Metabolic demands of slacklining in less and more advanced slackliners

ABSTRACT

Walking or balancing on a slackline has gained increasing popularity as a recreational and school sport, and has been found to be suitable for developing neuromuscular control. The metabolic requirements for neuromuscular control on slackline, however, have not been well described. Therefore, the aim of the study was to determine the metabolic demands of slacklining in less and more advanced slackliners. Nineteen slackliners performed several 4 min balance tasks: parallel and one-leg stance on stable platform (2LS and 1LS), 1 leg stance on a slackline (1LSS), walking at a self-selected speed and at a given speed of 15 m min⁻¹ on a slackline (WSS and WGS). Expired gas samples were collected for all participants and activities using a portable metabolic system. During1 LS and 1LSS, there were 140% and 341% increases in oxygen uptake (V̇O₂) with respect to V̇O₂ rest, respectively. During slackline walking, V̇O₂ increased by 460% and 444% at self-selected and given speed, respectively. More advanced slackliners required mean metabolic demands 0.377 ± 0.065 and 0.289 ± 0.050 kJ·kg⁻¹·min⁻¹ (5.7 ± 0.95 and 3.9 ± 0.6 MET) for WGS and 1LSS, respectively, whilst less advanced slackliners, 0.471 ± 0.081 and 0.367 ± 0.086 kJ·kg⁻¹·min⁻¹ (6.4 ± 1.2 and 5.0 ± 1.1 MET) for WGS and 1LSS, respectively. Our data suggest that balancing tasks on slackline require V̇O₂ corresponding to exercise intensities from light to moderate intensity. More advanced slackliners had a ∼25% reduced energy expenditure when compared with lower ability counterparts during simple balance tasks on the slackline.

Highlights

• Balancing on a slackline is metabolically demanding and slackline training is suitable not only to develop neuromuscular control but also to meet cardiovascular fitness demands.

• Improved postural control demonstrated by skilled slackliners reduces by ∼25% metabolic cost of balancing tasks on a slackline when compared to less skilled counterparts.

• Falls during slacklining increase the metabolic demands of the activity. Three falls per minute during walking on a slackline increase the oxygen uptake by ∼50%.

KEYWORDS:

• balance
• energy expenditure
• stability
• posture
• oxygen

Introduction

It is well established that the energy demands of exercise increase with increased muscle activity; speed of contractions and muscle mass involved (Åstrand et al., 2003). From that perspective, it may be considered that balancing tasks in standing positions or during slow movements require minimum amount of energy and therefore may not be considered beneficial to developing cardiovascular fitness.

Walking or balancing on a narrow ribbon tightened between two anchors – a slackline – has gained increasing popularity as recreational and school sport, and was found suitable to develop neuromuscular control in soccer, handball, basketball players and judoka (Jäger et al., 2017; Ringhof et al., 2019; Santos et al., 2014; Santos et al., 2016; Trecroci et al., 2018). To date, however, the metabolic demands have not been reported, although time spent slackline training during interventions has been reported at rates of 135–225 min per week (Granacher et al., 2010; Pfusterschmied et al., 2013; Pfusterschmied et al., 2013), which may represent sufficient time to promote cardiovascular health if the working intensity was sufficiently high (Bull et al., 2020).

Balancing on a slackline requires horizontal and vertical stabilization in order to respond to the dynamics of the challenge aiming to maintain contact with the slackline in a line between two anchors (Paoletti & Mahadevan, 2012; Stein & Mombaur, 2022). It has been shown that simple tandem standing on stable and unstable surface increased energy cost by 24% and 56% compared to parallel stance, respectively (Houdijk et al., 2009). During walking on a treadmill, lateral stabilization was responsible for 9.2% (Donelan et al., 2004) and enforced step pattern for 13% increases in metabolic cost (Wezenberg et al., 2011). Anecdotally, these stabilization demands appear more pronounced during basic slackline tasks such as walking and one-leg standing. Moreover, balancing tasks over the ground with a potential risk of fall result in greater effort for postural control, perceived exertion and metabolic cost (Baláš et al., 2021; Gajdošík et al., 2020; Pijpers et al., 2006).

Taken together, these factors may increase the metabolic demands of slacklining when compared with simple balance tasks on stable surface. These increases may attain values considered sufficient for health promotion. On the other hand, regular slackline training leads to improvements in postural control and decreases in H-reflex (Keller et al., 2012; Santos et al., 2016; Serrien et al., 2017), reducing, therefore, the metabolic cost of slacklining. It is, therefore, questionable, whether advanced slackliners would attain a sufficient metabolic expenditure during simple balance tasks on a slackline to contribute to improved cardiovascular fitness. The aim of the study was to determine the metabolic demands of slacklining for less and more advanced slackliners. We hypothesized that slacklining was a sufficiently demanding activity to meet recommended guidelines for maintaining or developing cardiovascular fitness (Bull et al., 2020; Garber et al., 2011).

Material and methods

Participants

Nineteen slackliners volunteered to participate in the study (14 males and 5 females and, mean age 29.7 ± 9.6 years; body mass males: 78 ± 12 kg; females: 65 ± 8 kg; height males: 180 ± 7 cm; females: 170 ± 5 cm). The entrance criterion was to have walked a slackline of at least 20 m without a fall in the last 3 months and to have a minimum 1-year experience with slacklining (mean distance without a fall 64 ± 35 m, experience 5.4 ± 3.0 years). These inclusion criteria were developed, as there was no agreement how to assess slackline ability level (e.g. long distance covered in a slackline may not correspond to a performance on a trickline (short elastic slackline) and vice versa). We assigned slackliners into one of two groups, less (N = 9; 3 females and 6 males) and more advanced (N = 10; 2 females and 8 males) ex post, based on median of the number of the fall (median 4; mean 10; range 0–36) from the slackline during the whole testing. All subjects were healthy non-smokers and were asked to refrain from exercise 24 h, caffeine 4 h and food 2 h drinks prior to testing. At the beginning of the study, the participants were informed of the study design and gave written informed consent. The study conformed to the recommendations of the local Research Ethics Committee in accordance with the Declaration of Helsinki.

Procedure

After completing anthropometric assessment and a skills questionnaire, participants completed a standardized warm-up: 5 min of mobilization exercises, 10 min of intermittent walking on different slackline types and length. The testing protocol with balance tasks is depicted in Figure 1: standing on both legs on a solid surface (2LS); standing on a solid surface alternately on the left and right leg with a duration of 20 s (1LS), standing on a slackline alternately on the left and right leg with a duration of 20 s (1LSS), walking on a slackline at self-selected speed (WSS); walking on a slackline at a given speed 15m·min⁻¹ (WGS). The testing was performed in an outdoor park during the summer with an ambient temperature (23.4 ± 5.3°C) and humidity (44.5 ± 11.2%).

Figure 1. Design of the study.

Stable surface tests

To provide a baseline, participants completed 4 min 2LS and 1LS with open eyes on a flat solid surface (concrete). The feet distance 2LS was set shoulder width. For 1LS, right and left leg were changed alternatively after 20 s.

Slackline tests

For slackline tests, a 50 mm wide slackline (Equilibrium Slacklines, Jablonec nad Nisou, CZ) was anchored between two fixed points spaced 10 m. The middle point of the slackline was 76 cm over the ground and the tension was set that after loading with a 20 kg weight, the middle point lowered by 14 cm. Generally, participants were around 40 cm over the ground when walking in the middle of the slackline.

Firstly, participants performed 1LSS in a place called “sweet spot”. They completed this using their right and left leg alternatively for 20 s. Then two slackline walking tests were completed: the first one at a self-selected speed, the second one at a given/controlled speed (15 m min⁻¹). The slackline was labelled with coloured marks every 2 m. These marks had to be attained within 8 s. The researcher monitored adherence to the given speed and verbally encouraged participants to accelerate or decelerate during the WGS. At the end of slackline, a small balance support from the tree was allowed for turning. In the case of fall from the slackline, participants returned to the slackline at the place of the fall and continued in walking. A participant’s return to the slackline had to be completed as fast as possible and the participant had to accelerate to comply with the expected time before the next turning during WGS condition. Both touching the ground with a foot or a complete fall from the slackline were considered as a fall.

Gas analysis

Oxygen uptake (V̇O₂), expired minute ventilation (V̇_E), tidal volume (VT) and breath frequency (BF) were measured using a breath-by-breath portable metabolic system (MetaMax 3B, Cortex Biophysik, Germany). Heart rate (HR) was monitored using a chest belt (Polar Electro OY, Finland) which transmitted data automatically to the MetaMax 3B. The MetaMax 3B device was worn on the chest with a harness (total weight 1.4 kg). Participants wore a face mask. Gas calibration was performed using a reference gas (15% O₂ and 5% CO₂). Before each test, ambient air and volume calibration were undertaken. Resting V̇O₂ (V̇O_{2 rest}) was taken from the last minute of the seated rest before the 1LS. Gas samples and HR values during standing and slackline tests were averaged over the whole 4 min intervals for data analysis.

Statistical analysis

Descriptive statistics (mean ± SD) were used to characterize anthropometric data and physiological responses in all slackliners. The magnitude of differences between ability groups was assessed using the Cohen’s d. Based on its value, the ability effect was rated as small (d < 0.5), moderate (d = 0.5–0.8) and large (d > 0.8). Only large effect was considered meaningful in the current study. Inferential statistics was not applied as the ability categorization was made post-hoc and the differences would be difficult to generalize. The effect of number of falls on V̇O₂ variability during 1LSS and WGS were calculated using linear regression model $\dot{V}{\mathrm{O}}_{2}1\mathrm{LSS}\mathrm{or}\mathrm{WGS}=a+b\times F$, where the F indicates number of falls, the y-intercept (a) represents V̇O₂ at 1LSS or WGS without the effect of fall, and the slope(b) indicates the V̇O₂ increases due to number of falls. To have a simple indicator of metabolic demands, MET as a ratio of energy expenditure during an activity to the rate of energy expended at rest was used. [One] MET is the rate of energy expenditure while sitting at rest (by convention) and is equal to V̇O₂ of 3.5 ml·kg⁻¹·min⁻¹. Energy expenditure (EE) in kJ·kg⁻¹·min⁻¹ was calculated using metabolic equivalent for O₂ 20.9 kJ (Bertuzzi et al., 2007).

Results

During the standing tests on a stable surface and the slackline, there was 140% and 341% increase of V̇O₂ with respect to V̇O₂ rest for 1LS and 1LSS, respectively (Figure 2(A)). On stable surface, there were no meaningful differences between less and more advanced slackliners for 1LS and 2LS, however, large differences were found for 1LSS (d = 1.23) (Table 1, Figure 2(B)).

Figure 2. (A) Oxygen uptake (V̇O₂) during seated rest, parallel two leg stance on a stable surface (2LS), one-leg stance on a stable surface with 20s right and left foot alternation (1LS), one-leg stance on slackline with 20 s right and left foot alternation (1LSS), walking at self-selected speed on slackline (WSS); walking at given speed (15 m·min⁻¹) on slackline (WGS). Percentages represent V̇O₂ increases with respect to seated rest. (B) V̇O₂ in less and more advanced climbers during balancing on stable surface and slackline. * represents large effect of slackline ability (Cohen’s d > 0.8).

Table 1. Oxygen uptake (V̇O₂), pulmonary ventilation (V̇_E), tidal volume (VT), breathing frequency (BF) and heart rate (HR) during balancing tasks in less and more advanced slackliners. Bold format represents large effect of slackline ability (Cohen’s d > 0.8).

	Less advanced	More advanced	Cohen’s d
	N = 10	N = 9
Standing stable surface - both legs
V̇O2 (l·min^−1)	0.37 ± 0.06	0.38 ± 0.08	0.17
V̇O2 (ml·kg^−1·min^−1)	4.9 ± 0.8	5.1 ± 0.8	0.18
V̇E (l· min^−1)	12.7 ± 2.3	13.2 ± 1.9	0.23
VT (l)	0.84 ± 0.33	0.80 ± 0.27	0.13
BF (breaths·min^−1)	17 ± 5	18 ± 5	0.17
HR (beats·min^−1)	88 ± 15	88 ± 16	0.00
EE (kJ·kg^−1·min^−1)	0.103 ± 0.016	0.106 ± 0.016	0.21
Standing stable surface– one leg
V̇O2 (l·min^−1)	0.47 ± 0.08	0.49 ± 0.10	0.21
V̇O2 (ml·kg^−1·min^−1)	6.3 ± 0.9	6.6 ± 0.9	0.26
V̇E (l· min^−1)	14.7 ± 2.2	15.3 ± 2.7	0.25
VT (l)	0.91 ± 0.32	0.85 ± 0.32	0.19
BF (breaths·min^−1)	19 ± 5	20 ± 5	0.24
HR (beats·min^−1)	91 ± 16	89 ± 14	0.10
EE (kJ·kg^−1·min^−1)	0.132 ± 0.019	0.137 ± 0.018	0.26
Standing slackline – one leg
Falls (n)	6.5 ± 3.7	0.3 ± 0.7	1.50
V̇O2 (l·min^−1)	1.30 ± 0.26	1.02 ± 0.19	1.23
V̇O2 (ml·kg^−1·min^−1)	17.6 ± 4.1	13.8 ± 2.4	1.12
V̇E (l· min^−1)	35.9 ± 7.0	27.2 ± 5.9	1.35
VT (l)	1.43 ± 0.47	1.28 ± 0.30	0.40
BF (breaths·min^−1)	27 ± 6	23 ± 7	0.68
HR (beats·min^−1)	129 ± 20	117 ± 21	0.60
EE (kJ·kg^−1·min^−1)	0.367 ± 0.086	0.289 ± 0.050	0.98

During walking on slackline, V̇O₂ increased by 460% and 444% at self-selected and given speed, respectively (Figure 2(A)). While the V̇O₂ was similar between more and less advanced slackliners at self-selected speed, more advanced slackliners demonstrated lower V̇O₂ at given speed than less advanced (d = 1.28). (Table 2).

Table 2. Oxygen uptake (V̇O₂), pulmonary ventilation (V̇_E), tidal volume (VT), breathing frequency (BF) and heart rate (HR) during walking on slackline in less and more advanced slackliners. Bold format represents large effect of slackline ability (Cohen’s d > 0.8).

	Less advanced	More advanced	Cohen’s d
	N = 10	N = 9
Walking on slackline at self- selected speed
Speed (m·min^−1)	12.2 ± 4.1	16.5 ± 4.7	0.97
Falls (n)	6.0 ± 0.8	3.0 ± 1.0	2.30
V̇O2 (l·min^−1)	1.67 ± 0.29	1.48 ± 0.36	0.57
V̇O2 (ml·kg^−1·min^−1)	22.4 ± 4.3	19.8 ± 3.5	0.66
V̇E (l· min^−1)	44.7 ± 9.3	36.4 ± 8.4	0.93
VT (l)	1.59 ± 0.48	1.42 ± 0.33	0.41
BF (breaths·min^−1)	30 ± 7	27 ± 6	0.49
HR (beats·min^−1)	137 ± 20	125 ± 17	0.65
EE (kJ·kg^−1·min^−1)	0.469 ± 0.089	0.415 ± 0.074	0.64
Walking on slackline 15m·min^−1
Falls (n)	5.5 ± 0.7	3.2 ± 0.7	2.04
V̇O2 (l·min^−1)	1.68 ± 0.32	1.34 ± 0.29	1.13
V̇O2 (ml·kg^−1·min^−1)	22.5 ± 3.9	18.0 ± 3.1	1.28
V̇E (l· min^−1)	45.8 ± 9.5	34.2 ± 7.9	1.32
VT (l)	1.54 ± 0.47	1.35 ± 0.32	0.46
BF (breaths·min^−1)	32 ± 6	27 ± 7	0.76
HR (beats·min^−1)	137 ± 22	121 ± 20	0.78
EE (kJ·kg^−1·min^−1)	0.471 ± 0.081	0.377 ± 0.065	1.09

Similarly, all physiological variables in less advanced slackliners were more elevated during 1LSS and WGS than in more advanced slackliners, although only V̇_E and EE reached meaningful difference (Tables 1 and 2).

More advanced slackliners required mean metabolic demands 28.0 ± 6.9 and 21.4 ± 4.7 kJ·min⁻¹ (5.7 ± 0.95 and 3.9 ± 0.6 MET) for WGS and 1LSS, respectively, whilst less advanced slackliners, 35.2 ± 6.0 and 27.2 ± 4.0 kJ·kg⁻¹·min⁻¹ (6.4 ± 1.2 and 5.0 ± 1.1 MET) for WGS and 1LSS, respectively.

The effect of number of falls on V̇O₂ variability during 1LSS and WGS was estimated by regression equations $\dot{V}{\mathrm{O}}_{21LSS}=13.4+0.66F$ (adjusted R²= 0.49) and $\dot{V}{\mathrm{O}}_{2WGS}=17.9+0.76F$ (adjusted R²= 0.36), respectively. Hence, falls explained 36–49% of V̇O₂ variability during WGS and 1LSS, respectively, across the sample group. The first model shows the mean V̇O₂ 13.4 ml·kg⁻¹·min⁻¹ for 1LSS without any effect of fall and V̇O₂ increases by roughly 0.7 ml·kg⁻¹·min⁻¹ per 1 fall from slackline during a 4 min task. The second model indicates the mean V̇O₂ 17.9 ml·kg⁻¹·min⁻¹ for WGS without any effect of fall and V̇O₂ increases by roughly 0.8 ml·kg⁻¹·min⁻¹ per 1 fall.

Discussion

The main finding of the study was that simple standing and walking tasks such as 2LS and WSS on slackline require on average 3.3-fold and 4.5-fold V̇O₂ rest during outdoor conditions, respectively. Moreover, less advanced slackliners attain V̇O₂ over 20 ml·kg⁻¹·min⁻¹ when walking on a slackline at given 15 m·min⁻¹ or self-selected speed and have roughly by 1/4 greater metabolic demands for the same balancing task then more advanced slackliners.

The metabolic demands of the balancing tasks on slackline corresponded to moderate intensity domain for this young population (20–39 yr; 4.8–7.1 MET) (Garber et al., 2011). Only 1LSS in advanced slackliners was rated as light exercise. The observed elevation in metabolic cost was surprising since maintaining balance control during tandem stance on a spherical balance board was responsible for a 160% increase when compared to parallel stance (Houdijk et al., 2009). This, 160% increase, represented a relatively smaller rise when compared to over 300% metabolic increase observed in our study. It appears that slacklining requires a considerably greater metabolic response than stabilization tasks on unstable surfaces. This may be the result of the greater task difficulty or greater effort required for balance control, which in turn is perhaps related to perceived fall stress (Baláš et al., 2021; Pijpers et al., 2006). On the other hand, regular falls occur during slacklining, which may increase the metabolic cost. We have intentionally included falls in the calculation of the metabolic demands, as the falls are integral part of slacklining. These falls (touching the ground with one leg or a complete fall from the slackline) explained 36–49% of V̇O₂ variability during WGS and 1LSS, respectively, in the whole research sample. Falls, however, cannot explain the substantial increase of V̇O₂ during 1LSS when compared to 1LS, since practically no falls occurred for the advance slackliners. Therefore, it is most likely that increased postural control during slacklining is responsible for the surprisingly elevated metabolic demands which was confirmed by the linear model showing the net V̇O₂ (without a fall) averaged ∼13 ml·kg⁻¹·min⁻¹ and ∼18 ml·kg⁻¹·min⁻¹ for 1LSS and WGS.

Although elevated V̇O₂ meets the intensity criteria to maintain cardiovascular fitness, the intensity is likely not sufficient to develop maximal oxygen power or capacity in healthy fit individuals (Garber et al., 2011). Slacklining is typically an intermittent activity whereby short bursts of activity are interspersed with rest periods. Novice slackliners with low specific balance skills may encounter fatigue after a few minutes of training. However, the sum of net slackline balancing time during slackline training in slackliners with sufficient skills may represent a considerable exercise volume to increase daily energy expenditure and contribute, therefore, to meet recommended cardiovascular fitness criteria.

Walking on a slackline was metabolically more demanding than balancing on one leg and more falls occurred during the 4 min tasks. More advanced slackliners walked faster during the self-selected speed trial and performed more efficiently during WGS than the less advanced slackliners. We did not assess the metabolic demands of walking at 15 m·min⁻¹ (0.9 km·min⁻¹) on stable surface. However, if we approximate the energy expenditure from V̇O₂ – walking speed relationship equations in adults (Waters & Mulroy, 1999), the speed 15 m·min⁻¹ corresponds to the V̇O₂ 4.5 ml·kg⁻¹·min⁻¹. On the contrary, the V̇O₂ 18 ml·kg⁻¹·min⁻¹ of WGS (no falls) corresponds to a walking speed 103 m·min⁻¹ (6.2 km·min⁻¹) on stable surface (Waters & Mulroy, 1999).

More and less advanced slackliners differed in regard to the metabolic demands of the same task on a slackline. More advanced slackliners were defined as participants with a lower number of falls during all balance tasks on a slackline used (≤4 falls during 1LSS, WSS, WGS). The testing slackline used was perceived as very tight and some “high level” slackliners experienced difficulties to walk it without a fall (e.g. the highest number of falls 36 was reported in a slackliner with 130 m distance covered without a fall on a slackline). This outcome serves to underline the specificity of balance control with respect to slackline types and exercises. These results also highlight the difficulties of slackline ability assessment and confirm the statement about limited transfer of slackline-specific skills to static and dynamic standing balance performance tasks (Donath, Roth, Zahner, & Faude, 2017; Giboin et al., 2018; Giboin et al., 2019; Volery et al., 2017). We used the number of falls on a specific 5 cm wide slackline to distinguish ability level. Although the number of falls was related to V̇O₂, the falls explained only 1/3 of V̇O₂ variability during WGS. Therefore, mechanisms associated with less developed postural control such as greater centre of mass acceleration, arm movements in all three directions or increased rotations around vertical axis (Stein & Mombaur, 2022) were likely responsible for the increased metabolic demands during slacklining rather than stepping up the ribbon from the ground after a fall.

Some limitations and strengths of the study have to be acknowledged. The measurement was undertaken in an ecologically valid setting with a relatively high number of participants. The participants wore a face mask which might have reduced visual control and, therefore, increased the difficulty of the task (Houdijk et al., 2009). We did not assess metabolic excess of V̇CO₂ and excess post-exercise oxygen consumption (EPOC). A V̇O₂ steady state (no increases in V̇O₂) was attained in each testing condition and the V̇CO₂ /V̇O₂ exchange ratio did not exceed 1.00 in any participant. Hence, we suggest that any metabolic excess of V̇CO₂ and an EPOC would have only minor effect on the current results. Moreover, the study design necessitated six testing conditions and additional rest between conditions. To allow for monitoring of EPOC values would have greatly increased the test time for each participant, which would have increased the risk of participants encountering central fatigue (due to increased attention demands and time wearing face mask, etc.). As a consequence, we completed the study with the described methodology for each participant.

All participants represented relatively experienced slackliners as the entrance criterion was set to 20 m without a fall on whatever type of slackline. In less experienced participants, we can expect more elevated metabolic demands due to a greater number of falls and postural stabilization movements. On the other hand, non-standardized temperature during outdoor testing may have increased V̇O₂ due to thermal regulation of the body. Moreover, choosing a different slackline width with a lower tension may have provided different results. The mean V̇O₂ rest in the current study (4.7 ± 0.6 ml·kg⁻¹·min⁻¹) was higher than conventional 3.5 ml·kg⁻¹·min⁻¹, therefore the V̇O₂ increases with respect to V̇O₂ rest during the balance tasks do not correspond to MET increases. This discrepancy may be due to insufficient rest after the warm-up (5 min), psychological expectations and not standardized environmental conditions. However, these limitations do not have an overall effect on the conclusions of the study.

Conclusions

Our research suggests that simple balancing tasks on slackline necessitate oxygen demands corresponding to exercise intensity in the light to moderate intensity domain. Therefore, regular slacklining could contribute to meeting the recommended daily exercise doses required to maintain cardiovascular fitness. However, the advanced slackliners in our study exhibited 25% reduction in metabolic costs when compared with less advanced counterparts likely due to more effective body posture stabilization. More demanding balance tasks or other forms of aerobic activity may be required to maintain or develop cardiovascular fitness for advanced slackliners.

Acknowledgement

This work was supported by Charles University Programme Cooperatio, research area Sport Sciences - Biomedical & Rehabilitation Medicine.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Charles University Programme Cooperatio - Sport Sciences - Biomedical & Rehabilitation Medicine.