Fatigue during sustained maximal voluntary contraction of different muscles in humans: dependence on fibre type and body posture

Muscle Strength Preservation During Repeated Sets of Fatiguing Resistance Exercise: A Secondary Analysis

ABSTRACT

Nuzzo, JL. Muscle strength preservation during repeated sets of fatiguing resistance exercise: A secondary analysis. J Strength Cond Res 38(6): 1149-1156, 2024-During sustained or repeated maximal voluntary efforts, muscle fatigue (acute strength loss) is not linear. After a large initial decrease, muscle strength plateaus at approximately 40% of baseline. This plateau, which likely reflects muscle strength preservation, has been observed in sustained maximal isometric and repeated maximal isokinetic contractions. Whether this pattern of fatigue occurs with traditional resistance exercise repetitions with free weights and weight stack machines has not been overviewed. Here, the aim was to determine whether the number of repetitions completed across 4 or more consecutive repetitions-to-failure tests exhibits the same nonlinear pattern of muscle fatigue. A secondary analysis was applied to data extracted as part of a recent meta-analysis on repetitions-to-failure tests. Studies were eligible if they reported mean number of repetitions completed in 4-6 consecutive repetitions-to-failure tests at a given relative load. Twenty-nine studies were included. Overall, the results show that the number of repetitions completed in consecutive repetitions-to-failure tests at a given load generally decreases curvilinearly. The numbers of repetitions completed in sets 2, 3, 4, 5, and 6 were equal to approximately 70, 55, 50, 45, and 45% of the number of repetitions completed in set 1, respectively. Longer interset rest intervals typically attenuated repetition loss, but the curvilinear pattern remained. From the results, a chart was created to predict the number of repetitions across 6 sets of resistance exercise taken to failure based on the number of repetitions completed in set 1. The chart is a general guide and educational tool. It should be used cautiously. More data from a variety of exercises, relative loads, and interset rest intervals are needed for more precise estimates of number of repetitions completed during repeated sets of fatiguing resistance exercise.

A mathematical model-based approach to optimize loading schemes of isometric resistance training sessions

Individualized resistance training is necessary to optimize training results. A model-based optimization of loading schemes could provide valuable impulses for practitioners and complement the predominant manual program design by customizing the loading schemes to the trainee and the training goals. We compile a literature overview of model-based approaches used to simulate or optimize the response to single resistance training sessions or to long-term resistance training plans in terms of strength, power, muscle mass, or local muscular endurance by varying the loading scheme. To the best of our knowledge, contributions employing a predictive model to algorithmically optimize loading schemes for different training goals are nonexistent in the literature. Thus, we propose to set up optimal control problems as follows. For the underlying dynamics, we use a phenomenological model of the time course of maximum voluntary isometric contraction force. Then, we provide mathematical formulations of key performance indicators for loading schemes identified in sport science and use those as objective functionals or constraints. We then solve those optimal control problems using previously obtained parameter estimates for the elbow flexors. We discuss our choice of training goals, analyze the structure of the computed solutions, and give evidence of their real-life feasibility. The proposed optimization methodology is independent from the underlying model and can be transferred to more elaborate physiological models once suitable ones become available.

Inter-joint coordination changes during and after muscle fatigue

People produce multi-joint movements by organizing many degrees of freedom into a few major covarying relationships, indicating a high level of inter-joint coordination. These relationships can be identified using data decomposition analyses (e.g. principal components analysis, non-negative matrix factorization). The purpose of this study was to determine how movement coordination changes during muscle fatigue by analyzing the covariance structure of multi-joint movements. Sixteen (16) healthy adults completed a continuous, timed ratcheting task with the right arm for three 1-min intervals before, during, and after an intermittent shoulder fatigue protocol. Joint angles from the right arm and trunk were tracked for subsequent principal components analysis. Principal component waveforms were constructed from the original joint angles, and changes in the waveforms during fatigue were assessed using cross-correlations. The variance explained by the first four principal components reached a maximum of 90.5% in the second minute of the pre-test and decreased to a minimum of 86.0% in the last minute of fatigue (p = .033). In the last minute of the post-test, explained variance (87.1%) did not differ from any other pre, fatigue, or post-test time point (p > .23). These results suggest that inter-joint coordination decreased during fatigue. Changes in the movement patterns and principal component waveforms suggest that subjects adopted a more rigid movement strategy when fatigued. However, the rigid movement strategy was not observed during the post-test. The results suggest that people adopted a new pattern of inter-joint coordination while using novel kinematics.

Influence of joint position on synergistic muscle activity after fatigue of a single muscle head

INTRODUCTION

We investigated synergistic muscle activity after fatigue of a single muscle in different joint positions.

METHODS

Two experimental groups (n = 12 each) performed maximal voluntary contractions (MVCs) before and after fatiguing the gastrocnemius lateralis (GL), using neuromuscular electrical stimulation (NMES). Neuromuscular tests, including muscle activity during MVC, H-reflex, and twitch interpolation, were performed. One group completed the experiment in a knee-extended position with the second group in a knee-flexed position.

RESULTS

In the knee-flexed position, the muscle activity increased in non-stimulated synergistic muscles. In contrast, in the knee-extended position, muscle activity of the synergistic muscles remained unaltered. The MVC force remained unaltered in the flexed position and decreased in the extended position.

CONCLUSIONS

Synergistic muscles compensate for the fatigued muscle in the flexed position but not in the extended position. Compensation mechanisms seem to depend on joint position.

Muscle fatigue: what, why and how it influences muscle function

Much is known about the physiological impairments that can cause muscle fatigue. It is known that fatigue can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex, and that there is no global mechanism responsible for muscle fatigue. Rather, the mechanisms that cause fatigue are specific to the task being performed. The development of muscle fatigue is typically quantified as a decline in the maximal force or power capacity of muscle, which means that submaximal contractions can be sustained after the onset of muscle fatigue. There is even evidence that the duration of some sustained tasks is not limited by fatigue of the principal muscles. Here we review experimental approaches that focus on identifying the mechanisms that limit task failure rather than those that cause muscle fatigue. Selected comparisons of tasks, groups of individuals and interventions with the task-failure approach can provide insight into the rate-limiting adjustments that constrain muscle function during fatiguing contractions.

Energy metabolism in intensively exercising calf muscle under a simulated orthostasis

We conducted non-invasive methods to investigate the mechanisms how an orthostasis improves fatigue resistance in human calf muscle during intense exercise. Eleven healthy volunteers performed two series of ten intervals of maximum dynamic exercise (15 s) and recovery (45 s) at almost horizontal body position under both, control conditions (CON) and lower body negative pressure (LBNP, -40 mbar). As from the second work interval, LBNP significantly improved fatigue resistance shown as a lower reduction in work and in contraction velocity (P < 0.01). During each work interval, EMG showed a small increase in amplitude (P < 0.01) and a steep drop by 20% in median frequency (P < 0.01). Under LBNP, both EMG parameters completely recovered during subsequent rest, whereas under CON recovery was incomplete (P < 0.01). During the first work interval, consumption of phosphocreatine (PCr) was almost the same for both conditions. In periods of recovery under LBNP, resynthesis of PCr and inorganic phosphate were significantly faster. PCr reached 10 to 20% higher levels (P < 0.01). LBNP caused an initial increase in intracellular pH (0.08 U (P < 0.01)). The subsequent time courses of pH were similar for CON and LBNP. During work, pH steeply increased by about 0.3 U. During subsequent recovery, pH dropped to values between 6.3 and 6.5. LBNP caused significantly higher levels of total haemoglobin and oxy-haemoglobin (P < 0.05). A simulated orthostasis increased fatigue resistance during high intense interval exercise because of a faster PCr resynthesis and may be because of improvements in the maintenance of motoneuronal activity.

The properties and interrelationships of various force-time parameters during maximal repeated rhythmic grip

The purpose of this study was to examine the properties and interrelationships of various force-time parameters including the inflection point for the rate of decline in force during a maximal repeated rhythmic grip. Fifteen healthy males (age M=21.5, SD=2.1 yr, height M=172.4, SD=5.7 cm, body mass M=68.2, SD=9.2 kg) participated in this study. Subjects performed a maximal repeated rhythmic grip with maximal effort with a target frequency of 30 grip.min(-1) for 6 min. The force value decreased linearly and markedly until about 70% of maximal strength for about 55 s after the onset of a maximal repeated rhythmic grip, and then decreased moderately. Because all parameters showed fair or good correlations between 3 min and 6 min, they are considered to be able to sufficiently evaluate muscle endurance for 3 min instead of 6 min. However, there were significant differences between 3 min and 6 min in the integrated area, the final force, the rate of the decrement constant (k) fitting the force decreasing data to y=ae(-kx)+b and the force of maximal difference between the force and a straight line from peak force to the final force. Their parameters may vary generally by the length of a steady state, namely, a measurement time. The final force value before finishing and the rate of the decrement constant (k) reflect the latter phase during a maximal repeated rhythmic grip. Although many parameters show relatively high mutual relationships, the rate constant (k) shows relatively low correlations with other parameters. We inferred that decreasing the time until 80% of maximal strength and the amount of the decrement force for the first 1 min reflect a linear decrease in the initial phase.

The effect of measurement time when evaluating static muscle endurance during sustained static maximal gripping

The purpose of this study was to examine the useful measurement time when evaluating static muscle endurance by comparing various parameters during sustained static gripping for 1, 3 and 6 min. Fifteen males (mean +/- SD age 20.8 +/- 1.3 yr, height 172.9 +/- 4.6 cm, body mass 67.7 +/- 5.7 kg) and fifteen females [mean +/- SD age 20.2 +/- 0.9 yr, height 158.5 +/- 3.2 cm, body mass 55.9 +/- 4.6 kg] volunteered to participate in this study. The subjects performed the sustained static maximal grip test with a sagittal and horizontal arm position for 1, 3 and 6 min on different days. Eleven force-time parameters were selected to evaluate static muscle endurance. The trial-to-trial reliability of each measurement time of sustained static maximal gripping was very high (rxy = 0.887-0.981 (1 min), 0.912-0.993 (3 min), 0.901-0.965 (6 min)). The errors of exertion values between trials were very small (below 10%). A significant correlation was found in the following parameters: the final strength and the exponential function between 1 min and 3 min, all parameters except for the time required to reach 80% of maximal grip, the regression coefficient at post-inflection between 3 min and 6 min, and the decreasing rate between all measurement times (1 min, 3 min, and 6 min). Significant differences between the measurement times were found in all parameters except for the time to 60, 70, and 80% force decreases, and the regression coefficient of pre-inflection. There was a tendency that the longer the measurement time, the larger the decreasing force. It is suggested that for the 6 min measurement, the subjects unconsciously restrained the maximal gripping force, influenced by a psychological factor as the pain became greater. The 1 min measurement may evaluate only the remarkable decreasing phase of the decreasing force, and not evaluate the phase of an almost steady state.

Fatigue responses of human triceps surae muscles during repetitive maximal isometric contractions

Nine healthy men (22-45 yr) completed 100 repetitive maximal isometric contractions of the ankle plantar flexor muscles in two knee positions of full extension (K0) and flexion at 90 degrees (K90), positions that varied the contribution of the gastrocnemii. Electromyographic activity was recorded from the medial and lateral gastrocnemii and soleus muscles by using surface electrodes. Plantar flexion torque in K0 was greater and decreased more rapidly than in K90. The electromyographic amplitude decreased over time, and there were no significant differences between muscles and knee joint positions. The level of voluntary effort, assessed by a supramaximal electrical stimulation during every 10th contraction, decreased from 96 to 70% (P < 0.05) with no difference between K0 and K90. It was suggested that a decrease in plantar flexion torque was attributable to both central and peripheral fatigue and that greater fatigability in K0 than in K90 would result from a greater contribution and hence more pronounced fatigue of the gastrocnemius muscle. Further support for this possibility was provided from changes in twitch torque.

Effects of fatigue duration and muscle type on voluntary and evoked contractile properties

The effects of fatigue duration and muscle type on voluntary and evoked contractile properties were investigated with an isometric, intermittent, submaximal fatigue protocol. Four groups performed contractions of the plantar flexors and quadriceps at various intensities to produce long (LDF; 19 min 30 s)- and short-duration fatigue (SDF; 4 min 17 s). The LDF group had a significantly greater decrease in muscle activation than did the SDF group (12 vs. 5.8%) during recovery, although there was no difference in the impairment of maximum voluntary contraction force beyond 30 s of recovery. The significant decrease in the compound muscle action potential of the LDF group (M-wave amplitude; 14.7%) contrasted with the M-wave potentiation of the SDF group (15.7%), suggesting changes in membrane excitation may affect LDF. The quadriceps group performing contractions at 50% MVC experienced a smaller decrease in agonist electromyograph activity than did other groups, indicating both muscle and fatigue duration specificity. Impairments in excitation-contraction coupling were indicated by changes in quadriceps peak twitch and time to peak twitch while decreases in PF M-wave amplitudes suggested a disruption of membrane potentials. Results suggest that fatigue mechanisms may be duration (activation, half relaxation time) or muscle specific (electromyograph, twitch torque) or a combination of both (M wave, time to peak twitch torque).

Endurance time characteristics of human ankle dorsiflexors and plantarflexors

The endurance time provides a convenient means to assess muscle fatigue resistance. The purpose of the experiments was to study endurance time characteristics of human ankle dorsiflexion and plantarflexion as a function of level of contraction. Nine subjects (four men, five women) were examined. The experimental protocol consisted of determining each individual maximum voluntary contraction (MVC) before each fatiguing experiment and undertaking only one fatiguing isometric contraction (dorsiflexion or plantarflexion) per day. Each subject produced at least six plantarflexion and six dorsiflexion contractions chosen from 15% to 90% MVC. An exponential model was fit to data for each individual and was then fit to the pooled data. The variance accounted for was over 99% for both dorsiflexion and plantarflexion mean values. As expected, endurance time declined as the contraction level increased. Plantarflexing responses presented a higher variation from subject to subject than for dorsiflexing. Inter-subject variability primarily seemed to involve a change in the shape of the endurance curve rather than a shift of the curve. When the data were pooled, the exponential curve had parameters similar to the average of the individual fits. Human ankle dorsiflexion and plantarflexion endurance times as a function of level of contraction were found to be similar under the present experimental conditions.