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In this study, we investigated the combined effects of age, dual-tasking (DT) and a passive hip exoskeleton on gait patterns among senior (SA) and young adults (YA). It was hypothesized that SA will be more affected by DT and that wearing the exoskeleton will improve gait patterns for both groups during DT. Twenty-two SA and twenty-six YA performed a single task (normal walking) and DT walking at their preferred speed with an exoskeleton (EXO), without (noEXO), and a sham version (SHAM) in a randomized and balanced order. Speed, cadence, double support time (DST), step length, hip joint power, range of motion (ROM), and moments (mom), as well as DT performance, were extracted using mocap, force plates (1000 Hz), and a voice recorder. Three-way MANOVA with group × device × condition was conducted (p < .05, inferred significance). Results showed a predominantly significant main effect of group for step length, speed, DST, ROM, and mom (p ≤ .01), main effect of condition for cadence, DST, speed, and mom (p < .01) and a main effect of the device for ROMz and mom (p < .05). Age-related changes were seen by decreased walking speed and step length, independent of DT and use of exoskeleton. Wearing the EXO aided the SA group to maintain similar levels of cadence from single to DT and decreased the hip internal rotation mom by 65%. There was no difference in DT performance between groups. In conclusion, SA showed a decline in gait patterns during DT that was somewhat mitigated by wearing an EXO.

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The aim of the present investigation was twofold. (1) to assess test-retest reliability of normalized mutual information (NMI) values extracted from the surface electromyography (sEMG) signal of muscles pairs of the upper body during dynamic bench press at a high load, and (2) to assess changes in NMI values from before to after a five-week quasi-randomized controlled bench press training intervention. For test-retest reliability, 20 strength trained males (age 25 ± 2 years, height 1.81 ± 0.07 m) performed two three-repetition maximum (3RM) tests in bench press, while sEMG was recorded from six upper body muscles. Tests were separated by 8.2 ± 2.9 days. For the training intervention, 17 male participants (age 26 ± 5 years, height 1.80 ± 0.07 m) trained bench press specific strength training for 5 weeks (TRA), while 13 male participants (age 23 ± 3 years, height 1.80 ± 0.08 m) constituted a control group (CON). 3RM bench press test and sEMG recordings were carried out before and after the intervention period. The NMI values ranged from poor to almost perfect reliability, with the majority displaying substantial reliability. TRA displayed a significant decrease in NMI values during the concentric phase for two agonist-agonist muscle pairs, while one agonist-agonist and two agonist-antagonist muscle pairs increased the NMI values during the eccentric phase. The observed changes did not exceed the minimal detectable threshold, and we therefore cannot surely ascertain that the changes observed in NMI values reflect genuine neural adaptations.

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ABSTRACT: Kristiansen, M, Sydow Krogh Pedersen, A-M, Sandvej, G, Jørgensen, P, Jakobsen, JV, de Zee, M, Hansen, EA, and Klitgaard, KK. Enhanced maximal upper-body strength increases performance in sprint kayaking. J Strength Cond Res 37(4): e305-e312, 2023-The association between upper-body strength and performance in 200-m flat-water sprint kayak is not fully elucidated. Therefore, the aim of study 1 was to investigate the relationship between upper-body strength and kayaking performance. In study 2, the aim was to perform a randomized training intervention to investigate whether a causal relationship was present between an increase in strength and an actual change in 200-m kayaking performance. In study 1, 37 (22 men and 15 women) elite kayak paddlers performed tests of maximal power output, isometric force, 1 repetition maximum (1RM), and 40 seconds of maximal repetition number in bench press and bench pull and a 30-second all-out on-water sprint kayak test. In study 2, 26 (16 men and 10 women) national elite junior A, U23, and senior kayak paddlers were allocated into 2 groups: a training group (TRAIN) and a maintenance group (MAIN). Each group completed a 6-week strength training intervention with the purpose of either increasing 1RM in bench press (TRAIN) or maintaining strength (MAIN). Pre- and posttests were performed in 200-m kayak ergometer sprint, 1RM bench press, and 1RM bench pull. In study 1, 1RM in bench press was the best predictor of 30-second on-water kayaking performance with a regression coefficient of 0.474. In study 2, TRAIN significantly increased 1RM strength in bench press (pre: 87.3 ± 21.2 kg, post: 93.9 ± 21.3 kg, p = 0.001) and bench pull (pre: 84.2 ± 15.3 kg, post: 86.0 ± 15.1 kg, p = 0.025). In the 200-m kayak ergometer sprint test, TRAIN significantly decreased the time to complete the test (pre: 44.8 ± 4.3 seconds, post: 44.3 ± 4.3 seconds, p = 0.042). In bench press, 1RM was the best predictor of 200-m kayaking, and an increase in bench press 1RM resulted in increased kayaking performance.

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Investigations of behavior and control of voluntary stereotyped rhythmic movement contribute to the enhancement of motor function and performance of disabled, sick, injured, healthy, and exercising humans. The present article presents examples of unprompted alteration of freely chosen movement rate during voluntary stereotyped rhythmic movements. The examples, in the form of both increases and decreases of movement rate, are taken from activities of cycling, finger tapping, and locomotion. It is described that, for example, strength training, changed power output, repeated bouts, and changed locomotion speed can elicit an unprompted alteration of freely chosen movement rate. The discussion of the examples is based on a tripartite interplay between descending drive, rhythm-generating spinal neural networks, and sensory feedback, as well as terminology from dynamic systems theory.

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The present study investigated whether the duration of the first tapping bout, which could also be considered 'the priming', would play a role for the occurrence of the behavioral phenomenon termed repeated bout rate enhancement. Eighty-eight healthy individuals were recruited. Sixty-three of these demonstrated repeated bout rate enhancement and they were assigned to two different groups, which performed either active or passive tapping as priming. The durations of the first tapping bouts, which acted as priming, were 20, 60, 120, and 180 s. Following the first bout there was a 10 min rest and a subsequent 180 s tapping bout performed at freely chosen tapping rate. Vertical displacement and tapping force data were recorded. Rate enhancement was elicited independently of the duration of the first bout in both groups. Rate enhancement occurred without concurrent changes of the magnitude of vertical displacement, time to peak force, and duration of finger contact phase. The peak force was reduced when 180 s of tapping had been performed as priming. The increased tapping rate following priming by as little as 20 s active or passive tapping, as observed here, is suggested to be a result of increased net excitability of the nervous system.

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In this study we investigated motor variability in individuals who showed (responders) and who did not show (non-responders) a behavioural phenomenon termed repeated bout rate enhancement. The phenomenon is characterized by an increase of the freely chosen index finger tapping rate during the second of two consecutive tapping bouts. It was hypothesized that responders would perform (i) tapping with a lower magnitude, but more complex structure of variability than non-responders and (ii) bout 2 with a lower magnitude and increased complexity of variability than bout 1, as opposed to non-responders. Individuals (n = 102) performed two 3-min tapping bouts separated by 10 min rest. Kinetic and kinematic recordings were performed. Standard deviation (SD), coefficient of variation (CV), and sample entropy (SaEn), representing magnitude and complexity of variability, were computed. For responders, SaEn of vertical displacement of the index finger was higher than for non-responders (p = .046). Further, SaEn of vertical force and vertical displacement was higher in bout 2 than in bout 1 for responders (p < .001 and p = .006, respectively). In general, SD of vertical displacement was lower in bout 2 than in bout 1 (p < .001). SaEn of vertical force was higher in bout 2 than in bout 1 (p = .009). The present lower SD and higher SaEn values of vertical force and displacement time series in bout 2 as compared to bout 1 suggest differences in the dynamics of finger tapping. Further, it is possible that the increases in SaEn of vertical displacement reflected a greater adaptability in the dynamics of motor control among responders compared with non-responders.

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Kristiansen, M, Samani, A, Vuillerme, N, Madeleine, P, and Hansen, EA. External and internal focus of attention increases muscular activation during bench press in resistance-trained participants. J Strength Cond Res 32(9): 2442-2451, 2018-Research on the effects of instructed attentional focus during execution of strength training exercises is limited and has thus far only been performed on single-joint exercises. The aim of this study was to compare the effects of instructed internal (INT) and external (EXT) focus of attention with a baseline measurement of no instructed focus of attention (BASE) on the surface electromyographic (EMG) amplitude during a free-weight bench press exercise in resistance-trained participants. Twenty-one resistance-trained male participants performed bench press at 60% of their 3-repetition maximum, with BASE, EXT, and INT. The order of EXT and INT was randomized and counterbalanced. Electromyographic data were recorded from 13 muscles of the upper and lower body. Subsequently, mean and peak EMG amplitudes were computed. EXT and INT resulted in significantly increased mean EMG amplitude of 6 upper-body muscles as compared with BASE (p ≤ 0.05). In addition, EXT and INT also resulted in increased peak EMG amplitude of 3 upper-body muscles as compared with BASE (p ≤ 0.05). These results show that muscular activation is increased during bench press, when applying an instructed focus of attention compared with a baseline measurement with no focus instructions (BASE).

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The transition from walking to running has previously been predicted to occur at a point where the stride frequency starts getting closer to the running attractor than to the walking attractor. The two behavioural attractors were considered to be represented by the freely chosen stride frequencies during unrestricted treadmill walking and running. The aim of the present study was to determine the relative and absolute test-retest reliability of the predicted walk-to-run transition stride frequency. Healthy individuals (n=25) performed walking and running on a treadmill in a day-to-day test-retest design. The two behavioral attractors were determined during walking and running at freely chosen velocities and stride frequencies. Subsequently, the walk-to-run transition stride frequency was predicted using camera recordings and a previously reported equation for prediction. The walk-to-run transition occurred at a velocity of 7.7±0.5kmh-1 at day 1 as well as at day 2. Besides, the predicted walk-to-run transition stride frequencies were 69.7±3.3 strides min-1 and 70.5±3.4 strides min-1 on day 1 and day 2, respectively (p=0.08). A further comparison between the predicted walk-to-run transition stride frequencies at day 1 and day 2 showed an ICC3,1 of 0.89, which indicated almost perfect relative reliability. The absolute reliability was reflected by a%-value of the standard error of the measurement (SEM%) of 1.6% and a%-value of the smallest real difference (SRD%) of 4.4%. In conclusion, the predicted walk-to-run transition stride frequency can be considered reliable across days.

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It remains unclear why humans spontaneously shift from walking to running at a certain point during locomotion at gradually increasing velocity. We show that a calculated walk-to-run transition stride frequency (70.6 ± 3.2 strides min-1) agrees with a transition stride frequency (70.8 ± 3.1 strides min-1) predicted from the two stride frequencies applied during treadmill walking and running at freely chosen velocities and freely chosen stride frequencies. The agreement is based on Bland and Altman's statistics. We found no essential mean relative difference between the two transition frequencies, i.e. -0.5% ± 4.2%, as well as limits of agreement of -8.7% and 7.7%. The particular two freely chosen stride frequencies used for prediction are considered behavioural attractors. Gait is predicted to be shifted from walking to running when the stride frequency starts getting closer to the running attractor than to the walking attractor. In particular, previous research has focussed on transition velocity and optimisation theories based on minimisation of, e.g., energy turnover or biomechanical loadings of the legs. Conversely, our data support that the central phenomenon of walk-to-run transition during human locomotion could be influenced by behavioural attractors in the form of stride frequencies spontaneously occurring during behaviourally unrestricted gait conditions of walking and running.

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The present study analyzed (a) whether a recently reported phenomenon of repeated bout rate enhancement in finger tapping (i.e., a cumulating increase in freely chosen finger tapping frequency following submaximal muscle activation in the form of externally unloaded voluntary tapping) could be replicated and (b) the hypotheses that the faster tapping was accompanied by changed vertical displacement of the fingertip and changed peak force during tapping. Right-handed, healthy, and recreationally active individuals (n = 24) performed two 3-min index finger tapping bouts at freely chosen tapping frequency, separated by 10-min rest. The recently reported phenomenon of repeated bout rate enhancement was replicated. The faster tapping (8.8 ± 18.7 taps/min, corresponding to 6.0 ± 11.0%, p = .033) was accompanied by reduced vertical displacement (1.6 ± 2.9 mm, corresponding to 6.3 ± 14.9%, p = .012) of the fingertip. Concurrently, peak force was unchanged. The present study points at separate control mechanisms governing kinematics and kinetics during finger tapping.

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Muscle synergies have been investigated during different types of human movement using nonnegative matrix factorization. However, there are not any reports available on the reliability of the method. To evaluate between-day reliability, 21 subjects performed bench press, in two test sessions separated by approximately 7days. The movement consisted of 3 sets of 8 repetitions at 60% of the three repetition maximum in bench press. Muscle synergies were extracted from electromyography data of 13 muscles, using nonnegative matrix factorization. To evaluate between-day reliability, we performed a cross-correlation analysis and a cross-validation analysis, in which the synergy components extracted in the first test session were recomputed, using the fixed synergy components from the second test session. Two muscle synergies accounted for >90% of the total variance, and reflected the concentric and eccentric phase, respectively. The cross-correlation values were strong to very strong (r-values between 0.58 and 0.89), while the cross-validation values ranged from substantial to almost perfect (ICC3, 1 values between 0.70 and 0.95). The present findings revealed that the same general structure of the muscle synergies was present across days and the extraction of muscle synergies is thus deemed reliable.

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Understanding of behavior and control of human voluntary rhythmic stereotyped leg movements is useful in work to improve performance, function, and rehabilitation of exercising, healthy, and injured humans. The present study aimed at adding to the existing understanding within this field. To pursue the aim, correlations between freely chosen movement frequencies in relatively simple, single-joint, one- and two-legged knee extension exercise were investigated. The same was done for more complex, multiple-joint, one- and two-legged pedaling. These particular activities were chosen because they could be considered related to some extent, as they shared a key aspect of knee extension, and because they at the same time were different. The activities were performed at submaximal intensities, by healthy individuals (n = 16, thereof eight women; 23.4 ± 2.7 years; 1.70 ± 0.11 m; 68.6 ± 11.2 kg). High and fair correlations (R-values of 0.99 and 0.75) occurred between frequencies generated with the dominant leg and the nondominant leg during knee extension exercise and pedaling, respectively. Fair to high correlations (R-values between 0.71 and 0.95) occurred between frequencies performed with each of the two legs in an activity, and the two-legged frequency performed in the same type of activity. In general, the correlations were higher for knee extension exercise than for pedaling. Correlations between knee extension and pedaling frequencies were of modest occurrence. The correlations between movement frequencies generated separately by each of the legs might be interpreted to support the following working hypothesis, which was based on existing literature. It is likely that involved central pattern generators (CPGs) of the two legs share a common frequency generator or that separate frequency generators of each leg are attuned via interneuronal connections. Further, activity type appeared to be relevant. Thus, the apparent common rhythmogenesis for the two legs appeared to be stronger for the relatively simple single-joint activity of knee extension exercise as compared to the more complex multi-joint activity of pedaling. Finally, it appeared that the shared aspect of knee extension in the related types of activities of knee extension exercise and pedaling was insufficient to cause obvious correlations between generated movement frequencies in the two types of activities.

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The authors investigated the effects of 2 weeks of heavy index finger strength training on characteristics of freely chosen finger tapping including tapping frequency, tap force, and finger displacement. One group (n = 12) performed index finger extension and flexion strength training. A control group (n = 12) performed no intervention. Results showed that the training group increased strength in both extension (19.5 ± 22.2%, p = .015) and flexion (9.4 ± 9.8%, p = .001) from pretest to posttest. Furthermore, training did not affect the frequency and the pattern of the tapping movement (Fs = 0.004-3.441, ps = .077-.954). The present results are in contrast to previous findings of reduced movement frequency and altered movement pattern during ergometer pedaling after strength training. This difference may be explained by the dissimilarity between the 2 tasks.

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Healthy individuals (n = 40) performed index finger tapping at freely chosen frequency during repeated bouts and before and after near-maximal muscle action consisting of 3 intense flexions of the index finger metacarpal phalangeal joint. One experiment showed, unexpectedly, that a bout of tapping increased the tapping frequency in the subsequent bout. Thus, a cumulating increase of 8.2 ± 5.4% (p < .001) occurred across 4 bouts, which were all separated by 10 min rest periods. Follow-up experiments revealed that tapping frequency was still increased in consecutive bouts when rest periods were extended to 20 min. Besides, near-maximal muscle activation, followed by 5 min rest, did not affect the tapping frequency. In conclusion, freely chosen tapping frequency was increased in repeated bouts of tapping, which were separated by 10-20 min rest periods. The observed phenomenon is suggested to be termed repeated bout rate enhancement.

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It was tested whether a marathon was completed faster by applying a scientifically based rather than a freely chosen nutritional strategy. Furthermore, gastrointestinal symptoms were evaluated. Nonelite runners performed a 10 km time trial 7 weeks before Copenhagen Marathon 2013 for estimation of running ability. Based on the time, runners were divided into two similar groups that eventually should perform the marathon by applying the two nutritional strategies. Matched pairs design was applied. Before the marathon, runners were paired based on their prerace running ability. Runners applying the freely chosen nutritional strategy (n = 14; 33.6 ± 9.6 years; 1.83 ± 0.09 m; 77.4 ± 10.6 kg; 45:40 ± 4:32 min for 10 km) could freely choose their in-race intake. Runners applying the scientifically based nutritional strategy (n = 14; 41.9 ± 7.6 years; 1.79 ± 0.11 m; 74.6 ± 14.5 kg; 45:44 ± 4:37 min) were targeting a combined in-race intake of energy gels and water, where the total intake amounted to approximately 0.750 L water, 60 g maltodextrin and glucose, 0.06 g sodium, and 0.09 g caffeine per hr. Gastrointestinal symptoms were assessed by a self-administered postrace questionnaire. Marathon time was 3:49:26 ± 0:25:05 and 3:38:31 ± 0:24:54 hr for runners applying the freely chosen and the scientifically based strategy, respectively (p = .010, effect size=-0.43). Certain runners experienced diverse serious gastrointestinal symptoms, but overall, symptoms were low and not different between groups (p > .05). In conclusion, nonelite runners completed a marathon on average 10:55 min, corresponding to 4.7%, faster by applying a scientifically based rather than a freely chosen nutritional strategy. Furthermore, average values of gastrointestinal symptoms were low and not different between groups.

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The purpose of the present study was to compare the effect of 12 weeks of strength training combined with a large volume of endurance training with the effect of strength training alone on the strength training adaptations. Well-trained cyclists with no strength training experience performed heavy strength training twice a week in addition to a high volume of endurance training during a 12-week preparatory period (S + E; n = 11). A group of non-strength trained individuals performed the same strength training as S + E, but without added endurance training (S; n = 7). Thigh muscle cross-sectional area, 1 repetition maximum (1RM) in leg exercises, squat jump performance, and peak rate of force development (RFD) were measured. Following the intervention period, both S + E and S increased 1RM strength, thigh muscle cross-sectional area, and squat jump performance (p < 0.05), and the relative improvements in S were greater than in S + E (p < 0.05). S increased peak RFD while S + E did not, and this improvement was greater than in S + E (p < 0.05). To the best of our knowledge, this is the first controlled study to demonstrate that the strength training response on muscle hypertrophy, 1RM strength, squat jump performance, and peak RFD is attenuated in well-trained endurance athletes during a period of concurrent endurance training.

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We investigated the effects of strength maintenance training on thigh muscle cross-sectional area (CSA), leg strength, determinants of cycling performance, and cycling performance. Well-trained cyclists completed either (1) usual endurance training supplemented with heavy strength training twice a week during a 12-week preparatory period followed by strength maintenance training once a week during the first 13 weeks of a competition period (E + S; n = 6 [♂ = 6]), or (2) usual endurance training during the whole intervention period (E; n = 6 [♂ = 5, ♀ = 1]). Following the preparatory period, E + S increased thigh muscle CSA and 1RM (p < 0.05), while no changes were observed in E. Both groups increased maximal oxygen consumption and mean power output in the 40-min all-out trial (p < 0.05). At 13 weeks into the competition period, E + S had preserved the increase in CSA and strength from the preparatory period. From the beginning of the preparatory period to 13 weeks into the competition period, E + S increased peak power output in the Wingate test, power output at 2 mmol l(-1) [la(-)], maximal aerobic power output (W (max)), and mean power output in the 40-min all-out trial (p < 0.05). The relative improvements in the last two measurements were larger than in E (p < 0.05). For E, W (max) and power output at 2 mmol l(-1) [la(-)] remained unchanged. In conclusion, in well-trained cyclists, strength maintenance training in a competition period preserved increases in thigh muscle CSA and leg strength attained in a preceding preparatory period and further improved cycling performance determinants and performance.

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The purpose of this study was to investigate the effect of heavy strength training on thigh muscle cross-sectional area (CSA), determinants of cycling performance, and cycling performance in well-trained cyclists. Twenty well-trained cyclists were assigned to either usual endurance training combined with heavy strength training [E + S; n = 11 (male symbol = 11)] or to usual endurance training only [E; n = 9 (male symbol = 7, female symbol = 2)]. The strength training performed by E + S consisted of four lower body exercises [3 x 4-10 repetition maximum (RM)], which were performed twice a week for 12 weeks. Thigh muscle CSA, maximal force in isometric half squat, power output in 30 s Wingate test, maximal oxygen consumption (VO(2max)), power output at 2 mmol l(-1) blood lactate concentration ([la(-)]), and performance, as mean power production, in a 40-min all-out trial were measured before and after the intervention. E + S increased thigh muscle CSA, maximal isometric force, and peak power in the Wingate test more than E. Power output at 2 mmol l(-1) [la(-)] and mean power output in the 40-min all-out trial were improved in E + S (P < 0.05). For E, only performance in the 40-min all-out trial tended to improve (P = 0.057). The two groups showed similar increases in VO(2max) (P < 0.05). In conclusion, adding strength training to usual endurance training improved determinants of cycling performance as well as performance in well-trained cyclists. Of particular note is that the added strength training increased thigh muscle CSA without causing an increase in body mass.

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PURPOSE: The aim of the present study is to examine the association between physical activity and perceived job demand, job control, perceived stress and energy, and physiological arousal reflected by morning and evening concentrations of cortisol in saliva among white-collar workers. METHODS: Physical activity during the last week was assessed during work and leisure time by a Danish version of the International Physical Activity Questionnaire and saliva samples were collected. The study group comprised 389 white-collar workers, aged 25-67 years, and of which 257 were women. RESULTS: We found that physical activity during leisure time was associated with higher perceived energy, and for men also with lower perceived stress. Further, we found that physical activity at leisure time affected the association between salivary cortisol and perceived stress and energy so that respondents being physically active at leisure time and perceiving higher energy showed higher evening saliva cortisol. CONCLUSION: Physically active employees perceive less stress and more energy. The association between stress-energy and salivary cortisol was affected by vigorous physical activity. No association between job control-demand and the degree of physical activity was found. Based on the present data, we recommend office workers-exposed to high job strain and inactivity at the job-to perform physical activity, preferably of high intensity, in order to reduce stress and increase energy.

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The development of noncircular chain wheels for the enhancement of cycling performance has been in progress for a long time and continues apace. In this study we tested whether submaximal cycling using a non-circular (Biopace) versus a circular chain wheel resulted in lower peak crank torque at preset pedal rates as well as resulting in lower pedal rate and metabolic response at freely chosen pedal rate. Ten trained cyclists (mean+/-SD: 27+/-3 years of age, 182+/-4 cm tall, 77.5+/-7.0 kg of body mass, and peak oxygen uptake of 61.7+/-4.4 ml kg(-1) min(-1)) cycled with a Biopace and a circular chain wheel at 180 W at 65 and 90 rpm for recording of crank torque profiles, and at their freely chosen pedal rate for recording of pedal rate and metabolic response, including oxygen uptake and blood lactate concentration. Crank torque profiles were similar between the two chain wheels during cycling at preset pedal rates. During cycling at the freely chosen pedal rate (being 93+/-6 and 93+/-4 rpm for the Biopace and circular chain wheel, respectively), blood lactate concentration was significantly different between the two chain wheels, being on average 0.2 mmol l(-1) lower with the Biopace chain wheel. A musculoskeletal simulation model supported the idea that a contributing factor to the observed difference in blood lactate concentration may be slightly reduced muscle activity around the phase where peak crank torque occurs during cycling with the Biopace chain wheel. In that particular phase of the crank revolution, the observed slightly lower muscle activity may result from larger transfer of energy from the legs to the crank.

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