The Non-linear Relationship between Muscle Voluntary Activation Level and Voluntary Force Measured by the Interpolated Twitch Technique

Interpolated twitch technique (ITT) is a non-invasive method for assessing the completeness of muscle activation in clinical settings. Voluntary activation level (VA), measured by ITT and estimated by a conventional linear model, was reported to have a non-linear relationship with true voluntary contraction force at higher activation levels. The relationship needs to be further clarified for the correct use by clinicians and researchers. This study was to established a modified voluntary activation (modified VA) and define a valid range by fitting a non-linear logistic growth model. Eight healthy male adults participated in this study. Each subject performed three sets of voluntary isometric ankle plantar flexions at 20, 40, 60, 80 and 100% maximal voluntary contraction (MVC) with real-time feedback on a computer screen. A supramaximal electrical stimulation was applied on tibia nerve at rest and during contractions. The estimated VA was calculated for each contraction. The relationship between the estimated VA and the actual voluntary contraction force was fitted by a logistic growth model. The result showed that according to the upper and lower limit points of the logistic curve, the valid range was between the 95.16% and 10.55% MVC. The modified VA estimated by this logistic growth model demonstrated less error than the conventional model. This study provided a transfer function for the voluntary activation level and defined the valid range which would provide useful information in clinical applications

INVESTIGATIONS INTO THE FATIGUE-RELATED REDUCTION IN TORQUE, SHORTENING VELOCITY, AND JOINT RANGE OF MOTION IN HUMANS

Studies in humans and animals have derived much understanding of neuromuscular function from isometric (static) contractions. In comparison, fewer studies have evaluated dynamic contractions, which are relevant to everyday movements and activities of daily living. The primary purpose of this thesis was to investigate and compare the contributing factors to fatigue during different voluntary contraction tasks. The interpolated twitch technique is commonly used to assess voluntary activation, but with changes in muscle length, musculotendinous slackness can diminish the amplitude of electrically-evoked twitches used to calculate voluntary activation. This might result in erroneous measurements of voluntary activation. Chapter 2 describes an experiment in which at the short muscle lengths, when voluntary activation is 80% or lower, actual activation will be underestimated. Maximum voluntary isometric contraction (MVC) torque is often used to assess overall neuromuscular function, and any activity-induced decline in MVC torque is indicative of fatigue. However, a reduction in shortening velocity is also an important feature of fatigue. Results from Chapter 3 indicate that shortening velocity was an important and perhaps more sensitive measure of fatigue following both isometric and dynamic contraction tasks than MVC torque per se. These findings are further supported in Chapter 4, in which, following comparable repetitive shortening contraction tasks in two different muscles, shortening velocity was reduced to a greater extent at task failure but was restored more rapidly than MVC torque. Shortening contractions are also characterized by a fatigue-related reduction in joint range of motion (ROM) and it was suggested that the reduction in ROM might be due to length-dependent alterations in torque or contractile slowing with fatigue. Results iii presented in Chapter 5, suggest that length-dependent alterations in torque or contractile slowing cannot explain the fatigue-related reduction in dorsiflexion ROM. Thus, in addition to fatigue-related reductions in torque, decreases in shortening velocity and joint range of motion are important indicators of a fatigue-induced impairment in muscle shortening capacity

Conventionally assessed voluntary activation does not represent relative voluntary torque production

The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT), which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects (n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship between superimposed and voluntary torque. This includes very high contraction intensities (90–100%VA), which are difficult to consistently obtain in regular healthy subjects (VA of ∼90%). Subjects were tested at 30, 60, and 90° knee angles on two experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation (triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles. Maximal VA was similar and very high across knee angles: 97 ± 2.3% (mean ± SD). At high contraction intensities, the increase in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque increased from 79.6 ± 6.1 to 100%MVC, superimposed torque decreased from 8.5 ± 2.6 to 2.8 ± 2.3% of resting triplet. Therefore, an increase in VA of 5.7% (from 91.5 ± 2.6 to 97 ± 2.3%) coincided with a much larger increase in voluntary torque (20.4 ± 6.1%MVC) and EMG (33.9 ± 6.6%max). Moreover, a conventionally assessed VA of 91.5 ± 2.6% represented a voluntary torque of only 79.6 ± 6.1%MVC. In conclusion, when maximal VA is calculated to be ∼90% (as in regular healthy subjects), this probably represents a considerable overestimation of the subjects’ ability to maximally drive their quadriceps muscles

Do changes in neuromuscular activation contribute to the knee extensor angle-torque relationship?

The influence of joint angle on knee extensor neuromuscular activation is unclear due in part due to the diversity of surface electromyography (sEMG) and/or interpolated twitch technique (ITT) methods employed. The aim of the study was to compare neuromuscular activation, using rigorous contemporary sEMG and ITT procedures, during isometric maximal voluntary contractions (iMVC) of the quadriceps femoris (Q) at different knee-joint angles and examine if activation contributes to the angle-torque relationship. Sixteen healthy active males completed two familiarization sessions and two experimental sessions of isometric knee extension and knee flexion contractions. The experimental sessions included the following at each of four joint angles (25°, 50°, 80° and 106°): iMVCs (with and without superimposed evoked doublets); submaximal contractions with superimposed doublets; evoked twitch and doublet contractions whilst voluntarily passive and knee flexion iMVC at the same knee joint positions. Absolute Q EMG was normalised to MMAX peak-to-peak amplitude and the doublet-voluntary torque relationship was used to calculate activation with the ITT (ACTITT ). Agonist activation, assessed with both normalised EMG and ACTITT , was reduced in the more extended compared to the more flexed positions (25 & 50 vs. 80 & 106°; P ≤ 0.016), whereas antagonist co-activation was greatest in the most flexed compared to the extended positions (106 vs. 25 & 50°; P ≤ 0.02). In conclusion, both agonist and antagonist activation differed with knee joint angle during knee extension iMVCs and thus both likely contribute to the knee extensor angle-torque relationship

Neuromechanics of maximum and explosive strength across knee-joint angles

The primary purpose of this thesis was to assess the effect of knee-joint angle on the neuromechanics of maximal and explosive contractions, specifically torque and neuromuscular activation, as well as the influence of isometric resistance training (RT) on these variables and thus joint angle specificity of training adaptations. It was found that electrode location had a pronounced effect on surface electromyography (sEMG) amplitude during maximum isometric voluntary contractions (MVCs) and moderate relationship between subcutaneous tissue thickness and sEMG amplitude (R2=0.31 up to 0.38) was reduced but not consistently removed by maximal M-Wave (MMAX) normalization [up to R2= 0.16 (peak-to-peak) and R2= 0.23 (Area)]. Thus, MMAX peak-to-peak was the better normalization parameter that removed the influence of electrode location and substantially reduced the influence of subcutaneous tissue thickness. Maximal torque-angle relationship presented an inverted U shape with both, agonist (measure by two different techniques) and antagonist neuromuscular activation both differing with knee-joint angle and thus, both likely contributing to the torque-angle relationship. Absolute explosive torque-angle relationship exhibited higher torques at mid-range knee joint angles in a similar manner to maximal strength, whilst the ability to explosively express the available torque (i.e. relative to maximal strength) revealed only subtle differences between joint angles. Agonist neuromuscular activation showed increases from extended to flexed positions during both maximum and explosive contractions (at all time points; ~6% to ~34%) and evoked contractile properties presented opposite patterns with twitch torque increasing (~5% to ~30%) and octet torque decreasing (~2% to ~14%) with knee flexion. Finally, after 4 weeks of RT at a 65° knee-joint angle evidence of joint angle specificity was provided from both within-group (greater gains at 3 angles than others) and between-group evidence (greater gains at 2 angles than others) for maximal strength but not for explosive strength and neuromuscular activation. In summary, this thesis demonstrated: (1) higher strength values at middle knee-joint positions than more flexed and/or extended positions during maximal and explosive contractions; (2) how agonist neuromuscular activation contributes to the beforementioned changes in strength; (3) how muscle contractile properties contribute to the explosive strength across knee-joint angles; and finally (4) that joint angle specificity has a neural basis

The influence of training and athletic performance on the neural and mechanical determinants of muscular rate of force development

Neuromuscular explosive strength (defined as rate of force development; RFD) is considered important during explosive functional human movements; however this association has been poorly documented. It is also unclear how different variants of strength training may influence RFD and its neuromuscular determinants. Furthermore, RFD has typically been measured in isometric situations, but how it is influenced by the types of contraction (isometric, concentric, eccentric) is unknown. This thesis compared neuromuscular function in explosive power athletes (athletes) and untrained controls, and assessed the relationship between RFD in isometric squats with sprint and jump performance. The athletes achieved a greater RFD normalised to maximum strength (+74%) during the initial phase of explosive contractions, due to greater agonist activation (+71%) in this time. Furthermore, there were strong correlations (r2 = 0.39) between normalised RFD in the initial phase of explosive squats and sprint performance, and between later phase absolute explosive force and jump height (r2 = 0.37), confirming an association between explosive athletic performance and RFD. This thesis also assessed the differential effects of short-term (4 weeks) training for maximum vs. explosive strength, and whilst the former increased maximum strength (+20%) it had no effect on RFD. In contrast explosive strength training improved explosive force production over short (first 50 ms; +70%) and long (>50 ms; +15%) time periods, due to improved agonist activation (+65%) and maximum strength (+11%), respectively. Explosive strength training therefore appears to have greater functional benefits than maximum strength training. Finally, the influence of contraction type on RFD was assessed, and the results provided unique evidence that explosive concentric contractions are 60% more effective at utilising the available force capacity of the muscle, that was explained by superior agonist activation. This work provides a comprehensive analysis of the association between athletic performance and RFD, the differential effects of maximum vs. explosive strength training, and the influence of contraction type on the capacity for RFD

Optimisation of a neuromuscular electrical stimulation paradigm for targeted strengthening of an intrinsic foot muscle

The intrinsic foot muscles stabilise and stiffen the foot during posture and locomotion. Since they are placed under continued load, these muscles merit training to meet the weight-bearing demands of everyday activities. Their strengthening is however a largely neglected area and furthermore, the occurrence of common foot-related pathologies is associated with their dysfunction. Indeed, atrophy and dysfunction of the strongest intrinsic foot muscle, abductor hallucis (AbH), is symptomatic to pes planus and Hallux Valgus. AbH’s oblique mechanical action along with an inability for its voluntary activation in many individuals limits the strengthening capacity of existing training modalities. Due to the superficial location of AbH, neuromuscular electrical stimulation (NMES) offers a solution to this problem; however, its efficacy for muscle strength gains relies on high stimulation-intensity protocols, which are uncomfortable and limit participant adherence. Therefore, the purpose of this thesis was to develop an optimised NMES paradigm that is tolerable and efficacious for a targeted strengthening intervention of AbH. The studies reported in this thesis were undertaken with the overarching aim to systematically establish a tolerable and low stimulation-intensity NMES paradigm to train AbH. With this motivation in mind, four sequential experimental studies were designed to identify the optimal mode of NMES application (muscle vs nerve) and stimulation pulse duration (Chapter 3), pulse frequency and train duration (Chapter 4), training stimulus intensity (Chapter 5), and duty-cycle (Chapter 6), respectively. A major finding from the work undertaken in this thesis was the prevalent inability to voluntary activate AbH that exists in healthy participants. Since this inability also limits the measurement of voluntary force generation following an intervention, this thesis also developed a methodological approach that overcomes this limitation. Collectively, the studies in this thesis demonstrated that NMES successfully evokes contractions from AbH irrespective of ability for its voluntary activation and can therefore be used as a training modality. The optimised NMES paradigm presented in this thesis targets the motor point of AbH using 22s-trains of 1ms pulses at 20-100-20Hz with an intensity of 200% motor threshold and a 1:4 duty-cycle. This wide-pulse, high-frequency, low-intensity paradigm promotes adherence and has the potential to depolarise sensory axons due to their lower rheobase, and evoke contractions with a contribution of the central nervous system. When delivered using long trains and an alternating frequency pattern, it can take advantage of post-tetanic potentiation to produce force, which is then preserved across trains using a duty-cycle with long rest periods. This thesis intended to bind the aforementioned experimental chapters together with a final chapter investigating the effectiveness of the developed NMES paradigm instrengthening AbH following long-term exposure. However, the implementation of this study was not possible in light of the COVID-19 pandemic and is therefore not reportedin this thesis. Nevertheless, future work in this area can benefit from the extensive methodological work undertaken in this thesis and implement a longitudinal study to better understand the clinical implications for targeted AbH strengthening via NMES

Central and peripheral determinants of fatigue in acute hypoxia

This thesis was submitted for the degree of Docter of Philosophy and awarded by Brunel University on 24th March 2011.Fatigue is defined as an exercise-induced decrease in maximal voluntary force produced by a muscle. Fatigue may arise from central and/or peripheral mechanisms. Supraspinal fatigue (a component of central fatigue) is defined as a suboptimal output from the motor cortex and measured using transcranial magnetic stimulation (TMS). Reductions in O2 supply (hypoxia) exacerbate fatigue and as the severity of hypoxia increases, central mechanisms of fatigue are thought to contribute more to exercise intolerance. In study 1, the feasibility of TMS to measure cortical voluntary activation and supraspinal fatigue of human knee-extensors was determined. TMS produced reliable measurements of cortical voluntary activation within- and between-days, and enabled the assessment of supraspinal fatigue. In study 2, the mechanisms of fatigue during single-limb exercise in normoxia (arterial O2 saturation [SaO2] ~98%), and mild to severe hypoxia (SaO2 93-80%) were determined. Hypoxia did not alter neuromuscular function or cortical voluntary activation of the knee-extensors at rest, despite large reductions in cerebral oxygenation. Maximal force declined by ~30% after single-limb exercise in all conditions, despite reduced exercise time in severe-hypoxia compared to normoxia (15.9 ± 5.4 vs. 24.7 ± 5.5 min; p < 0.05). Peripheral mechanisms of fatigue contributed more to the reduction in force generating capacity of the knee-extensors following single-limb exercise in normoxia and mild- to moderate-hypoxia, whereas supraspinal fatigue played a greater role in severe-hypoxia. In study 3, the effect of constant-load cycling exercise to the limit of tolerance in hypoxia (SaO2 ~80%) and normoxia was investigated. Time to the limit of tolerance was significantly shorter in hypoxia compared to normoxia (3.6 ± 1.3 vs. 8.1 ± 2.9 min; p < 0.001). The reductions in maximal voluntary force and knee-extensor twitch force at task-failure were not different in hypoxia compared to normoxia. However, the level of supraspinal fatigue was exacerbated in hypoxia, and occurred in parallel with reductions in cerebral oxygenation and O2 delivery. Supraspinal fatigue contributes to the decrease in whole-body exercise tolerance in hypoxia, presumably as a consequence of inadequate O2 delivery to the brain

Dynamic muscle quality of the plantar flexors is impaired in claudicant patients with peripheral arterial disease and associated with poorer walking endurance

Objective Peripheral arterial disease and intermittent claudication (PAD-IC) negatively affects physical activity and function. There is evidence for plantarflexor muscle dysfunction and weakness; however, the extent to which this dysfunction can be attributed to reduced muscle size or quality, or both, is not yet known. This study investigated whether in vivo plantarflexor muscle quality during static and dynamic contractions is altered by PAD-IC and whether such changes are associated with impaired walking endurance according to initial and absolute claudication distances. Methods The study recruited 22 participants, consisting of 10 healthy controls and 12 claudicant patients with occlusion of the superficial femoral artery (seven unilateral and five bilateral). Muscle quality of the combined gastrocnemius muscles during static contractions was calculated by normalizing the estimated maximal potential muscle force to the physiological cross-sectional area of the lateral and medial gastrocnemius. Muscle quality during dynamic contractions of the combined plantarflexor muscles was calculated as the ratio of peak voluntary concentric plantarflexor power and the summed volume of lateral and medial gastrocnemius. Results Dynamic muscle quality was 24% lower in the claudicating-limb and asymptomatic-limb groups compared with controls (P = .017 and P = .023). The differences were most apparent at the highest contraction velocity (180°/s). Dynamic muscle quality was associated with reduced walking endurance (R = 0.689, P = .006 and R = 0.550, P = .042 for initial and absolute claudication distance, respectively). The claudicating-limb group demonstrated a trend toward reduced static muscle quality compared with controls (22%, P = .084). The relative contribution of the soleus muscle to plantarflexion maximum voluntary contraction was significantly higher in the claudicating-limb and asymptomatic-limb groups than in controls (P = .012 and P = .018). Conclusions The muscle strength of the plantarflexors in those with PAD-IC appears to be impaired at high contraction velocities. This may be explained by some reduction in gastrocnemii muscle quality and a greater reliance on the prominently type I-fibered soleus muscle. The reduced dynamic capability of the plantarflexor muscles was associated with disease severity and walking ability; therefore, efforts to improve plantarflexor power through dynamic exercise intervention are vital to maintain functional performance

Age-Related Reductions of Motor Unit Discharge Rates in the Human Hamstrings

Neuromuscular age-related differences of human limb muscles have been widely described with the notable exception of the hamstring muscles. The purpose was to assess contractile function and spinal motor neuron output expressed as motor unit discharge rates in the hamstrings of 11 young (26 ± 4 y) and 10 old (80 ± 5 y) men. Maximal voluntary isometric contractions (MVC), stimulated contractile properties and motor unit discharge rates from sub-maximal to MVC were recorded from the lateral (biceps femoris) and medial (semimembranosus-semitendinosus) posterior thigh. In the old men, knee extension and flexion at MVC were lower (P \u3c 0.05) and voluntary activation as assessed by the twitch interpolation technique was reduced (P \u3c 0.05) compared with the young. Electrically evoked twitches were lower in amplitude and increased in duration of old hamstrings (P \u3c 0.05) compared with the young. At sub-maximal to maximal contraction intensities the old had lower motor unit discharge rates as compared to the young (P \u3c 0.001). At MVC, mean motor unit discharge rates in the biceps femoris and semimembranosus-semitendinosus of old hamstrings were 15.6 ± 6.4 and 15.3 ± 5.9 Hz, as compared to 26.1 ± 10.1 and 27.9 ±7.8 Hz in the young, respectively (P \u3c 0.001). To date, the hamstrings show the greatest age-related reductions in motor unit discharge rates of any major limb muscle. These findings, in relation to motor unit discharge rates from other flexors and extensors support that in ageing, greater reductions are associated with limb flexor muscles