 22-143% (mean average voltage) and 41-106% (rate of EMG rise) in the early contraction phase (0-200 ms). In conclusion, increases in explosive muscle strength (contractile RFD and impulse) were observed after heavy-resistance strength training. These findings could be explained by an enhanced neural drive, as evidenced by marked increases in EMG signal amplitude and rate of EMG rise in the early phase of muscle contraction.

Fatigue-induced dissociation between rate of force development and maximal force across repeated rapid contractions

Aging is associated with a progressive impairment in postural balance control, which results in an increased incidence of falls at old age. Specifically, a reduced rapid force capacity (rate of force development, RFD) of the lower limb muscles results in an impaired ability to reverse a fall during sudden stumbling. We recently showed that explosive-type heavy-resistance strength training (HRT) elicits increased RFD in old (60 yr) and very old (80 yr) women. However, the effect on postural stability remains unknown. PURPOSE: To examine the effect of explosive-type HRT on postural stability in old and very old women. METHODS: Old (62.7 ± 2.2 yrs, n=17, TG60) and very old (81.8 ± 2.7, n=10, TG80) women performed 12 wks of explosive-type HRT for the leg muscles (24 sessions) using 75-80% 1-RM loads. Non-training age-matched women served as controls (n=17, CG60; n=12, CG80). Center of pressure (CoP) excursion was analyzed during 60-s bilateral static stance using force plate analysis (Kistler 9281B) and average values were obtained in the final 15-s epoc. Data were normalized to individual base of support (BoS) dimensions. RESULTS: Prior to training 80-yr women demonstrated 32%, 102% and 42% greater CoP sway length (L), sway area (A) and velocity-moment (VM), respectively, than 60-yr women. Post training TG60 showed decreased A (-21%) and VM (-28%) (p<0.01), while L tended to decrease by 8% (p=0.108). CG60 showed no changes, and % pre-to-post changes were greater (p<0.05) in TG60 than CG60 for all parameters. Post training changes in TG80 for A (-18%), VM (-7%) and L (-2%) failed to reach statistical significance, however % pre-to-post changes for A were greater in TG80 than CG80 (p<0.05). A similar tendency emerged for L and VM (p=0.103-0.108). M-L CoP variability (SD[CoPx]) decreased 23% in TG60 along with 28% reduced VM variability (p<0.01). Likewise, A-P VM variability tended to decrease in TG80 (p=0.089) after HRT. CONCLUSION: When contrasted to age-matched sedentary controls, explosive-type resistance training led to improved postural stability in women aged 60 and 80 years, respectively. However, within-group changes were less manifest in 80-yr old individuals, suggesting that HRT should be supplemented with specific postural balance exercise to more effectively improve postural stability in very old individuals.

Cancer-related fatigue (CRF) is the most reported side effect of cancer and its treatments. Mechanisms of CRF are multidimensional, including neuromuscular alterations leading to decreased muscle strength and endurance (i.e., fatigability). Recently, exercise fatigability and CRF have been related, while fatigability mechanisms remain unclear. Traditionally, fatigability is assessed from maximal voluntary contractions (MVC) decrease, but some authors hypothesized that the rate of force development (RFD) determined during a rapid contraction could also be an interesting indicator of functional alterations. However, to our knowledge, no study investigated RFD in cancer patients. The purpose of this study was to determine whether RFD, fatigability amplitude, and etiology are different between fatigued and non-fatigued cancer patients. Eighteen participants with cancer, divided in fatigued or non-fatigued groups according their CRF level, completed a 5-min all-out exercise in ankle plantar flexor muscles composed of 62 isometric MVC of 4s with 1s rest, to assess fatigability amplitude as the force-time relationship asymptote (FA). Before and after exercise, fatigability etiologies (i.e., voluntary activation (VA) and evoked forces by electrical stimulation (Db100)) were assessed as well as RFD in 50 and 100ms (RFD50 and RFD100, respectively) during rapid contractions. FA is significantly lower in fatigued group. Significant differences were found between pre- and post-exercise VA, Db100, RFD50, and RFD100 for both groups, with no statistical difference between groups. During treatments, fatigability is higher in fatigued patients; however, the mechanisms of fatigability and RFD alterations are similar in both groups. ClinicalTrials.gov, NCT04391543, May 2020.

Although 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 4 wk 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. Although strength training did not change motoneuron behavior 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 behavior following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.NEW & NOTEWORTHY Although maximal force increases with strength training, the rate of force development may remain unaffected. For the first time, we associated motor unit population behavior during rapid force contractions before and after a 4-wk isometric strength training intervention. We found that strength training combined with slow and rapid contractions does not change rate of force development. The specific mechanisms include similar discharge rate during the initial phase of contraction and similar recruitment speed of motoneurons.

Aging impairs the recovery in mechanical muscle function following 4 days of disuse